Low engine compression, strange symptoms.

[Cal]

I have a chevy 6.0 that is going through oil (about a liter per fill up) so I did a compression test which it failed miserably. The odd thing is that it is missing most of the symptoms of blow by. All the plugs looked good, there was some oil on the threads but only two plugs showed any signs of burning any oil and oddly enough those two cylinders had decent compression. I cant detect any pressure to speak of either negative or positive at the oil fill cap. Carefull inspection showes there is a VERY small amount of air movement there which fluctuates in and out but it doesnt seem proportionate to the amount of oil loss. I'm not getting any smoke out the exhaust pipe or finding any leaks either, wondering if anyone can tell me whats going on.

[Robmcleod82]

Its possible that it is getting burnt up in the catalytic converter, I just had to replace the engine in my truck, it went through 4 liters of oil in 5000 km and no smoke or leaks, it was all getting burnt up (and probably plugging up) the cat. If you feel like getting crazy unbolt your exhaust and see if there is any smoke. Unless you have a guage that reads in inches of mercury it is hard to tell what your crankcase vacuum or pressure is, unless the blowby is extreme.

[sako1]

Check your pcv valve,there is a chance an updated one is available as well.

[Cal]

Originally Posted by Robmcleod82

Its possible that it is getting burnt up in the catalytic converter, I just had to replace the engine in my truck, it went through 4 liters of oil in 5000 km and no smoke or leaks, it was all getting burnt up (and probably plugging up) the cat. If you feel like getting crazy unbolt your exhaust and see if there is any smoke. Unless you have a guage that reads in inches of mercury it is hard to tell what your crankcase vacuum or pressure is, unless the blowby is extreme.

Thats a pretty good theory right there, thanks! I have one cylinder that makes only 25 psi and a few that only make 70-80, I thought that would have showed up in the crankcase pressure but I'm no expert.

[Robmcleod82]

yeah i would think the 25 would show up in the crank case too thats strange

[gitrdun]

Loss of compression can happen thru the top end too. Here's a quick test you can run. Record your compression pressure for the lowest cylinder. Pull the plug and squirt some SAE30 oil in there. Repeat the compression test, if the compression is higher, then it's likely the piston rings or cylinder wall because the oil will have temporarily sealed the leak. If the compression is unchanged, then it's a bad valve, likely exhaust.

A leak down tester is also a better way to check, but you probably don't have one. I got one years ago from Princess Auto. You can also build your own, but you need a small shop compressor.





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[berjerkin]

Originally Posted by gitrdun

Loss of compression can happen thru the top end too. Here's a quick test you can run. Record your compression pressure for the lowest cylinder. Pull the plug and squirt some SAE30 oil in there. Repeat the compression test, if the compression is higher, then it's likely the piston rings or cylinder wall because the oil will have temporarily sealed the leak. If the compression is unchanged, then it's a bad valve, likely exhaust.

A leak down tester is also a better way to check, but you probably don't have one. I got one years ago from Princess Auto. You can also build your own, but you need a small shop compressor.

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This is the correct way to to a compression test. Probably losing compression through a valve.

[RandyBoBandy]

you've done the compression test, now go do the right thing and do a "cylinder leakage" test. That should give you the results you seek. Albeit, cylinder leakage test will not confirm siezed oil rings. Just sayin'

[Cal]

It had occured to me that I might be losing pressure through the valves but I didnt think that this would result in major oil loss. If I'm wrong about that then it would deffinatly make more sense.