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87 Comanche 4.0L Compression Test

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My son and I have just started on a 1987 Comanche. Had the fuel pump and all fuel injectors professionally replaced, and just completed a compression test. Here's what we came up with:


1 - 65 psi

2 - 90 psi

3 - 90 psi

4 - 60 psi

5 - 90 psi

6 - 80 psi


Although the engine only shows 97K, it was not well-maintained by the previous owner. We changed the oil immediately upon purchase, and the oil was the blackest I've ever seen. It even felt sticky. No telling when the last oil change took place.


Any opinions on what the readings above are telling us?



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You might be able to get away with using a can of the Engine Restore stuff. The 4.0 is pretty big for a 6, so get the 8 cylinder formula.


But yeah, those numbers are kinda crap. Wouldn't hurt to dump the oil pan, take a good look inside of it, get it cleaned out, get the oil pump strainer cleaned out, and then you can take a look at at least part of the cylinder walls.

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To help get sludge out of the engine prior to taking it all apart you can put a mixture of 50% new oil and 50% kerosene in the engine and start it up. ONLY let the engine idle, DO not rev it. Idle it for about 1 minute and shut it down. Drain it immediately while all of the sludge is still warm. Won't hurt anything as long as you idle it and don't touch the gas.


My Dad and I did this to a 304 in a CJ-5 we once had and it cleaned it out good. We removed the valve covers and the oil pan and painted them then put them back on with new gaskets. Prior to running the mix, the engine didn't leak. After we ran it, it leaked out of every old seal that there was. We wished we didn't clean the gunk out after that.

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Those numbers are in the toilet. The LOW number per the FSM is 120 psi and you are WAAAAAY under that. The next step is to figure out if it's because of rings or valves. Desludging the engine won't do anything for compression, so before you even waste time on that ... check things out and decide if the engine can be saved or if it needs a rebuild/replacement.

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We only paid $500 for the MJ, and I knew there might be problems with the engine. I mainly was after the body, as it was straight with no rust. A solid engine would have just been gravy.


Doesn't look like we got the solid engine.


However, we also got the 4.0L, 4WD, SWB, and 5-speed manual.


Good luck with your '90.

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