Where is the spark advance

Tiny
88
  • MEMBER
  • 1988 CHEVROLET CAPRICE
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 1,200,000 MILES
Poor acceleration, bad gas mileage, slight stutter when taking off from a complete stop. If it was an older car I would say the vacuum hose has slipped off the diaphragm on the vacuum advance, but I cannot find the diaphragm. It used to be on the side of the distributor (which I recently tuned up).
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Monday, April 17th, 2017 AT 6:19 PM

23 Replies

Tiny
CARADIODOC
  • EXPERT
Mechanical and vacuum advance are built into the car's computer. An elusive cause for retarded spark timing is signals from knock sensors. That could be caused by a sloppy and slapping timing chain. At the mileage you listed, the chain would have needed to be replaced a dozen times. That slapping can mimic a spark knock.
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Monday, April 17th, 2017 AT 7:37 PM
Tiny
88
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I am not getting a knock. The car was running extraordinarily well just a few weeks before. Even now the engine operates smoothly on acceleration and has a smooth and typical intake sound when you open the secondaries and kick it down. But it is slow to accelerate, as if there is no spark advance.

Is there is an electronic device in the distributor which signals for an advance or retard on the spark? If so, what is it called, where is it, and how is it serviced.

If spark advance and retard is completely controlled by the management computer how is it wired to the distributor and how can I tell if the trouble is in the computerized management system or in the wiring?
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Monday, April 17th, 2017 AT 9:31 PM
Tiny
CARADIODOC
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You have that backward for the vacuum advance. We add in vacuum advance only under very light load and when cruising at a steady speed. When you accelerate, you get a momentary unusually-rich mixture to prevent a stumble or hesitation. With so many molecules of fuel packed so closely together in the cylinder, it is easy for the flame front to jump from one molecule to the next. The fuel can burn so rapidly and easily that it causes a knocking sound as the flame front bounces off the cylinder wall. To reduce the chance of that happening, vacuum advance is lost, and spark timing is retarded to start the mixture burning a little later.

When cruising at a steady speed, best fuel mileage is what we're gong for. The mixture is as lean as possible. That makes the fuel take longer to burn. To get the most power from it, the fuel has to be done burning before the piston gets near the bottom of its stroke. To achieve that, the vacuum advance fires the spark sooner, to get the burning started earlier.

The mechanical advance is strictly an engine speed thing. The fuel must not explode in the cylinder. It has to burn and expand gradually and smoothly, but it has to be done burning by the time the piston is about 70 or 80 percent of the way to bottom dead center. At faster engine speeds, the fuel still burns at the same rate. Since it has less time to complete the burn, it has to be ignited earlier. That is the job of the mechanical advance.

The ignition system is triggered to fire a spark plug by a signal pulse from a crankshaft position sensor and / or camshaft position sensor. The cam sensor can be a pickup coil or assembly inside the distributor. The best example of that was GM's High Energy Ignition (HEI) system that first showed up in 1975. That system was totally self-contained in the distributor, and included the vacuum and mechanical advance. After adjusting base timing, the two advance systems inserted the amount of advance they wanted for varying load and speed. The built-in ignition coil was fired by the built-in ignition module the instant the trigger signal appeared.

The problem was when used with a carburetor, those could only be calibrated to be as efficient as possible for two conditions; idle and mid-throttle. At any other speed, the mixture had to be a little rich to insure against those stumbling and hesitation problems. We would like to have a perfect fuel/air mixture, but we could not risk having the performance problems a slightly-lean mixture would cause. Since fuel metering was at best, a compromise, there was no need to develop an ignition system that had advantages that could not be utilized. That all changed with fuel injection. Now we can adjust the mixture for varying coolant temperature, air temperature, engine speed, throttle position, throttle direction of change, throttle rate of change, barometric pressure, and load on the engine. To take advantage of all the things we can fine-tune the mixture for, adjusting spark timing also became necessary for more than just engine speed and load. There are dozens of things the Engine Computer is constantly looking at to determine when to fire the spark plugs, but the majority of those decisions are to achieve the lowest tail pipe emissions.

When you have a distributor, there will usually still be a pickup assembly in it. The problem is we cannot get a trigger pulse, then say we would like to fire the spark plug ten degrees sooner. It is too late then. Instead, we start out with an extremely high base advance, often as much as 30 to 45 degrees. The signal pulse still triggers part of the ignition module, but instead of the module firing the ignition coil, it sends the trigger pulse to the Engine Computer. The computer looks at a whole pile of information to calculate the amount of delay to achieve the desired timing, then it sends a pulse to the module in the distributor that tells it to fire the ignition coil.

By adding the computer control, we get a trigger pulse much too soon, then the computer says, "wait for it, wait for it, wait, okay, now". If we start with thirty degrees of advance, and the computer calculates we need ten degrees of advance, it will get a trigger pulse, then wait twenty degrees before telling the ignition module to fire the ignition coil.

The way to see if the system is working is to view live data on a scanner. That will show the "target advance", meaning the desired or calculated amount of spark advance needed, and it will show the actual advance currently being calculated. If you see the amount of advance is always rather low or is bouncing around a lot, that is when to consider the loose timing chain. There is a very good chance you will not hear that slapping, but the knock sensor picks up the vibrations and incorrectly interprets that as spark knock. If you unplug the knock sensor, you will set a fault code, (big deal), but you will get your timing advance back and engine speed and power will come back.

One thing that can confuse the issue is a sloppy timing chain retards valve timing. Late valve timing increases low-end torque. That is how an engine's personality is adjusted to get a big heavy motor home going from a stop sign. That increased torque can mask the reduced power from retarded spark timing.
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Monday, April 17th, 2017 AT 11:05 PM
Tiny
88
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That was very informative. Thank you. There is a module in the distributor that requires the distributor be removed to change it for a new one (I forgot the name of it and there is no name on the box, just the part number). That is the only part I did not change because I did not want to mess with the timing. Could that be the problem?

As for unplugging the knock sensor, where would I find that and is there any 'trick' to unplugging it (my Haynes manual does not cover models after 1981. Besides, I do not find Haynes to be comprehensive enough)?

Thanks for the help so far. 88
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Tuesday, April 18th, 2017 AT 9:26 AM
Tiny
CARADIODOC
  • EXPERT
There's two different ignition modules, depending on which engine you have. The module is held into the distributor with two small bolts. The more common one will have two spade terminals on one end, and three or five on the other end.

I suspect the part you're referring to is the pick-up coil. That plugs into the two spade terminals, and is a common failure part. Failures are higher on the older distributors with the vacuum advance assembly because its operation flexes the wires repeatedly. The shaft has to be removed to replace that coil. That requires removing the distributor.
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Tuesday, April 18th, 2017 AT 9:58 PM
Tiny
88
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Thanks for your reply. OK sounds like the pick-up coil should be replaced just to be on the safe side as all other parts were in or close to failure at the time I changed them out. It also sounds like if it isn't working it could cause the kind condition I described. Please correct me if I'm wrong.

***But what about the location of the knock sensor plug in? I'll need a little help to find it and unplug it. Also, is this just a temporary fix and will it have any detrimental affects on the engine?

Just to clarify. Are you saying that the timing chain should have been replaced every 10,000 miles (reply said ". Should have been changed a dozen times." In 120,000 miles)? What about the older cars? Didn't they have the same timing chains and weren't they pretty well permanent for the effective life of the engine, like the pistons?

Thanks. 88
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Wednesday, April 19th, 2017 AT 11:27 AM
Tiny
CARADIODOC
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The timing chains on a lot of GM engines in this era became sloppy and caused slapping problems as early as every 75,000 miles. You listed 1.2 million miles. No timing chains would last that long.

Timing chains become sloppy on all engines, but they don't make a slapping noise on most of them. Also, Chryslers and many other engines don't use a knock sensor, so erratic spark timing isn't an issue. The fastest way to identify this on GMs is to watch the "target spark advance" on a scanner. If it is bouncing around, unplug the knock sensor and see if timing becomes steady. You can also watch the timing marks with a timing light. If the marks are bouncing around, you could have worn bushings around the distributor shaft, but the timing chain is the much better suspect.

Back in the late '70s and early '80s, GM had a real lot of trouble with spark knock. Their excuse was "Spark knock is the sound of high fuel mileage". Well, okay, ... I guess, but that is what led to their use of knock sensors. They'd rather avoid complaints about the noise then explain why their fuel mileage was so low. Today we have better additives in gas to reduce the occurrence of spark knock.

I've never replaced a knock sensor, so I only know of a few locations based on other replies. The typical place to find them on GM V-8 engines in on the side of the block, under the cylinder head and exhaust manifold.
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Wednesday, April 19th, 2017 AT 3:28 PM
Tiny
88
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Alright! Things are getting clearer. I'll take a look on each side and see if I can find it.

Now. That's 120,000 miles, not 1.2 million, therefore I can safely say it may be due for its FIRST chain re&re (how do I get into the profile and change that mileage?)

I'm guessing the re&re on the chain is the same as older models. True? If so I can look it up in my manual. If not, looks like I'm going to need a little more info on how to.

Thanks. 88

PS: Should I be using a higher octane fuel? I use 87. Seems to run smooth at the moment but when the knock sensor is disconnected, will it knock?
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Wednesday, April 19th, 2017 AT 6:18 PM
Tiny
CARADIODOC
  • EXPERT
Disconnecting the knock sensor will not CAUSE a knock, but it can fail to reduce the occurrence of knocking. Higher octane gas is harder to ignite, and that will reduce knocking in the cylinder. It won't help your power loss if that is due to the knock sensor picking up timing chain slap. If the engine ran fine before, changing to a higher octane rating should only be considered if you just moved to a different part of the country. Thanks to our brilliant politicians, the gas companies have to supply all kinds of different formulations to different areas. People are getting sick from the fumes from the gas that's sold in parts of California, and in the southern counties of Wisconsin.
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Wednesday, April 19th, 2017 AT 10:43 PM
Tiny
88
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California was first to bring in reg's re. Pollution. Seems strange they would approve a formula that makes people sick. They better get on that right away before it does permanent damage I think.

I think I'll leave that knock sensor for now and, as you suggest, keep using the octane that's working (and cheaper). Knocking can do serious damage to pistons and cylinder walls. Best to leave it alone for now. It must be working because the engine isn't knocking (and it might knock if I disconnect the knock sensor). It's very smooth, actually, just slower than it was. If the timing chain is overdue, I think that might be the wise place to start. Is it a big job?

Thanks again. 88
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Thursday, April 20th, 2017 AT 12:13 AM
Tiny
CARADIODOC
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The job depends on your skill with tools, and experience level. There's two different 5.0L engines for your car. If I was doing this job, I'd get a copy of the manufacturer's service manual, and read through the pages of instructions with line drawings. I can do a '70s Chrysler small-block V-8 in my sleep, but any other engine would be a learning experience. I'd rather read about the job first than rely on trial and error on the car.

From what I heard on the news a few years ago, the fuel problem in CA involved an additive getting into the ground water due to leaking tanks at gas stations. The problem we have in WI is from breathing the additives in the exhaust fumes. The politicians, (those people who know nothing about the things for which they make rules the rest of us have to live by), require that special formulation in the seven southern counties, which doesn't affect me. They think the exhaust fumes are going to stay in those counties and not be blown to other areas by the wind, ... I guess.
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Thursday, April 20th, 2017 AT 3:29 PM
Tiny
88
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I looked through my old manual just to get an idea. It's a job just getting to it. And I'll have to buy at least a timing light just to make sure the timing is right after I'm done (I can't find my old one). Maybe I'll see if I can afford to have someone with more experience do it (like a real mechanic). There's also the pick up coil that should probably be changed.
. How much do you think those two jobs would cost me (I already have the pick up coil)?

By the way. The car drove perfectly this afternoon. Go figure?
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Thursday, April 20th, 2017 AT 9:04 PM
Tiny
CARADIODOC
  • EXPERT
We don't get involved with costs here because there's too many variables. Flat rate time calls for 5.1 hours, then if necessary, add 0.4 hours with air conditioning, 0.2 hours if it has an air pump in the way, 0.5 hours if "electronic engine controls interfere", (I don't know what that means), add 0.3 hours if the power steering interferes. That comes to 6.5 hours for an experienced mechanic. It would probably take me 8 to 10 hours. For me, that's a 2-hour job on the old Chrysler engines without AC.

If your engine uses a carburetor and mechanical fuel pump, the pump can be removed, then you can fashion a tool from a piece of coat hanger wire to reach in and tug on the timing chain to see how much slack it has. You can also watch how long it takes for the distributor shaft to start to turn when you turn the crankshaft back and forth by hand. When you change direction, if the chain is sloppy, you have to turn the crank far enough to take up that slack, then the distributor shaft will start to turn.

If dollars are an issue and time is not, you might look for a nearby community college with an Automotive program. We were always looking for live work to give the kids real-world learning experiences. We charged ten dollars for what the job was supposed to take, and we got parts at real good discounts, then marked them up ten percent to form a "breakage" fund in case we damaged something. Be aware the students are in the classroom part of the day, so it could take a couple of weeks to get the car back. Also, most schools only teach each subject once, or maybe twice per year. They will only consider working on your car while they're teaching Engine Repair. They won't do engine work during Brakes or Electrical class, for example because that would take work away for the shop owners who we want to hire the graduates.
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Friday, April 21st, 2017 AT 8:10 PM
Tiny
88
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Thanks so much for that info. We do have a university with an auto mechanics program nearby. I'll check with them. I've noticed over the past two days (because I've been paying special attention) that the car is very smooth when I put my foot into it and open the secondaries (though no real hot rod, if you get my meaning) but rather tentative at the lower speeds, sometimes going and slowing when just feathering the gas. Does that narrow it down to anything specific? Does that sound like the variation of spark advance/retard you described earlier?

Is there a 'stop leak' solution that works (really works) on a leaky crankcase gasket? I've torqued the bolts on the oil pan twice (to make sure). They're tight and I'm still getting a little seepage. I don't want to raise the engine to take off the oil pan. That's not an option.

And yeah, for the good old days when pulling a timing chain wasn't a major project- among other things. Or rebuilding a carburetor was something you did without hesitation and without special instruction. When changing spark plugs wasn't a job for a contortionist. When doing a simple re&re on a part didn't involve taking half the car apart. When you looked forward to tuning up the engine on a Saturday morning or showing off the new intake you put on before lunch. Yeah!

Thanks and best wishes. 88
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Friday, April 21st, 2017 AT 8:59 PM
Tiny
CARADIODOC
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I'm not an expert on "mechanic-in-can" or other additives, but I can share a great and wondrous story on oil leaks. All engine oil has to meet the latest ratings that might be required with a certain engine. That is shown by the "S" and "C" ratings on the container. "S" stands for "spark ignition", meaning a gas engine, and "C" is for "compression ignition", meaning diesel engines. Each time there's a new set of standards, the second letter will increase by one, so "SG"-rated oil is an improvement over the no-longer-produced "SF"-rated oil.

Beyond those ratings, every motor oil producer has their own proprietary formula for the additives they use. Those include detergents to remove deposits, dispersants to hold the bad stuff in suspension so it can be carried to the filter, anti-foaming agents to prevent air bubbles from being whipped into the oil when it gets hit by the spinning crankshaft, viscosity index improvers to maintain the oil's thickness so it will continue to isolate moving parts from each other, (think the.006" clearance between an engine bearing and the journal it rides on), seal conditioners, to keep them soft and pliable, and corrosion inhibitors, to, ... Uhm, ... Prevent corrosion.

Where the problem comes in quite often is when you switch to a different brand of oil during an oil change. The typical V-8 engine holds around seven quarts of oil, but only five of that drains out when you do the oil change. The other two quarts is stuck in the oil pump, passages, lifers, and places in the heads where it pools. Most of the additives wear out in about 3,000 miles, but what remains might not be compatible with the additives in the new oil. If the new detergent attacks the seal conditioners in what's left of the old oil, it could expose a leak, or make some minor seepage turn into a noticeable leak.

Another source of trouble appears to show up when switching to synthetic oil. I've heard about this from a number of people, and witnessed it with two students, but I've never read about this anywhere yet. Engines have developed fairly bad oil leaks within a couple of weeks of switching to synthetic oil. My students switched back after a couple of months, then it took a second oil change after another couple of months, then the leaks slowed down a lot. The leaks never totally dried up, but they were small enough to ignore.

As for your oil pan, by '88, GM had been using gasket sealer from a tube on their rear axle differential covers for quite a while. That eventually became standard on many other applications, including oil pans. You can still buy cork or fiber gaskets, but you still need to clean the surfaces, so you might as well just use the sealants. If you can unbolt and drop the pan enough to fish the fiber gasket into the pan, then under and around the oil pump's pick-up tube and screen, you can add it that way. It is going to have to cut into and seal through the sealant residue. This is an even bigger problem on rear-wheel-drive Fords. The oil pans rust out and leak, so they have to be removed, and that requires lifting the engine, often to the point of damaging the wiper motor, if you aren't paying attention.

The best way to do this is obviously to lift the engine, if necessary, then scrub the surfaces clean, then use the sealant. I'm very familiar with Chrysler's products. Other manufacturers have the same stuff under their own name. Chrysler's black sealant cures more rubbery, but it absolutely will not bond and seal anywhere there is a hint of oil residue on either surface. This can be a problem on the four-speed automatic transmission pans. Transmission fluid will still be draining and running across part of the gasket surface a day later if you care to wait that long. There's some easy ways to overcome that, but instead, they have a gray sealant that WILL seal through a little film of oil. That stuff cures a little harder, and in some applications that makes it harder to scrape off next time. I like the gray stuff the best because when you make a piggy-mess of yourself, it is stickier and not as slimy compared to the black stuff. Once it cures on your fingers, it's easier to rub off.

You don't have to wait the hour or so for either one to cure. You can add the oil or transmission fluid right away.

I'm not opposed to you using an additive to try to stop the oil leak, but I'd start by looking at the oil you're using. Some oils advertised for "high-mileage" engines claim to have extra additives to address leaks, which are more common in older engines. A lot of these types of additives can be left out of cheaper brands of oil without negatively affecting the oil's ability to do the important parts of its job, just like leaving power windows off a car doesn't affect its ability to get you back home.

Mechanics don't use additives for things like this, so it doesn't pay to ask their opinions. Instead, if you're going to go with a can of something, ask the people at an auto parts store what they recommend and have had good luck with. They receive training and related information from their suppliers, and they know when to warn you against using a certain product.
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Friday, April 21st, 2017 AT 10:54 PM
Tiny
STEVE W.
  • EXPERT
Has the carb ever been rebuilt to handle the ethanol in modern fuel? Your description sounds like it's running very rich, especially when you say it runs better when you open the secondaries. There are a bunch of rubber seals and O-rings in that carb. None of them are alcohol safe. It may simply be a case that the carb is flooding the engine at lower speeds.
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Friday, April 21st, 2017 AT 11:07 PM
Tiny
88
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That's very interesting. I usually use Castrol GTX- 5-30 in winter, 10-30 in summer. But I have used other oils like Valvoline and Quaker State (even store brands occasionally) if the GTX isn't on sale and others are.

I looked at a couple of stop leak additives today. Bardol was one of them but I'm a little hinky about crankcase additives, that's why I asked. That stuff usually goes into old oil burners and I wouldn't want to stress the seals on the rings, valves, and guides with anything I'm not sure of.

No- I've never used a carb kit on this one. It's been fine up to now. Also, this thing looks a lot more complex than the old ones I've done before. I'm afraid if it comes apart in too many pieces I might not be able to get it back together and I can't have the car down right now. If it were flooding, though, wouldn't it stall at idle or is there some computer program to compensate for that, too? Remember, I did start with an idle problem.

Thanks & Best wishes. 88
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Saturday, April 22nd, 2017 AT 12:15 AM
Tiny
88
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Reply for all three answers is above. Thanks to both of you. 88
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Saturday, April 22nd, 2017 AT 12:20 AM
Tiny
STEVE W.
  • EXPERT
It all depends on how much extra fuel is going into the engine. The symptoms you mentioned in the original question match a rich condition. Stumble on acceleration because it's already running rich and now your adding more fuel, Then it gets the extra air and keeps running. Poor mileage, again because it's running rich all the time.
Then you mention it runs better when you open the throttle, that dumps in more air.

Yes there are computer controls on that carb. That car has the Computer Command Control so the carb has a mixture control solenoid, an idle load compensator and idle speed controls on and in it. Plus a MP sensor, TPS and more. Basically everything a fuel injected engine has, but it uses a carb.

There are a LOT of tests and measurements involved that can show you what is going on, something like 24 pages just on the carb! You need the better manual though. I actually like the Mitchell online service. For something like 20-30 dollars a year per car, you get the same information a shop gets and that we have. Shows all the tests and numbers.
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Saturday, April 22nd, 2017 AT 3:53 AM
Tiny
88
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Looks like I'll need an education to work on that carb. I'm beginning to think the expression "leave well enough alone" applies.

Thanks for the info, though. If I'm feeling brave I'll take a look through Mitchell or see if there's a Chilton's in the library that covers that model.

Just a thought. Know anything about converting over to sequential fuel injection on this model? Not the throttle body but the sequential injectors. Possible? Practical? Too costly? Too much work? All of the above?

Thanks again. 88
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Saturday, April 22nd, 2017 AT 11:09 PM

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