Two months ago a friend of mine bought a 91 si, stock motor with about 170k. When he bought it, I did a compression test and all cylinders were right around 180. So I drove the car the other day and noticed it was pinging and he said it was burning a lot of oil. You can see the car smoking too. So I again tested the compression and it was over 200 PSI on all four! The only reason that I could come up with is that the engine is burning oil and lots of carbon is building up in the combustion camber. I just checked the plugs again and there doesn't seem to be any signs of oil being burned. I thought the the PCV valve might be clogged, but I checked it and it seems fine. There doesn't appear to be any oil in the intake manifold. The plugs are new, the air filter is new, there are no engine codes. What else could this be? Tomorrow I'm going to retest the compression. I'm going to try another gauge, just in case too.

Does this sound like carbon build up? If so what could be causing this? Could this be a valve that is sticking or some thing like that? Any thing else I should check? Any other possible causes? Thanks for the help!

 

I used to buy the cheapest gas I could find, that is until I pulled the head off and saw a thick (i mean like 0.25 inch in places) layer of carbon buildup. It must have been raising my compression. Try filling up with Chevron Techron supreme for a few tanks. I cleaned the piston tops off as well as I could and switched to techron. When I had the opportunity to pull the head again the piston tops were cleaner than I had left them and they were cleaner near where the fuel-air would come into the combustion chamber from the intake mani.

If your buildup is bad, this might be too slow. I've heard that putting the pistons at the same height and filling the bores with ATF through the spark plug hole, letting it soak, and then sucking it out with a turkey baster works. Be sure to change your oil afterwords. There are some threads about doing this here on H-T.

 

Thanks, I'm going to try the ATF treatment in a few days. Do you know what could be causing this to happen in such a sort period of time? Thanks again for the help.

Noah

 

Actually, the best thing to pour in there to clean out carbon is GM top end cleaner. I know it's sold by GM dealerships, but it's not actually made by GM so rest easty. Anyway, it gets rid of carbon asap, just let it sit overnight and all of it will be gone. Suck it out, then change the oil. and how do you know the PCV valve is ok, you said you checked it, how did you check it?

 

How do you "suck it out"? I just check to see that the PCV didn't seem to be clogged, I sucked on the hose and I checked that there wasn't any oil in the intake manifold. How are you suposed to check the PCV? Thanks for the help.