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Compression Test results - Could this be a problem?

1997 Ford F150 - 133K Miles - 6cyl - 4.2L. This is a truck that gets less than 2K/year put on it. Has had “issues” with worn bearings and holding oil pressure for a few years now. Yesterday, the check engine light started flashing (as opposed to solidly lit) and the code came back as a cylinder #4 misfire (only the one code). The truck was clearly not firing on all cylinders and at idle, ran terribly (couldn’t be driven). I just did a compression test on all the cylinders and all measured around 140-145psi except the #4 which read almost 170psi. I rechecked this a few times as well as a couple of the others and am now sure of the results. Would anyone have any info as to what could cause this one cylinder to have a higher compression ? Is whatever is causing this increase in the compression, also be the cause of them mis-fire ?? I’m stumped here and quite frankly, expected the compression on the #4 cylinder to be really low as opposed to the others.

I would replace the #4 plug and coil and see what happens…Don’t worry about the compression for now…

The compression is low but I agree with replacing the plug and possibly the coil. A bad plug or plug connection in the boot can kill a coil.

If you go back and perform a wet compression test on the 140ish PSI cylinders and the numbers rise by 20 PSI or whatever then you know there’s a piston ring problem.
A 170 is bit low for a 133k miles engine unless there’s a bad history behind it.

If the compression is higher in one cylinder than the others, then it may be caused from a liquid leaking into that cylinder. This could be oil, gas, or coolant… You can’t compress a liquid.


I would pull a valve cover and check to see if that exhaust valve is opening…170 is high…145 is good. you might have a flat cam lobe or a broken rocker/push rod

“145 is good”

I would think this engine has at least 8:1 compression, so I would expect at least 160 psi when all is well.

Although I have never run across an exhaust valve causing an increase in cranking compression it is a possibility. And an excessive clearance on the intake valve also. But my first guess would be heavy carbon deposits in the cylinder. Coolant in the cylinder would cause the high pressure but other more ominous symptoms would have been too obvious to overlook if that were the cause.

I’ll suggest that rather than considering the 170 as high, you consider the other three as low. It would not ne unusualy on a worn engine to find variations in the compression.

Rod, I’m not challanging your post, only “mining” your knowledge to learn from. I’m having sifficulty envisioning how an exhaust valve could increase cranking compression. Can you elaborate? Carbon buildup I absolutely agree with as a possibility, and one that could be isolated to one cylinder if that cylinder were not burning as completely as the others or the injector were not operating as efficiently or slightly leaking.

The 140 PSI is a problem and it should be determined with a wet compression test if a ring problem exists. The 170 could just be an anomaly and I’d wet test the 170 cylinder also.

The misfire could also be due to an oil fouled spark plug on the 170 cylinder. It’s quite possible to have a 170 cylinder pumping oil like a grounded tanker on a reef and the 140 cylinders consuming little oil at all.
Heavy oil consumption on the 170 cylinder can also create heavy deposits on the piston top but this should also be noticeable on the spark plug itself.

Once a Honda came to me with a no start complaint. The compression was 215 psi. The problem was a totally blocked catalytic converter that was integrated into the exhaust manifold. I never tried to do the physics to know exactly how that happened but replacing the converter got the compression down to normal and the car left under its own power.

That’s logical, Rod. I thought you were referring secifically to an ehaust valve problem raising compression.

I can only assume that a closed exhaust valve give a similar result, mountianbike.

I see your point. A valve that didn’t open at all could. That scenerio never even occurred to me.

I have to wonder, the truck has 130k on it and bearing problems, if the oil has ever been changed. Or if it has been changed but rarely. Sounds like the engine is likely trashed internally. I like the idea of the wet test on all cylinders, and perhaps a quick look under the oil filler cap. If the cap and surroundings are black and crusty, it could be inferred that the engine may be close to passing away.