Compression Test Results... need help interpreting them - Defender Source
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  #1  
Old March 2nd, 2005, 08:30 PM
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Marty Marcinczyk
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Compression Test Results... need help interpreting them

So I did a compression test today and my results are as follows:

1: ~155psi
2: ~155psi
3: ~100psi
4: ~155psi
5: ~155psi
6: ~145psi
7: ~155psi
8: ~145psi

clearly #3 has some issues....but what is normal for a 3.9 V8??? All i could find in my manual is that the compression ratio is 9.35:1.

I think my next test will be a leak down test....not sure yet how to do it...but I will learn.......any suggestions? And any thoughts as to what is up with #3...could it be a head gasket problem or do you believe it is the valves/rings or something else.....all input is welcome.

please help!!! btw....my 3.9 has about 130K....any other info that might be helpful for troubleshooting????

Marty
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  #2  
Old March 2nd, 2005, 08:54 PM
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Jim Cheney
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Typically you are less interested in the actual PSI than you are the differential between cylinders (because the engine wears, unpredicably but hopefully evenly). Usually up to around 5% deviation is considered no problem at all, so the 145-155 measurements dont sound too bad, especially since it appears your instrument is calibrated in increments of 5.

If #3 is for certain 100 PSI, then you clearly have a problem. A number of things could cause it really.

Is the engine doing anything unseemly or did you just want to learn to do a compression check? Is there oil in your coolant or anything else undesireable? Is the "change engine" light on? It may be time for a Tdi...
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Old March 2nd, 2005, 09:14 PM
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Hans Haase
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Here is quickie way to better identify the problem. Pour a couple spoonfulls of oil through the sparkplug hole on 'ole #3 there, and run the compression test again. If the compression comes up, it's blowby around the piston and means a full rebuild to eliminate the problem. If the compression doesn't come back up, it's a valve or head problem.

-Hans
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Old March 2nd, 2005, 10:24 PM
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The truck was running a bit hot....never above red...but nearing it. Also it does have some white smoke when I start it up.....does not appear to be oil....more like steam comin out of the exhaust.


I'll try the other test tomorrow...pouring oil in #3 and seeing what happens.
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Old March 2nd, 2005, 10:48 PM
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Use a very small amount of oil or you will have a problem when it hydrolics the cylinder.
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Old March 2nd, 2005, 11:26 PM
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Yep, I will second that. When I say a couple spoonfuls, I mean 2 or 3 max. Just enough to coat the upper surface of the piston.
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Old March 3rd, 2005, 02:14 PM
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ok...so i did the test with the 2 spoons of oil in #3 and there was basically no change....maybe 1-2 psi but still reading around 90-95....all others read around 150 (i tested them again)....so what does this mean? any suggestions?
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Old March 3rd, 2005, 02:38 PM
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Could be valves, could be head gaskets. Either way, I would recommend doing both at the same time.
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Old March 3rd, 2005, 02:55 PM
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Peter Sherman
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If you do valves ah you do head gaskets?
It could be a bent push rod a lifter that is not pumping up fully or a host of other nasties. I doubt you will know till you tear into it. I am not aware of your budget but if it runs well leave it. [ unless you have some CASH burning a hole in your pocket] If you have oil in the coolant or coolant in the oil then head gaskets are an easy remedy. Enjoy the thing & save money for the rebuild.
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Old March 3rd, 2005, 08:56 PM
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You might see something when you pull off the valve cover. If it's a bent pushrod you might notice something at that point... if it's a burnt/cracked valve or worn valve guides you won't be able to tell without pulling off the head.

It could even be a warped or cracked head, but let's hope not! If you blow white smoke regularly you are getting coolant in the combustion chamber, which could be caused by a bad head gasket or one of the two more severe situations...

If you do any maintenance discussed here you need to pull the head and replace the head/intake gaskets, so that's a minimum. If you have it off it'd be best to get the head done while it's apart & have it checked for cracks or warping if you don't find any other obvious causes (eg valve problems).
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Old March 3rd, 2005, 09:24 PM
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if the the white smoke smells sweet it is coolant.
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Old March 3rd, 2005, 09:30 PM
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Ok, the oil trick changed nothing, that means it's not worn piston rings.

What else is the truck doing wrong? As the others suggested, it might be better to just let it go the way it is, and save up for a full rebuild. At that amount of mileage, it's almost not worth it to just do a head-job. You'll have rebuild heads, but still have a 130k short-block that will eventually need to be dealt with. Why go through the trouble of doing head gaskets, only to do new ones again later.

-Hans
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  #13  
Old March 3rd, 2005, 09:46 PM
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Thanks...i'm heading out of town tomorrow (not with the truck!!)...so I will not operate until next week. I think it will be on wed. I'll take your advice and open it up and see if anything is apparent. I'll get the head done and have it checked for warping and cracks. Hope that is all it will take....I really want to convert it to a TDI next year....so I hope that I will not have to do too much work on it.

Follow-up Post:

hans...i posted prior to seeing your post......the truck does not appear to be doing much else wrong now...but I have to go thru inspection before the end of April....but with the smoke I figured I would not pass.....perhaps I should just risk it and see if I pass....
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