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Trying to get the engine to last until march

Hello All.

1989 GMC Sierra (S15) 4.3L v6

I’m trying to get the engine to last me until March. I have low compression in the #1 cylinder. I’m guessing it’s the rings but it’s hard to tell.

I did a compression check on all of the cylinders Sunday. To test I installed the compression gage and turned the engine for 4 seconds. This typically provided me with 2 full cycles of compression. The #1 cylinder came in at 65 on the first compression and 95 on the second with about a 2-3 second lag before falling back to around 65-70 and holding. All other cylinders came in 95 on the first and 115+ and hold on the second. Over the 2-3 seconds the drop in compression was negligible on cylinders 2-6.

I keep fouling plugs on the #1 cylinder. I have been running 20-50 but I am changing the oil back to 10-40 to hopefully allow the plug to function better and burn the oil completely. I’m also going to step up the plug heat 1 number and put a new anti fouling thingy on the plug. I’m hoping that this will keep the plug from fouling to often.

After changing the fouled plug I took it out on the freeway and ran it for about 10 miles. The smoking stopped after about 5 miles, but on a cold restart the plug was fouled again. I’m hoping the above changes and longer run (40+miles one way) will allow the cylinder to burn clean this time. On the way back from Charlotte to Columbia (maybe 1/2 to 3/4 of that distance one way) on a fouled plug I am sure has caused problems with the oil and possibly the valves. The oil has visibly thinned but did not burn (discoloration) appreciably. The power increase after changing the fouled plug is very good and there is little vibration coming from the engine.

Does anyone have any other thoughts. I know I need an ignition coil, but if I gotta get an engine, that’s first! Any other suggestions would be very greatly appreciated. Also any thoughts on whether this is a head or rings issue. The engine has 201k on it and has always smoked a little on firing up. Literaly just a puff of smoke and then it has run fine. Is it more likely I have a valve sticking open or some carbon on the valve seats? Hmm ?~? New heads would be a lot cheaper.

Frank Pytel

To determine what is causing the low compression reading a cylinder leak-down test should be performed.

A leak-down test is where compressed air is introduced into the cylinder thru the spark plug hole with the piston at TDC. Then if you hear air escaping from the tail pipe there’s a leaking exhaust valve. If you hear air escaping from the throttle body there’s a leaking intake valve. And if you hear air escaping out of the oil dip stick tube the rings are leaking.


@FrankPytel, why are you using 10W40? Doesn’t your engine call for 10W30?

All of those readings are low and it sounds like the engine is just worn out. Add a small squirt of motor oil to one of those cylinders and then recheck the compression. If it goes up the rings are at fault.

With the constant plug fouling going on you need to run anti-foulers and try increasing the spark plug gap a bit; say by 8 or 9 thousandths of an inch.
I would go back to a heavier weight oil also.
About anything you do is a crutch at this point and if there’s a good side to this, it’s that you should be able to find a good 4.3 for a reasonable price.

I have done this to get by. It may work or may not. Take all of the plugs out and turn the engine so some of the pistons are down at the bottom. Then fill them with a mix of 1/2 trans oil and 1/2 keroseen. let it set till it drain it to the oil pan. Repete till all cyl’s have been done. If the you have stuck ring’s it may free them up. I have had it work and slow down the fouling or stop it. Change the oil when done and hope for the best. ALso I would not put any money into this engine. 4.3 are cheep used.

The compression check should be done with the throttle blocked open and it should be 6 cycles, not two. I would also try oldbodyman’s trick, but just on the one bad cylinder. Also, instead of kerosene and ATF, you could use straight kerosene or SeaFoam.

If it isn’t too cold in your area, never below +20°F, then use 30HD oil, it is more resistant to burning. Otherwise 15w40 or 20w50.

Try a dose of sea foam or techron, just in case it works! Fouled injector my guess 1, it may help!

@Barkydog, I believe the poster’s engine has TBI

If so, wouldn’t that clogged TBI injector cause far more severe problems than 1 single fouled plug?

If 1 injector is fouled and constantly injecting gas it might help was my thought. Granted there are probably bigger things going on but stranger things have happened.

I go along with installing an anti-fowler on the #1 plug and keeping a close watch on the oil level while driving conservatively.

I think you are on the right track in going to a plug with a higher heat range and using an antifouler on the #1 cylinder spark plug. It is possible that your valve stem seals are bad. This would explain the smoke at start-up. I had plug fouling problems on my 1971 Ford Maverick wome years back and corrected the problem with new valve stem seals. The mechanic had an adapter that screwed into the spark plug hole and attached a hose from his air compressor. This kept the valve from falling into the cylinder while he removed the keeper and spring and then replaced the seal. However, if your compression is low, you may be losing oil past the worn rings. If you are getting good voltage to the plug, try the remedy of a higher heat range on the spark plug and an anti-fouling thing.

@FrankPytel, if the truck sits a few days, does it smoke like crazy when it’s started up?
I’m talking obvious blue smoke.