runner6 Posted October 18, 2009 Share Posted October 18, 2009 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? Thanks. Link to comment Share on other sites More sharing options...
Automan2164 Posted October 18, 2009 Share Posted October 18, 2009 Looks like it may be time for a rebuild... Those numbers are pretty low. Rob L. Link to comment Share on other sites More sharing options...
Geonovast Posted October 18, 2009 Share Posted October 18, 2009 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. Link to comment Share on other sites More sharing options...
Automan2164 Posted October 18, 2009 Share Posted October 18, 2009 Also, with that amount of "Abuse" from the PO, I would pull the valve cover and check out how sludged up the top end is. Rob L. Link to comment Share on other sites More sharing options...
dakal Posted October 18, 2009 Share Posted October 18, 2009 ya Link to comment Share on other sites More sharing options...
500 MJ Posted October 18, 2009 Share Posted October 18, 2009 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. Link to comment Share on other sites More sharing options...
Eagle Posted October 19, 2009 Share Posted October 19, 2009 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. Link to comment Share on other sites More sharing options...
deziped Posted October 19, 2009 Share Posted October 19, 2009 Cyl compression s/b between 120 - 150 psi. Your engine is tired. Rings are probally worn out. I'm betting cyl 1 & 4 have some valve problems (burnt, warped, bad cam lobes) . Dumping magical treatments in it ain't gonna help. Link to comment Share on other sites More sharing options...
HOrnbrod Posted October 19, 2009 Share Posted October 19, 2009 A perfect stroker candidate. :D Link to comment Share on other sites More sharing options...
mvusse Posted October 19, 2009 Share Posted October 19, 2009 Yeah, don't waste money on it. It needs rebuilt. And if rebuilding, might as well build a stroker. Get a used engine in the meanwhile. Link to comment Share on other sites More sharing options...
1987Comanche Posted October 19, 2009 Share Posted October 19, 2009 What kind of driveability problems did the truck have when you bought it? Any indications the compression was that low? I'm assuming you had some issues as you replaced the fuel pump and rail. I'm wondering because I'm going to look at a '90 4.0 on Tues. Link to comment Share on other sites More sharing options...
runner6 Posted October 19, 2009 Author Share Posted October 19, 2009 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. Link to comment Share on other sites More sharing options...
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