New Radiator and Upgraded Environmental System

CITYGUYUSA

MEMBER
1968 CHEVROLET CAMARO
7.4L
V8
2WD
MANUAL
123,000 MILES

I had a 4 core brass and copper radiator and a belt-driven fan since I've had the car. When the first Vintage Air system was installed my mechanic suggested lowering the thermostat because it was running a bit warmer than he liked. I wouldn't think that would do a whole when the weather was hot because it's not like the thermostat was opening and closing but I hadn't really thought about it at the time it was a small price for what he seemed to think was going to make some difference. I guess it worked, I never had an issue with overheating.

Then about 2 years ago the radiator started leaking. I replaced it with a 2-row aluminum with an electric fan which to me made sense because that electric fan was going to keep running fast while the car was sitting at lights and therefore would keep the car even cooler and that was their promise.

Well, the radiator didn't fit so back it went and I was looking around for a radiator that had more of a 45-degree angle where the filler neck was. There only appear to be 2 radiators available for the classic muscle car. You either get a really boxy looking one or you get a more curvy one that has a 45-degree neck and everyone just brands the radiators as their own.

No one seemed to know what to tell me regarding 2-row or 3-row or 4-row. I did my research as best I could with what was available on classic car replacements. The only thing I could find was that the 2-row's did a better job of cooling than the 4 rows which didn't make a whole lot of sense to me but it seemed to be the same pitch regardless of who I talked to. Come to find out now that the reason that the 3 or 4-row radiators don't cool any better and maybe even a bit worse was that they weren't increasing the size of the radiator they were just adding rows into the existing one size fits all radiators. You can't get a 1.25" row in anything but a 2-row radiator because if you go to a 3-row they just make the rows narrower and they take up a little more space for the extra row and ultimately have less room to store coolant. But to have the public believe that it was somehow unique to aluminum seems to me to be a scam.

My problem was that the 2-row was causing my car to overheat and yet everyone I talked to ensured me that the 2-row was my best bet. There were a few unscrupulous people that tried to sell me a new radiator with more rows which was actually when I realized that the supposedly bigger radiators weren't any bigger.

I went through everything. I flushed the radiator because I found out that when they installed it no-one flushed the system, a brand new radiator and no one flushed out the Stop Leak that was put in other than inserting a hose into the block. I don't know what happened to my mechanic's brain but he won't even talk to me anymore. I guess he's feeling bad just not bad enough to apologize so this is pushing all this work onto me except I have serious chronic pain problems and I can't always do the work. There was a lot of Stop Leak in the coolant it took 2 flushes to get most of it out and as of last week, I saw some small deposits collecting again.

Anyway, I've still not found a culprit for why I'm having overheating problems I bought the radiator and fan as a package deal and the so-called experts that I've talked to all seem to think that I have plenty of CFM although must of my research suggests a fan in the 3000 CFM range and I'm at 2600 but they tell me that is more than most people have. Hmm? What the heck is going on?

Then I came across a person's website were he was customizing a cooling system for his Jeep and he was explaining the actual principles he used in his process. It was the first time the sun shined. He was talking about the "area" of the radiator which got me thinking about cubic inches or volume. The first thing I realized was the 16" fan that I was sold was only big enough to cool less than 50% of the actual area which I never questioned because that's how most of these aluminum radiators and electric fans are sold. I don't know if the combination is even a good combination that should assure a cool engine because they sell the same radiator pretty much just changing the core size and slapping on a single fan or 2 fans and because most of the cars are probably smaller 350's or even less it works. I'm unfortunately not one of those people that it comes together for, however. If you added the depth of course it becomes even worse. Instead of having a big belt-driven fan here was this 16" fan that was tight against the back of the radiator with an even tighter shroud that wouldn't allow any extra air in around the sides. That got me thinking on how to increase my fan coverage of the radiator without having to go back to the noisy mechanical fan which was always so loud and annoying.

The problem is that the actual radiator support only has an area of 23" X 17" which prevents getting any type of a combination fan that would cool more area. This is when I discovered that 2 - 10" fans only cover 50% of the area that 1 - 20" fan does. I know it seems crazy but it makes no difference what size fan you are using it always takes 4 smaller fans to cover as much area as a single fan 2 times bigger than the size of one little fan.

It would take 4 - 10" fans to cover the same area as 1 - 20" fan. It drove me nuts for an entire day and I'm still not sure that it makes any sense to me but I verified the equation on other sites to ensure that it wasn't an error on the site I was using. That makes this problem incredibly difficult to resolve were 1 - 16" fan-only covers 200 square inches but my radiator is 396 square inches. Again that doesn't cover depth which would bring it to 978 cubic inches or a little less than 25% of the volume. That seems terrible to me.

The radiator opening is the same size as it was back when the cars were originally built of course they only had a 396 cubic inch engine then I've got a 454. They also used the mechanical fan which is the only thing I don't want. I spent my day doing calculations trying to figure what would be the best setup within my existing radiator. Unfortunately, that would be to 11" fans maybe an 11.5" depending but that doesn't do any better than the 16" fan I already have. 2 -11" fans cover an area of 189 square inches technically a few inches less than my 16" fan.

If I am willing to get a custom shroud built I could get 2 - 14" fans in but they would slightly exceed the size of the core and they would have to be cut out around the input and output hoses and attached using the same holes as the radiator itself. 2 - 14" fans would bring my area up to 308 still less than my actual radiator but 110+ more surface area excluding the hangover onto the radiator support on the sides. I also might be able to create a setup where the shroud would tilt the fans towards the side of the engine where the exhaust exits and probably the hottest part of the engine. That actually makes some sense and it might fit or it might not there's not a lot of room between the radiator and the engine which of course has the long water pump. Why exactly did they have a long water pump, to begin with? What was the thinking?

Today I went out and looked it over again and thought I might be able to get 2 - 14" fans between the condenser and the front of the radiator and just get rid of the shroud which I think is probably causing less air to flow since it's tight against the radiator and tight against the fan. In my mind that creates a 16" tunnel in the center of the radiator that gets cooled. I can't see that the fan would pull anything but the area directly in front of it the shroud is just a restriction on airflow unless like I said you would tilt or move the fan away from the radiator and more toward the engine then you might get some extra flow.

I saw some 14" fans that are high output pushing some 4000 CFM which is impressive of course I might have some power problems trying to run them. But a marginal change is not going to get me a running car when it's all but 100 degrees outside and I've turned on the A/C which I haven't even been able to do. The few times I've had it out when it was really hot I had to have the heat on high.

Right now I can run the car for short periods when the temperature is in the low 70's but once it gets into the upper 70's I may get stuck somewhere if I get caught up in a traffic snarl.

The other option would be to have someone make custom radiator support that would allow me to have a wider radiator then I could get like a Chevrolet C10 radiator or even a Chevelle radiator that would give me some more choices. I wonder if I could get a Chevelle radiator support into my Camaro?

Another option would be to get a 10" fan in the upper left corner where the heated coolant enters the radiator. Maybe it would get enough heat out.

Anyway, I'm getting paralysis by analysis and not being a professionally trained mechanic I can get stuck on some of the details that maybe aren't so crucial. What are your thoughts?

5 Replies

CARADIODOC

MECHANIC
33,752 POSTS

Finally I found someone who writes longer stories than I do!

I have a suspicion you're over-thinking this in the wrong area. Keep in mind the engine didn't overheat when the car was new, so you have to look at what is no longer original. Your story about the number of rows of cores has me scratching my head. In that era, a three-core radiator was an uncommon sight, but they were used when needed. Two-core radiators were sufficient for most cars. A three core radiator had 50 percent more cooling area, both by coolant volume and by surface area, than a two-core radiator. It sounds like you're running into people who have three or four-core radiators that use smaller cooling tubes. That would create an advertising gimmick without adding any cooling capacity.

To address the radiator itself, I would take your original one to a radiator repair shop. Every larger city has at least one. If you aren't aware of any near you, visit any large truck repair shop. They use their services all the time, and they'll be able to tell you where the best ones are. It used to be standard practice to unsolder the two tanks, them pop them onto a new core. That way you keep your mounting brackets and filler neck.

The things to look for related to age that you haven't mentioned include things that affect air flow around the radiator. Specifically, your comments about the fan and shroud suggest a lot of the radiator's surface area isn't covered. Years ago people thought if they removed the factory shroud, there would be more air flow and less chance of overheating. In fact, the opposite was true. That shroud forced air to be drawn through all of the core. Than fan blade was set back far enough that it drew air from all corners of the radiator. Any part of your radiator now that is not covered by the fan isn't giving up its heat, so in fact, you're only using a fraction it.

If your car came with a thermostatic fan clutch but it's missing now, put one back on. Those were used to address the complaints of loud fans. They'll engage gradually as the air around the sensor warms up. Those clutches can fail too if the silicone fluid leaks out. To prevent that, the units are supposed to be stored standing up in their "on-car" orientation. To test one of these, if you're wimpy, like me, tie a string around a fan blade to hold onto. Have a helper start the engine. You'll feel a little tug at first, but it will be real easy to hold the blade from spinning. As the engine warms up, you'll feel the tug get stronger and stronger if the clutch is working properly. I watched an instructor do this by holding the blade with his fingers, but the problem then is you have to trust that your helper will stop the engine. If you let go with the engine running, you might get your fingers slapped around a little.

The next thing to look at is the rubber seals under the hood. There is going to be a long one that runs from fender to fender, right under the front of the hood. That could be clipped to the bottom of the hood or it can be snapped onto the top of the core support, then it seals against the bottom of the hood. If that seal is missing or dry-rotted, air that has come through the radiator and been heated up can blow up, over the core support and go right back through the radiator again and again. Now you're cooling the radiator with air that can be well over 100 degrees.

The same is true of any holes in the core support. Some were designed-in, such as one in front of the battery to keep that cool. I've seen those holes filled with washer fluid bottle necks, charcoal canisters, AC pipes, and things like that. When people remove those things to turn a car into a race car, they use thin sheet metal patches screwed onto those holes to plug them. If left open, air will again cycle around through the radiator multiple times, which seriously reduces its cooling ability.

While this was very uncommon years ago, don't overlook the possibility of a leaking cylinder head gasket. Any combustion gas that sneaks into the cooling system could pool under the thermostat causing it to not open. Thermostats have to be hit with hot liquid. Hot air won't do it. A clue to this is the engine overheats shortly after starting it, just about the time you would expect the thermostat to open for the first time in that drive cycle. If you can drive for ten minutes or longer before the overheating starts to occur, the thermostat has already opened by that time, so other external causes are the better suspects.

Also consider the water / antifreeze mixture. Water carries more BTUs of heat to the radiator, but we need antifreeze for its water pump lubricant, seal conditioners, and corrosion inhibitors. The way people get into trouble is they incorrectly think if some antifreeze lowers the freeze point to minus 30 degrees, more antifreeze and less water must be better. In fact, straight antifreeze starts to turn to gel at around minus ten degrees, and of course water freezes at 32 degrees. It's when the two are mixed that the freeze point goes lower, to a point. Once you get past around minus 45 degrees, if you add more antifreeze, the freeze point actually starts to go back up again. The problem is the little freeze-point testers with floating balls or pointers only measure the weight of the coolant compared to the weight of water. Antifreeze is heavier than water, so straight antifreeze makes all the balls float, and it looks like the freeze point is somewhere around minus 80 degrees. In this half of the scale, the weight of the coolant being measured does not follow the actual freeze point. To know the true freeze point, you need a "refractometer", which a lot of shops use. The point of this part of my story is if there is too much antifreeze in the mixture, it doesn't have the heat-carrying capacity that water does, so overheating can result.

Perhaps take a look at this article:

https://www.2carpros.com/articles/how-a-radiator-works

to see if you get more ideas. There's a lot more articles related to the cooling system here that might help:

https://www.2carpros.com/articles

SPONSORED LINKS

Was this

answer

helpful?

Yes

Monday, October 21st, 2019 AT 7:36 PM

CITYGUYUSA

MEMBER
86 POSTS

So quickly. I bought the car used. We only had a small overheating issue when the first A/C upgrade was put in which we resolved with a secondary fan, a 160-degree thermostat, a larger mechanical fan, and a shroud the kind that attaches to the radiator while the fan attaches to the water pump, Then maybe 5 years later my 4-row brass and copper radiator sprang a leak. The first attempt at replacement failed due to a non-fit. The second radiator included a 16" fan. If you look at any of the new aluminum radiators they all have the shroud right up against the radiator with the fan center. It looks pretty but the assumption is that the fan/shroud combination is enough to cool the engine which is what they assert but it can't keep every engine cool even though some sell based on the original car specs which mine wouldn' t fall into because it has 454bb and was only ever built with a 396bb. Others sell-by horsepower but that only makes assumptions about the heat of the engine, not the actual work the engine is doing and the resulting heat. This car came with a 37 amp alternator it now has a 100-amp all these little adds have an impact. We attempted repair of the old radiator several times the local radiator shop wanted $600 just to look and if the core was gone it would be $800 or more. Plus the cost of removing it. It was ridiculous when you could buy a new brass & copper 4-row for that same $800. Everyone I knew was getting out of the big brass and copper radiators they didn't cool as well and the weight alone was enough reason to move on to a new aluminum radiator. At the same time the new radiator was installed I had an opportunity to have the latest version of Vintage Airs' environmental system which in my existing version had no control over the heat which annoyed the hell out of me. How can you sell an environmental system that didn't control the heat and the defrost had the reverse problem it would only run without heat. VA offered me a free upgrade. I was nervous about the upgrade because the last one took us a few years to find a small leak on the backside of the Schrader valve and so every year I'd have to refill the system so I didn't want to have to go through whatever problems may result but I couldn't tolerate what I had. The new radiator was installed in September so not the best time to know how it would perform in the hot months. The new VA got installed in October and had some big surprises. First of the installer refused to pick up the phone regarding anything to do with the new system even though I had plastered my phone number everywhere I'd end up calling him and saying well if you'd have called me I could have told you and saved you the time of blah, blah, blah, Just an example he rewired some circuits that he didn't like the way they were wired without even knowing what they were or why they may have been wired that way and of course now lights that shouldn't be lit are and lights that use to light don't. Fan's appeared to have been rewired in reverse so the secondary 12" fan was blowing all the time and the main fan only ran when the A/C was on. So there's a lot that I tried to avoid getting into in my original story that I have to still account for. I knew there was a large power draw when the secondary fan was coming on, so much so that it was stalling the engine. I tracked it back to what appears to be an intentional short from the 30 posts on the relay to the 85 I think the one on the side that's supposed to be the trigger when I disconnected the fan all well except that fan no longer runs which is fine because I haven't turned on the A/C so it shouldn't be an issue. There's also some question as to the proper installation of the fan control with the short on the relay maybe causing some feedback into the controller and causing it to misfunction. There' also some question on having solid grounds, etc all this takes time to sort and because of all the problems no mechanics want to get caught up in the middle. There are only 3 things at the moment directly on the cooling system itself that I don't know for sure how to verify: 1) the flow of the coolant. I know it's flowing but I don't know if it's flowing slow or normal or if that's even a possibility. I was told that in a GM the water pump works or it doesn't and if it wasn't working the weep hole would be dripping and you'd hear the bearings clunking. Is there still some Stop Leak inside the tiny radiator passages keeping me from getting all I should from my new radiator because they didn't flush the system before they installed the radiator. The one thing you mentioned was regarding a vinyl type strip that went in between the radiator support and the radiator or the frame. I don't recall which at the moment, we'll call it a preview of getting older but I think we did initially have a problem with the first radiator because it sat so far away from the support but this one is pretty tight. I did by a closeout for the space between the grill and the radiator which is not in place currently because I can't check things with that in the way so there's some loss there. Third, I don't know how to verify the fan is actually pushing 2600CFM's. The bigger problem is the new condenser that is no longer a 12' or 14'" taking up a small space in front and to the side of the radiator, it's a full-blown condenser running the entire length of my radiator core and probably a bit more. The little 12' fan that used to keep that cool is just a joke now, of course, no one called and said do you know that your condenser has grown over the entire front of the car and that 12' fan isn't going to cut it anymore. No, they just put it back where they found it and since the close-out was in place I didn't even know until the night I went to disconnect that secondary fan. There's no longer a side-space that the secondary can kind of blow into that isn't going to cause airflow problems. When it was a mechanical fan there was plenty of room to blow a path of air through to the other side of the radiator now it just hits the front of the shroud and blows back at itself. Before I forget I did check the thermostat and I replaced it in case it has some type of slow action or was intermittent for whatever reason.

The real problem here was that the radiator never got a shakeout before the next layer of complications was added and there's no way I'm going to try to go backward. I know there are people out there have aluminum radiators in a late 60's Camaro with a bigger than an original engine with a 4:11 rear and every accessory under the sun including a top-end stereo with multiple amps. What I don't know is what that cooling system looks like. I asked a few guys when I found cars online that had shiny aluminum radiators in their pictures. The problem is that they may live somewhere that temps aren't a real problem or they paid other people to do frame-off restorations and really don't have a clue and don't want to look like they didn't do their own work so they don't want to respond. I've got Vintage Air and Dakota Digital in my contact list they've had me check some things but so far nothings popping out. When I ask about other people that have similar setups they only seem to be able to give me information from people that maybe they had worked with. There doesn't seem to be a list that you could pull off the cars similar to mine that overheat on days where the temps exceed 70 degrees.

To address my radiator and it's shroud. The shroud covers the entire area of the radiator. If you look at the 1st picture you literally can't see even the width of the shroud because it's just a sheet of aluminum between the radiator and the fan. It's not a shroud that makes a whole lot of sense to me, but it's what everyone is selling. The airflow is going to be in the center and like you noted it's not going to pull from the corners and the upper left is the hot spot. If they centered the fan in the upper left maybe it would be very different. I know the car gets cooler when it moves it just doesn't get cool enough. Conceptually I shouldn't need a fan at highway speeds because the airflow should outflow any fan. The heat keeps growing degree by degree the longer I sit the more degrees build this is not like it's cold right to overheating. But the higher the outside temp the faster the process which is why I'm thinking of just removing the shroud to see what difference it makes. The whole idea of going electric was that it would cool better sitting at a light but that doesn't seem to be true and yet it should be and the other was to reduce road noise. When the car is sitting still there's very little difference in temp from top of the radiator to the bottom, maybe 10 degrees, that's not enough. I'm not losing any fluid so I'm pretty sure I don't have a block problem. The fluid is all new and all a 50/50 mixture with distilled water and like I said before I did 2 chemical flushes because the mechanics didn't flush it out before putting the radiator in. The coolant is still clear at this point you can see through the coolant with ease I see a very minor accumulation of Stop Leak but nothing like what I had seen originally. I should also mention in winter it over-cools. Intermittent heat to the inside when going over 40mph or so. That was last winter and I've have dropped in a new thermostat I don't think any of the other changes would affect that interior heat of which I get plenty in summer. Never had a clutch fan probably because of clearance problems. This was originally a small block 350 Part of what I did when I got it was to fix what they missed when they converted the car to a 454 but there have only ever been 2 size radiators and one size radiator support for either core. I have the largest the other is only slightly smaller by 2 inches in width if memory serves.

I think I covered all your questions or responses, unfortunately, it's still a long story. It's a lot to take in but each discussion gets both of us closer to being on the same page. I may tell more than I should but something I might say different might make it more obvious. My honest opinion is that it's going to come down to that fan placement and a reshaped shroud. Because the shroud can't be refitted to another fan unfortunately maybe I could make a model out of wood paneling or something cheap where I could do some other configurations, of course, the cost of the fans doesn't allow a whole lot of choices. Maybe the first trial is just to try moving that 16" up and over towards the left and at the same time pushing it as close to the engine as possible but there's not a lot of space to go that direction.

Pic 1 is the space between the back of the new fan and the front of the engine. Tiny would be the word that comes to mind. The bottom edge of that picture would be the sheet of aluminum.
Pic 2 Passenger's side of the radiator you can see the condenser starts here, you can see the radiator itself and the support.
Pic 3 Driver's side of the radiator you can see the disconnected secondary sitting in front of the condenser which I had said I might be able to put 2 pushers in between the condenser and the radiator and may have better results.

Images (Click to make bigger)

Was this

answer

helpful?

Yes

Tuesday, October 22nd, 2019 AT 8:23 PM

CARADIODOC

MECHANIC
33,752 POSTS

I have two more thoughts for your consideration. The first is while I've never run into this myself, we always taught an engine can overheat when the thermostat has been removed. That's because the coolant doesn't stay in the radiator long enough to give up its heat. If you're running a 160 degree thermostat, that is going to open real early and prevent the engine from reaching proper temperature. That could explain the low heater output too. The same thing might be happening with your car. Heat should be given up from the radiator faster than it is being produced in the engine. If the coolant flows too fast, there's not enough time for that to occur.

At 160 degrees, the upper radiator hose is not going to feel very hot. At 210 degrees, you can't hold onto a hose very long, but you can grab it for a few seconds. The engineers at GM run a lot of their cooling systems up to over 226 degrees before the electric radiator fan turns on. I'm not comfortable with that, but that presents a much higher temperature difference between the radiator and the air flowing through it. The higher the coolant temperature gets over ambient air temperature, the faster heat is going to be transferred out of that coolant.

The second issue was something that just happened this past summer to my friend. He has a body shop where he specializes in rebuilding one and two-year-old smashed Dodge trucks. One of his personal projects was to turn a 2500 Diesel megacab into an extended version with an 8' box, and he made it a dually. These diesels have been fairly well-known for being cold-blooded. It takes them a long time to get them up to normal temperature, and overheating is very uncommon. The problem he ran into this summer was the pressure relief valve for the AC system blew multiple times causing loss of refrigerant. Each time he recharged it, the problem occurred again, but only after the truck had been standing still and idling for a long time. Finally he noticed the coolant temperature was a few degrees higher than normal at the same time.

Once this started happening more frequently, he had the gauges hooked up and watched the high-side pressure go way too high, again, while the truck was idling. It finally occurred to us that this is the first summer he helped a neighbor drag hay trailers off their fields, and there was a lot of dust. That dust had collected on the condenser, resulting in its inability to give off its heat. By spraying the condenser out with compressed air, the high-side pressure dropped from almost 500 pounds to around 250 pounds within less then ten seconds. If a cold-blooded engine saw a rise in coolant temperature, the same thing would likely affect other vehicles, but even worse. Just thinkin' out loud; don't overlook dust or a butterfly collection on the front of the condenser.

For the water pump, coolant leaks from the weep hole when the seal is leaking, that doesn't have anything to do with the bushing or bearing the shaft rides on, although if that bearing gets sloppy first, the shaft will walk away from the seal. I did have a freshly-rebuilt engine overheat within 600 miles due to the timing belt being slightly loose. The water pump was driven off the backside of the timing belt, so the assumption was it was slipping over the pulley. This is less likely to occur with your serpentine drive belt, but keep it in the back of your mind.

I'll share one more story about an overheating Chevy Cavalier. The overheating came on gradually, getting worse the longer the car was driven, and outside temperature didn't seem to affect it. The clue came when someone pointed out the exhaust manifold was orange after the engine had been idling for about 15 minutes. Turns out this was all caused by ignition timing being significantly retarded from specs. A tweak of the distributor solved both problems. By being so late with the spark, the fuel was being ignited too late in the power stroke to develop much power, then it went out the exhaust, still burning. The fuel was burning as it entered the exhaust manifold, and much of that heat went into the cooling system. Just one more oddity I'll probably never see again.

Was this

answer

helpful?

Yes

Tuesday, October 22nd, 2019 AT 10:04 PM

CITYGUYUSA

MEMBER
86 POSTS

I saw a video the other day on water pumps. This particular water pump they pulled was circulating fine but the feeds in and out had been partially clogged up with silicone causing a flow deficit. Now I feel a need to check mine. I did 2 chemical flushes since they failed to flush it before putting in the new radiator leaving it full of Stop Leak. I'm starting to wonder if they really did attempt to solder the leaks or just put a second can of Stop Leak in? It's the only thing that I haven't checked the functionally of because I don't know how. Somewhere I read you should let it warm up then shut it off. Have a friend restart it while you're holding onto the upper hose which they say should let you feel how fast it is moving through the system. Thoughts?

Was this

answer

helpful?

Yes

Monday, November 4th, 2019 AT 4:00 PM

CARADIODOC

MECHANIC
33,752 POSTS

I'm not aware of a way to check flow except by watching with the radiator cap removed, when it is the middle of the radiator. GM likes to use cross-flow radiators with the cap on an upper corner. With those, if the level is down a few inches, sometimes you can see the coolant flowing out of the upper rows of tubes, into the tank. Other than that, we typically go by feeling the temperatures. Radiator and heater hoses should be too hot to hold onto for very long. If you feel from top to bottom on the front of the radiator, it should be pretty hot on top, and cool on the bottom. If it's cool on top, (or on the inlet side with cross-flow radiators), the heat has had time to be given up too quickly. That's an indication the coolant isn't reaching proper temperature, (verify by feeling the upper hose), or it's circulating so slowly that the heat volume is too low.

The only story I've ever heard of volume being too low was with some models of Volkswagens a few years ago. Seems the engineers chose to use a plastic impeller on their water pumps, and the blades either became worn away, or the fan cracked around the center and became loose on the shaft. I know it was one of those things.

Seems to me I also heard something a long time ago having to do with bypassing heater cores on GM products. They used some unusual or uncommon characteristics in the design of their cooling systems that required flow through one of the heater hoses to get hot coolant to the thermostat. There was some modification to those hoses that led to overheating. The advantage of that design was hot air was available from the heater much sooner than on other car brands. I can't remember if this was caused by plugging the two heater hoses, so there was no flow through them, or by removing them, then connecting them together, so there was no heater core restriction to limit flow through them.