Valve issue affect compression test?

Hi all,

If I have issues with my valves not seating correctly, would this cause a faulty compression reading on a test?

The reason for suspecting a valve issue is I'm getting a carbon trace on one side of my spark plug from the electrode to the base and someone mentioned that this could be due to a valve problem not seating correctly.

The interesting thing is though I'm getting this on all cylinders (the carbon trace).

I'm readying a 4.0L head to go on my jeep and wanted to test the engine so as not to put a good head on a suspect engine. The only test I know to determine engine strength is a compression test but if this could be in-accurate due to valve issues then I won't have a valid reading.

Thanks for any suggestions, comments.

Bill

Reply to
William Oliveri
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It's possible. The best engine test to tell the condition of an engine is a cylinder leakdown test. This involves bringing the piston to TDC on the compression stroke and then applying air pressure to the cylinder. The leakdown tester will have 2 pressure gages on it. The first gage will read whatever air pressure you apply to it, and th second gage will read a lower number depending on the condition of the engine. A freshly rebuilt engine will read about 5-8% leakdown due to ring end gaps and other imperfections. I believe the maximum recommended leakdown number is about 15-20%, but don't quote me on that. The nice thing is that when you use this test you can determine where the leakage is occuring. If the intake valve is leaking you will hear a hissing through the carb/throttle body, exhaust valve will be out the tailpipe, rings can be heard through the PCV hose or dipstick tube, and a cracked head or bad head gasket will bubble in the radiator. You actually don't even need the gages, you can get an adapter that connects to your air hose and screws into the spark plug hole. The gages are nice for telling how much leakage is occuring however.

Here are a couple links:

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Also, if you search the net, you'll find plans to make your own tester if you desire to do so.

Chris

Reply to
c

The 258's all seem to track the plugs the same as you describe.

If you do a compression test and all the cylinders come out the same, no worries. If one is low, then you put a tablespoon of oil in the spark plug hole and try again. If it then jumps up, you have ring issues. If it stays low, suspect valve issues.

If two side by side are low and the wet test doesn't raise them, suspect a blown headgasket between them.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

William Oliveri wrote:

Reply to
Mike Romain

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