compression #6

[Anthony10370]

Hey I ran a compression test on my '79 and got good overall numbers (8.2-8.6) except for cylinder number 6 where I got about 6.5:1. Its weird though because I can hear the compression leaking; when the piston comes up I can hear the "tsssssst" noise of air escaping and it's pretty clear. I cant figure out where it might be coming from. do head gasket leaks make that noise or are they still to muffled? Any ideas?

Thanks,
Anthony.

[Danglerb]

Valves or rings are my guess, try a leak down test. HF sells one cheap.

[s928s]

Headgaskets can make noise.

1. Try a leakdown test
2. use the adapter and a compressor and put about 90 psi in the cylinder with it at TDC for that cylinder and listen for an exhaust valve air leak or an intake air leak.

[Mrmerlin]

has the car been running?? if not then it may need to be run with some marvel mystery oil in the fuel that may help with the valves sealing better

[Fredfox]

Yes, as Merlin said was the car running?
I had low compression in two cyl. it turned out the lifters were pumped up more on those two. I brought the cam around and let it sit for a minute with the valves open and retested and it was fine.

[JK McDonald]

If you do not have a leak-down detector set up - as Danglerb and s928s said one quick way to determine if you have an intake or exhaust valve leaking is to remove the spark plug and run your #6 piston to TDC. Be careful when removing the plug because this area will accumulate a lot of crud which can easily fall down into your cylinder. Use compressed air to blow out the plug recess before pulling the spark plug. Then use a spark plug replacement type adapter to connect an air compressor to the problem cylinder. The adapters are not too expensive but you can also make one by breaking out the center of an old spark plug and welding on a air chuck to the top of the remaining base. Set your air pressure regulator somewhere around 90 psi or so. You can listen at the intake manifold and then at the tail pipe for a hissing sound. Normally the problem is a cylinder head exhaust port with a burnt seat or a slight carbon build up on the exhaust valve face it self.

A leak down detector is much more accurate in determining the extent of the leak but the rough test above can take some of the initial guess work out. If this doesn’t pin down your problem all these guys will tell you to add a teaspoon of oil to the cylinder and rerun the compression test. If your compression numbers improve you may also be looking for worn piston rings and/or cylinder surface damage.

Good Luck, Michael

[Anthony10370]

Well after the car sitting unstarted for a couple of days, I pulled the spark plug and inspecting around with a flashlight I found the problem. Unfortunately it was the green puddle of coolant in the cylinder that confirmed the head gasket as the issue :'(.

Thanks for the advise everyone.
Anthony

[Maleficio]

That sucks.

[Danglerb]

Sucks worse if you find it by hydrolocking.

I tend to think all the older cars that have sat for any length of time need new head gaskets, and sooner or later they make the problem known. Best to plan on pulling the heads, putting in new gaskets, doing a valve job with new guides etc.

[Anthony10370]

well I don't have access to a valve grinder anymore and valve guides adds $100 to my extremely limited budget. I think I might fix the head gaskets, do valve seals, do brakes then sell and buy a manual 928.

[JhwShark]

Originally Posted by Danglerb

I tend to think all the older cars that have sat for any length of time need new head gaskets, and sooner or later they make the problem known. Best to plan on pulling the heads, putting in new gaskets, doing a valve job with new guides etc.

Agreed, my e9 (BMW 3.0CSi) was neglected, just parked in the garage while I worked around the globe...a little crystallization at the head/block interface tells it all...time to pull the head and do a gasket refresh, fuel line/tank, radiator/thermo, brake/clutch...40 years old under 50k miles and it is due. I'll replace valve guides/seats...as well.

Though, pulling the head on the e9 is a lot easier than on the 928.
Jon