Significantly higher compression in one cylinder, jeep 258
reckboy
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- Location
- South San Francisco, CA
- Vehicle(s)
- 1984 AMC/Jeep CJ-7, 4.2L, T-5 Trans, Dana 300, Stock Diffs w/lockers
Hello Jeep-CJ Forums! I am a long time reader, first time poster. Lots of great info here so let me say "Thank You!" right off the bat for all the things I have learned from you folks as a passive reader.
On to meat of the post... My 84 Jeep CJ7 with AMC 258 i6 / 4.2l /MPFI at about 203k miles is losing oil, presumably from blow-by. I see pools around the oil filler cap, ccv and around the valve cover gasket. I decided to do a compression check to get an idea if my engine is just plain wearing out.
Admittedly, I did this when the engine not warmed up....
Disconnected the coil, disconnected the injectors, propped open the throttle body, pulled all the spark plugs...
Cylinders 1 thru 5 all tested at between 124 and 130 PSI.
Cylinder 6 tested at AMC 150 PSI.
Also, noteworthy is the fact that Cylinder 6 had the nastiest looking black, crusty, goopy spark plug.
My assumption based on the compression test results and plug reading is that there is significant carbon build-up in cylinder 6. I proceeded to do a three day Marvel Mystery Oil soak with about 2oz of MMO in each cylinder barrel. I turned the engine over every 24 hours and replaced the 2oz of MMO in each barrel for the soak.
After the 72 hour soak, I turned the engine over a few times to clear the oil and tested the compression again.
Cylinder 1 thru 5 tested at ~AMC 150 PSI
Cylinder 6 tested at 175 PSI
The MMO soak may have helped a little but I think the numbers are higher all around because of the "wet" rings and the fact that I forgot to prop open the throttle this time (D'oh!). In any case, that cylinder 6 variance is still there and fairly significant IMO.
I cleaned up the throttle body, intake tubes and air filter, wire bushed and replaced the spark plugs, hooked everything back up and she fired right up. Smoked for a few minutes while burning off any MMO still in the combustion chamber, but not bad. Once warmed up, she ran better than ever. Drove to the gas station and filled up with high-test and about 4 oz of MMO in the tank. Me and my CJ drove around town for about 20 minutes very nicely and running strong.
Not sure yet if I am still losing oil. I would assume so because not much has changed other than a good cleaning of the throttle body, air filter, spark plugs and whatever the MMO did for the combustion chamber.
I guess I just wanted to share with forum what I am doing, my results so far, and open the floor for suggestions about the cylinder 6 high compression issue and the blow-by.
I have read suggestions of "steam cleaning" the combustion chamber by spraying small amounts of water into the throttle body while the engine is running to clear out carbon build-up. That sounds a little crazy to me, but some of the old-timers swear by it. I have also read that some guys just leave the carbon and unbalanced compression alone and live with it for fear of causing more problems on an older engine.
Thanks guys. I love this forum and am proud to finally be a poster/contributor.
On to meat of the post... My 84 Jeep CJ7 with AMC 258 i6 / 4.2l /MPFI at about 203k miles is losing oil, presumably from blow-by. I see pools around the oil filler cap, ccv and around the valve cover gasket. I decided to do a compression check to get an idea if my engine is just plain wearing out.
Admittedly, I did this when the engine not warmed up....
Disconnected the coil, disconnected the injectors, propped open the throttle body, pulled all the spark plugs...
Cylinders 1 thru 5 all tested at between 124 and 130 PSI.
Cylinder 6 tested at AMC 150 PSI.
Also, noteworthy is the fact that Cylinder 6 had the nastiest looking black, crusty, goopy spark plug.
My assumption based on the compression test results and plug reading is that there is significant carbon build-up in cylinder 6. I proceeded to do a three day Marvel Mystery Oil soak with about 2oz of MMO in each cylinder barrel. I turned the engine over every 24 hours and replaced the 2oz of MMO in each barrel for the soak.
After the 72 hour soak, I turned the engine over a few times to clear the oil and tested the compression again.
Cylinder 1 thru 5 tested at ~AMC 150 PSI
Cylinder 6 tested at 175 PSI
The MMO soak may have helped a little but I think the numbers are higher all around because of the "wet" rings and the fact that I forgot to prop open the throttle this time (D'oh!). In any case, that cylinder 6 variance is still there and fairly significant IMO.
I cleaned up the throttle body, intake tubes and air filter, wire bushed and replaced the spark plugs, hooked everything back up and she fired right up. Smoked for a few minutes while burning off any MMO still in the combustion chamber, but not bad. Once warmed up, she ran better than ever. Drove to the gas station and filled up with high-test and about 4 oz of MMO in the tank. Me and my CJ drove around town for about 20 minutes very nicely and running strong.
Not sure yet if I am still losing oil. I would assume so because not much has changed other than a good cleaning of the throttle body, air filter, spark plugs and whatever the MMO did for the combustion chamber.
I guess I just wanted to share with forum what I am doing, my results so far, and open the floor for suggestions about the cylinder 6 high compression issue and the blow-by.
I have read suggestions of "steam cleaning" the combustion chamber by spraying small amounts of water into the throttle body while the engine is running to clear out carbon build-up. That sounds a little crazy to me, but some of the old-timers swear by it. I have also read that some guys just leave the carbon and unbalanced compression alone and live with it for fear of causing more problems on an older engine.
Thanks guys. I love this forum and am proud to finally be a poster/contributor.