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Old 12-28-2012, 08:44 AM   #1
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compression check....what next

Guys-

2001 TJ Auto, 4.0 been running rough with flashing CEL. Did a compression check and all cylinders 170-185 except #3 was 115 dry and 150 wet. Concerned about the cylinder at this point. This is a Jasper reman with @ 14K on it (with TUPY head) but out of warranty. What would be your next step.

Thanks!

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Old 12-28-2012, 09:38 AM   #2
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I'd probably take off the head, see what those valves look like on that cylinder. might need to be ground.

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Old 12-28-2012, 11:00 AM   #3
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Yep thining that but wanted to be sure I checked all that i should prior to pulling the head. Was trying to rent a leak down tester but can't find one and read the cheap harbour freight doesn't work so well. Thanks for the reply
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Old 12-28-2012, 12:34 PM   #4
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If the wet reading was 35# higher than the dry reading, I would lean towards rings over valves.
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Old 12-28-2012, 12:51 PM   #5
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If the wet reading was 35# higher than the dry reading, I would lean towards rings over valves.
true. either way, take the head off, valves are cheaper, and you'll need it off to do rings anyway. since its off I'd have them ground.
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Old 12-28-2012, 01:43 PM   #6
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Can I try just replacing the rings in #3? Can I replace while in jeep? I guess with the head off I would see if the cylinder walls are the issue, not real familiar with the internals but may open it up myself. Thanks for the advice guys
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Old 12-28-2012, 02:05 PM   #7
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Can I try just replacing the rings in #3? Can I replace while in jeep? I guess with the head off I would see if the cylinder walls are the issue, not real familiar with the internals but may open it up myself. Thanks for the advice guys
I would pull the head off first and see if there is evidence of visible scoring in the cylinder wall. You probably could get away with replacing rings in that one cylinder given the readings in the others and the fact that it only has 14K on it. My question would be why is there a problem at that kind of mileage? It would mean dropping the pan, but I don't see why you couldn't do it in the Jeep. Another thing that you might take a close look at first is if the head bolts are torqued to specs and the condition of the head gasket
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Old 12-28-2012, 02:23 PM   #8
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With that much drop it is most likely piston ring lands or rings. You can plug in air to the plug hole and listen for escaping air. Air cleaner hose or tailpipe is a valve. Oil fill hole is rings. Either way the head comes off. And yes you can rebuild some motors right in frame
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Old 12-28-2012, 03:51 PM   #9
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Thanks for the advice, not sure why the issue with such low milage. It was running rough, took it to the shop they tried a bunch of stuff (coil rail, plugs, injector, said compression was ok) thought it was computer and said take it to the dealer. Dealer did compression test and said #3 was low but wet and dry the same # so I thought head issue. I took it back home and did it myself and got the info I posted. I don't think I did anything wrong doing the test, did use 3&1 oil for the wet test. I am just trying to gather info before I dig in deeper than needed. I really think the dealer assumed the head was the issue since the 2000-2001 0331 heads had problems. The dealer did not know at the time this was a reman engine with the TUPY head. Still could be the head/gasket I guess but never ran hot while I had it. This did start (flashing CEL) on the way home from a camping trip running 75-80 on the highway. Thanks again
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Old 12-28-2012, 04:31 PM   #10
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As stated above a Leak Down Test is the only logical next step it's a waste of time tearing it apart if you don't know where to look for the problem Diagnose 1st then repair.
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Old 12-28-2012, 06:51 PM   #11
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I am just trying to gather info before I dig in deeper than needed.
1st question is... Does it smoke?

Low compression can be the valves not opening enough to get air in the cylinder. Bad cam, bad lifter, broke valve keeper, ...

I would remove the valve cover and compare the movement of the rockers.

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Old 12-28-2012, 09:21 PM   #12
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No smoke, just runs rough usually at red lights. I can shut it down and restart and seemed to run good again. don't notice it rough at higher speeds just really at idle. Planning to pick up a leak down and see if that can help steer me in the right direction. The wet compression seems to point to the ring/cylinder wall. Can a leak down be done on a cold engine or should it be up to running temp first? So if i do decide to pull the head i can pull the cover and run it to watch the rockers for clues. Thanks, keep any thoughts coming....I hate seeing the jeep in the garage and not able to run it.
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Old 12-28-2012, 09:41 PM   #13
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I would use a remote switch to watch the rocker action, You don't want oil blowing all over the place with the engine running.

If it doesn't smoke I wouldn't suspect rings.

I suppose I would do the leak down with it cold, If it leaks down, the do the oil in the plug hole trick, and see if it still leaks down as fast.
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Old 12-29-2012, 07:57 AM   #14
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ok, still have the plugs out from compression check. May put them back in and warm up before leak down. The only reason for thinking the rings was due to the compession jumping and the wet test. It still bugs me why this would be with just 14K on the motor, and why it doesn't seem to be constant. It acts up when idling at red lights and shutting it down and restart seems to clear it up for a short while. Thanks guys for all of the help, going to try to get a leak down tester today to get more clues.
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Old 12-29-2012, 08:20 AM   #15
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May try the block tester that changes fluid color, could have rented it the other day but didn't know what it tested. Just saw on another thread and think it is worth the test.
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Old 12-29-2012, 10:01 AM   #16
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The only reason for thinking the rings was due to the compession jumping and the wet test. It still bugs me why this would be with just 14K on the motor, and why it doesn't seem to be constant. It acts up when idling at red lights and shutting it down and restart seems to clear it up for a short while. Thanks guys for all of the help, going to try to get a leak down tester today to get more clues.
If you set off an o2 sensor, and the CEL is flashing, you're gonna need to tear into it. Just do the simple stuff first. Verify the compression/valve train first, spark second, and fuel third.

Depending on how much oil you used on the last ring test, a couple teaspoons could raise compression that much just by the volume change.
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Old 12-29-2012, 11:35 AM   #17
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what is the p code? Have you run the compression test again?
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Old 12-29-2012, 12:00 PM   #18
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Depending on how much oil you used on the last ring test, a couple teaspoons could raise compression that much just by the volume change.
yeah, i wondered if that could be the reason for the higher number. When the dealer did it they claimed no diff between dry and wet. I didn't really measure just had a small bottle of 3 and 1 oild with the little pointed nipple and pointed it in and squeezed sevarl times. got 115 first then wet with 150, waited a minute or so and got 135. I picked up a leak down tester and the fluid b;ock tester going to try these. Pcode was 0300 if i remember right with different cylinders listed at different times, cyl 3, then later got 4 or 5.
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Old 12-30-2012, 11:58 AM   #19
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Tried the block tester and the fluid didn't change color. So this is telling me there is not likely a head gaskets leak right? If there was a leak between the cylinder and one of the water jackets the color should have turned and A leak between cylinders would have low compression in both cylinders...sound right? No pro here try to get as much as I can before opening up. I need to borrow a compressor to run the leak down test, maybe later today for that.
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Old 12-30-2012, 12:12 PM   #20
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p300 is a random misfire. With no coolant contamination, and a random misfire instead of a specific cylinder, I would look into the o2 sensors, map, tps, vacuum leaks, exhaust leaks instead of the engine itself. Run the compression test again with a battery charger on, all the plugs out, and do it 3x for each cylinder. Watch the gauge and stop cranking when it doesn't go any higher.
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Old 12-30-2012, 01:14 PM   #21
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Tried the block tester and the fluid didn't change color. So this is telling me there is not likely a head gaskets leak right? If there was a leak between the cylinder and one of the water jackets the color should have turned and A leak between cylinders would have low compression in both cylinders...sound right? No pro here try to get as much as I can before opening up. I need to borrow a compressor to run the leak down test, maybe later today for that.
Your right on target. Here is the thing. If the plugs look good then sometimes a misfire will cause the cylinder to run cool and the lack of a burn will cause fuel wash on the cylinder walls. This washes off the oil seal. Plugs "out" on a cold motor. Spin it over about 15 seconds then do the leak down test.
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Old 12-30-2012, 05:53 PM   #22
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Thanks guys, put the plugs back in for the block test so will warm it prior to leak down. Will also double check the compression test as suggested above.
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Old 12-30-2012, 06:56 PM   #23
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You've told us how many miles are on the new engine. But was it a short block, and how many original miles?
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Old 06-17-2013, 11:19 AM   #24
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For the Jeep 4.0 what is the spec for compression (upper and lower limits).
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Old 06-17-2013, 11:20 AM   #25
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For the Jeep 4.0 what is the compression spec range?

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