I am at a loss here, I have a 95 GMC yukon 247,000 miles. I changed the spark plus and the wires about 2,000 miles ago. Running fine 2 weeks ago I put some AT205 Reseal on the engine oil to help real seal any leaks. Valve covers were covered with oil. I washed them and as of now dry.
Before I put it in I was driving and the engine felt like it was too low of a gear for the speed like in a manual. It hadn't done that again for about 3 days. Then my husband was driving me to work and it was supddering again. It got to the point of nothing over 35 mph. We parked it and I pulled all the plugs after I got all parts to replace distributor and coil and EGR. The check engine light only had the EGR value code.
Fast ward to today the #5 cylinder was oil fouled, it felt really gritty and was pretty thick. Not sure what to think here could it have started to foul before I put the AT205 in and has now sealed what ever needed to be sealed? I took all plugs out and cleaned them I have 8 new ones on hand just in case. Could this mean an over haul is necessary?
If that could be the case then I will park it and tear it apart and just replace the whole thing with new parts. What could I do to see if that cylinder is still the issue? Will that oil burn off or what? I will be pulling that plug at least 1 time a week to see if it is progressing any. This is our only vehicle right now I know we shouldn't be running it probably but just need some in sight to what you think.
Thank you
Paragraphs are your friends. (And ours.)
AT-205 will not seal valve cover gaskets (or any gaskets) and it will not prevent oil burning. It does not "seal whatever needs to be sealed." AT-205's sole purpose is to rejuvenate rubber seals such as crankshaft seals or transmission seals, and even then it is a temporary bandaid at best.
If you used a generic OBDII scan tool there are probably more codes that a professional-grade tool would find, as well as live data for diagnosis.
On the face of it, it sounds like you have bad piston rings in that #5 cylinder. Do a dry vs. wet compression test.
Replace the PCV valve to see if that helps. If not, get a dry/wet compression test to see if the piston rings are worn out. With nearly 250K miles, unfortunately, that is probably the case. You could do rings and rod bearings fairly easily with the engine in the vehicle, and get the heads rebuilt but if the mains are that old, it would either need a completed rebuild or a rebuilt engine.
