Significantly higher compression in one cylinder, jeep 258

Significantly higher compression in one cylinder, jeep 258

reckboy

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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 258/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 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 ~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.
 
I think you should cancel the idea about cleaning out the combustion chambers. There are a few gas additives that you can use to do this, MMO is a good one also. With as much blow by that you have in your engine I would advise using a breather cap on the rear hole of the valve cover instead of sending that :dung: back into your engine, just don't let the smog man see it.
 
I think you should cancel the idea about cleaning out the combustion chambers. There are a few gas additives that you can use to do this, MMO is a good one also. With as much blow by that you have in your engine I would advise using a breather cap on the rear hole of the valve cover instead of sending that :dung: back into your engine, just don't let the smog man see it.

Breather on the rear valve cover hole... Interesting suggestion! I thought that line went to a canister sitting below the break vaccum assist and not back into the engine. I will have a look when I get home.
 
I was encouraged to do a seafoam or water combustion chamber cleaning by a mechanic neighbor. Since I didn't have Seafoam handy, I slowly put about 32 oz of water thru the throttle body while she was running at fast idle. I didn't seen much soot come out of the tail pipe. A few small puffs. I took her for a hard drive around the neighborhood and she is driving and pulling hard. Not sure if there is any improvement though... I will do another compression test this weekend and see if Cylinder 6 is any more in-balance with it's 5 buddies. Will post an update with new numbers when I have them...

Yes, it felt kind of wrong putting water into my throttle body. :confused::eek:
 
Yes, dumping plain old water down the carb is fine. I hope I'm not considered an old timer, but a friend's dad showed me that trick when I was a teenager, and I've used it from time to time. It loosens up the carbon deposits in high mileage engines so they eventually find their way out of the chambers. It doesn't hurt anything on a carbed car. I'm not so sure on FI. It might not be good for the O2 sensor? Can anyone who's an FI guru please chime in?

As for the higher compression, that's a weird one. Usually you'll have one bad cylinder reading lower than the others. If she's still running great, I'd just drive her till she doesn't. :chug:
 
I was encouraged to do a seafoam or water combustion chamber cleaning by a mechanic neighbor. Since I didn't have Seafoam handy, I slowly put about 32 oz of water thru the throttle body while she was running at fast idle. I didn't seen much soot come out of the tail pipe. A few small puffs. I took her for a hard drive around the neighborhood and she is driving and pulling hard. Not sure if there is any improvement though... I will do another compression test this weekend and see if Cylinder 6 is any more in-balance with it's 5 buddies. Will post an update with new numbers when I have them...

Yes, it felt kind of wrong putting water into my throttle body. :confused::eek:

Reckboy

:)Pouring water down into the throttle body? I know there are people who say that's a great Idea and all............but keep in mind water does not compress and anything cold on hot valves could warp them. If you have excessive blow by that process will not fix anything. Look to your rings...............25 lbs +/- is not a big deal..............do a leak down test on each cylinder , you should be able to tell if its valves or rings..................my guess @ 200k miles "BOTH"
:D:D:D:D
 
Reckboy

:)Pouring water down into the throttle body? I know there are people who say that's a great Idea and all............but keep in mind water does not compress and anything cold on hot valves could warp them. If you have excessive blow by that process will not fix anything. Look to your rings...............25 lbs +/- is not a big deal..............do a leak down test on each cylinder , you should be able to tell if its valves or rings..................my guess @ 200k miles "BOTH"
:D:D:D:D

Good points all! :chug:

As for water, it won't hurt a carbed engine. You poor it very slow so you don't kill the engine or hydraulic lock it. Like I said before, I have no idea if it can harm an O2 sensor in a FI engine. It was just a thought so I figured I'd mention it and hope someone knows the answer.
 
I would inspect the valve train for excessive valve lash on the high cylinder. Look for worn rockers and balls, pulled or stretched studs. If you had enough carbon to jump the compression that much, the valves would impact the carbon.:)
 
I wish I knew enough about the valve train to feel comfortable digging into it. I guess it might be time to learn. :).

So here are my latest compression numbers. I made sure the engine was warm this time and WOT, plugs out, coil disconnected, injectors disconnected. Post "steam cleaning" combustion chamber (difference in parentheses).

1 - 142 (+12)
2 - 132 (+7)
3 - 140 (+16)
4 - 125 (-5)
5 - 132 (+8)
6 - 155 (+5)


Looks like a nice improvement except #4 lost some psi.

I put everything back together and went for a hard ride to get the engine hot. When I got home I immediately pulled the plugs and started another piston soak with MMO while she was hot. Maybe this will free up whatever is going on in #4... Maybe not. Either way, after this I think I have done what I can and will be back to just wheeling and enjoying until I can afford a rebuild or a reman engine. The blow-by is irriatating, but I can live with it for a while. I will post the compression numbers after this 24 hr MMO soak is done for comparison.
 
I wish I knew enough about the valve train to feel comfortable digging into it. I guess it might be time to learn. :).

So here are my latest compression numbers. I made sure the engine was warm this time and WOT, plugs out, coil disconnected, injectors disconnected. Post "steam cleaning" combustion chamber (difference in parentheses).

1 - 142 (+12)
2 - 132 (+7)
3 - 140 (+16)
4 - 125 (-5)
5 - 132 (+8)
6 - 155 (+5)


Looks like a nice improvement except #4 lost some psi.

I put everything back together and went for a hard ride to get the engine hot. When I got home I immediately pulled the plugs and started another piston soak with MMO while she was hot. Maybe this will free up whatever is going on in #4... Maybe not. Either way, after this I think I have done what I can and will be back to just wheeling and enjoying until I can afford a rebuild or a reman engine. The blow-by is irriatating, but I can live with it for a while. I will post the compression numbers after this 24 hr MMO soak is done for comparison.

Recboy,

:)I do not think those numbers are that far off for an old motor and just cause one is either +/- is no big deal...........there is really nothing you can do to improve that compression with a bottle of liquid.
In fact if I was you I would now change the oil do to the contaminates you have added.................one other thing I have seen in the past when folks run these snake oil remedy's down there intakes it sometimes washes the rings & cylinders away and the problem get's even worst!
You already know it has blow by..........which is caused by rings that are not sealing and pressurizing the crankcase.
Drive it and when it starts smoking or using oil plan on a rebuild.
:D:D:D:D
 
Recboy,

:)I do not think those numbers are that far off for an old motor and just cause one is either +/- is no big deal...........there is really nothing you can do to improve that compression with a bottle of liquid.
In fact if I was you I would now change the oil do to the contaminates you have added.................one other thing I have seen in the past when folks run these snake oil remedy's down there intakes it sometimes washes the rings & cylinders away and the problem get's even worst!
You already know it has blow by..........which is caused by rings that are not sealing and pressurizing the crankcase.
Drive it and when it starts smoking or using oil plan on a rebuild.
:D:D:D:D

Yeah tarry99, I think I am with you. I will collect and post my compression numbers tomorrow in the interest of completeness of data for the forum. I enjoy turning wrenches and testing but I don't think I can do my jeep any more good without a major engine overhaul. Time will tell if the blow-by is better or worse after all this.
 
final numbers after 22 hr piston soak in MMO starting with a hot engine. This morning I turned the engine over a few time to clear the MMO that might still be in there, reassembled and went for a 10 minute drive around the town to warm her up and burn off whatever was left in the combustion chamber. Got home, pulled the plugs, disconnected the coil, injectors, propped open the throttle and tested compression... difference from pre-MMO soak in parentheses.

1 - 142 (+-0)
2 - 140 (+8)
3 - 143 (+3)
4 - 134 (+9)
5 - 146 (+14)
6 - 156 (+1)

All in all, not bad numbers for a 258 engine with 203k miles on it. #4 is still the low and #6 is still the high, but the difference is less extreme across all the cylinder. I am dumping the oil and changing the filter now to get the MMO and water that condensed from the steam clean out. Refilling with my standard Rotella T and will run it and see how oil lose and blow-by are going forward.

Thanks for the advice everyone. I hope others find this on searches and can find use from it.

CB
 
Those numbers look pretty good for 200k. I'd run it until oil consumption gets too high or performance drops.
 
Update

Well, it has been almost a couple months since my compression experiment with MMO and it seems like a good time for an update. Although my compression numbers look good and saw some improvement with the mmo soak, the engine blow-by is still significant, about a quart every 500 miles or so... A little more if sustaining freeway speeds for long stretches. She is running good as long as I keep the air filter and throttle body clean. I did have an overheating event a couple weeks ago while climbing a very long, steep hill. No doubt it is almost time to drop in a reman engine or start a rebuild. Perhaps it will be my winter project if I have the funds together.

Any suggestions on rebuild vs remanufactured engine? Any recommended companies for the reman? Would love to get my reman from a local Northern California shop if there is one...


Thanks,
CB
 

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