1994 GMC Sierra oil changes in old trucks

Tiny
SAVANNAHSTRUCK21
  • MEMBER
  • 1994 GMC SIERRA
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 240,000 MILES
So my truck is up for an oil change. This truck has been passed down and my grandfather has had it for 7 or 8 years without changing the oil just the filters. (I know it sounds bad). But I've been fixing little things at a time. I take it to the same guy to do all the work and he says that my engine needs to be flushed out. However I have heard from other people that my engine is tight and if I flush all that out it will become loose and ill have more problems. I will be putting a new engine in it in a couple years but for now I want to keep it as sound as can be. I need opinions.
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Sunday, January 12th, 2014 AT 4:19 PM

4 Replies

Tiny
CARADIODOC
  • EXPERT
First of all, not changing the oil is really really stupid, and I should know. More on that later.

When it comes to advice on oil brands and oil changes, people are going to go by what they have experienced in the past, and those experiences are going to be as varied as the people who had them and the vehicles they involved. I can offer some generalizations that you can use when you hear these stories.

All oils contain a lot of additives that wear out over time, typically in about 3,000 miles. The oil is still oil after that time, and it still has the same lubricating properties, ... However, some of those additives that wear out improve on those lubricating properties. You may have lost the additional protection but you haven't lost all of the protection. Oil lubricates by isolating moving parts from each other when it's under pressure. It doesn't lose that ability from age.

Another function of oil is to remove heat, mainly from the pistons and cylinder walls. It can still do that as long as the level isn't too low and it can be pumped around freely. It also carries dirt away to be trapped in the filter. Here is where you can run into problems. All engines have some blowby past the piston rings, and that exhaust gas and unburned raw gas condenses in the oil to form sludge. In its worst form, that sludge sits in the bottom of the oil pan and doesn't do much, but when it is dissolved into the oil, it passes right through the filter just like the rest of the oil. Sometimes, when the engine is stopped, that sludge coagulates in the filter and plugs it. All engines have a bypass valve to allow unfiltered oil to reach the engine when the filter becomes plugged. That valve can be built into the filter or into the filter housing on the engine. Dirty, unfiltered oil is bad, but it's better than no oil at all. The problem is you can have soot and other particles circulating in the oil and never know it. That is a big cause of worn crankshaft and connecting rod bearings, but we just attribute that to normal wear when we tear the engine down to rebuild it.

You will often hear people say switching to a certain brand of oil caused a leak or blue smoke from the tail pipe. I had a former student with an early '70s Ford pickup truck, (which is rarely seen here due to road salt and rust), that had no oil leaks. He started using a popular synthetic oil and immediately had a puddle wherever he parked. When he switched back to his original brand, the leak cleared up. At issue here, as I've been told by the experts, is the mixing of the different additives in different brands of oils. Those include seal conditioners, rust inhibitors, detergents, and viscosity index improvers. I've heard similar stories years ago, but people who stick with one brand of oil seem to have fewer leaks.

Now for the other side of the story I eluded to. I have well over 400,000 miles on the engine in my '88 Grand Caravan, (another vehicle you never see around here due to rust), and under the assumption the engine was on its last legs 12 years ago, I stopped changing the oil. At the time I had students chasing leaks so I was always adding oil anyway. Each time they found and fixed a leak, the next leak was worse. Turned out fixing those leaks cause more pressure to build up and push the oil out of the last leak, a pin hole rusted in the top of the oil pan. With that fixed, I still never changed the oil to continue the "experiment" to show my students what engines are capable of. What's happening though is the engine still uses about a quart of oil every 800 to 1,000 miles, so I'm always adding oil with that quart of new additives. One quart of oil has enough additives for the engine. Five quarts of new oil has five times the new additives but those don't last five times longer. All the additives wear out at the same rate. The goal is to get them to last 3,000 miles. I have new additives every 1,000 miles.

That's the good part of the story. There's still the sludge and blowby to deal with, ... And I haven't. In this respect, that is not neglect; it's abuse, and I am definitely not suggesting you or anyone else do this. My engine has run for so long like this and for over 100,000 miles that it's pointless to grow a conscience now. I add the cheapest hardware store oil I can find but I've stuck with that brand all these years.

I also have a '93 Dynasty that has had only one oil change in its life, but that car has just over 4,200 miles. I plan on changing the oil soon, and much more often than the recommended 3,000 miles, because the additives do wear out too with age, even when the engine doesn't get run much. That one only gets Mopar oil because I have a lot of it and that's what I started it on when I worked at the dealership. It is no better or worse than any other brands. They just buy it from a supplier and put their own name on it.

I have an '80 Volare that gets a name brand oil at each oil change, but that one too doesn't get changed too often. The car only has 45,000 miles on it.

You won't find anyone who knows about engines to tell you to not change the oil. The oils today are continually getting better and they're developing better additives. Your engine won't need or benefit from most of those improvements, but it's still very cheap insurance to help your engine experience the least amount of wear. Don't waste time planning on replacing your engine. I did that based solely on high mileage, and as it turns out, the engine is going to outlast the body. I still use that van to drag around a tandem axle enclosed trailer that's bigger than the van. Most engines wouldn't tolerate that either.
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Sunday, January 12th, 2014 AT 5:50 PM
Tiny
CARADIODOC
  • EXPERT
I forgot to add one really important observation. If you ever have a no-start condition that results in a lot of raw gas washing down into the oil, or you have a running problem due to fuel leaking through the vacuum port in the fuel pressure regulator, as a lot of GM engines did, get that oil out of there right away, as soon as the running problem is fixed. That gas will kill the oil's ability to lubricate and isolate parts.

In my case, the original battery in my Caravan lasted eight years, and while I should have known better and replaced it, it was finally unable to start the engine one day when it got down to 30 below. Ma tried to start it multiple times throughout the day but the engine cranked too slowly to start. What I didn't learn until years later was on that engine, when the battery voltage got sucked down momentarily by the starter, the Engine Computer turned off the spark, but not the fuel pump or injectors. Even though the starter picked up speed in a couple of seconds, there still was no spark until the ignition switch was turned off and back on to reset the computer, then the slow cranking started all over again. I knew from working at the dealership what needed to be done, but I just popped in a used battery, the engine fired right up, and off she went, ... For two round trips to town totaling 50 miles, before the engine blew up. Come to find out the oil was two quarts overfull due to all the raw gas that ran down past the piston rings when she was trying to start the engine. One connecting rod was hammered to shreds and all the other bearings were badly chewed up. All because I didn't check and change the oil.
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Sunday, January 12th, 2014 AT 6:04 PM
Tiny
SAVANNAHSTRUCK21
  • MEMBER
Okay thanks that gave me a lot of info. I am changing the oil tomorrow and I want to know if I should just change the oil and filters or if I should flush the whole engine out?
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Sunday, January 12th, 2014 AT 6:28 PM
Tiny
CARADIODOC
  • EXPERT
Flushing is not necessary. Years ago sludge could plug the passages to the lifters and prevent them from pumping up. Today the additives are better and are going to clean the engine. That gunk is going to collect in the filter, so you might want to do the next oil change sooner than 3,000 miles.

One trick some people used many years ago was to add a half a quart of automatic transmission fluid to the oil a day or two before they changed the oil. I've seen it work because of the different detergents, but transmission fluid is a lot thinner than engine oil and doesn't have the same lubricating properties. I've never used it in an engine myself.

I bought a four-year-old car in 1976 with 66,000 miles and poor maintenance. The valve cover gaskets were leaking, and when I replaced them, I found a good 1/4" thick layer of hardened sludge inside the covers. I added a special add-on filter that uses a roll of toilet paper. Between that and regular oil changes, I had to replace those gaskets again at 120,000 miles, and the valve covers were spotless. No special chemicals were used at any time. It was just the detergents in the oil that were responsible for getting rid of that sludge.
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Sunday, January 12th, 2014 AT 9:27 PM

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