Low compression in one cylinder. - JeepForum.com
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post #1 of 7 Old 04-08-2009, 02:24 PM Thread Starter
Lone Star XJ
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Low compression in one cylinder.

I am trying to get my new to me 97 Cherokee on the road. I had a friend of mine at a shop replace all the fluids, do a tune up, and generally go through the vehicle. The engine was not idling very smooth, so he started check things out. He ran a compression check and found three cylinders at 130psi and one at 90psi. He did not do a wet compression test. I will do this once I get it home.

This engine only has 126K on it, so I am surprised at the numbers. If it turns out it is caused a bad valve, I will pull the head and have it fixed. However, I think the low compression could be from sticking rings. The rig has been parked for over a year and only started a half dozen times. If it is due to sticking rings, what would y'all suggest to get them to break free? Seafoam? Wide open throttle runs?

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post #2 of 7 Old 04-08-2009, 03:30 PM
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Pull the spark plugs and spray some PB Blaster, Deep Creep, or similar penetrating oil into the cylinder. Use the little red tube and get it all over. With the plugs still out, turn over the engine a few times. Wait for a while and give the spray a chance to work.

The 4.0 L can be prone to carbon build up on the valves if not driven wide open throttle now and then, so consider an oil additive to clean the top end, and then change the oil.


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post #3 of 7 Old 04-08-2009, 03:47 PM
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PB blaster, and letting it sit for a couple of days, restored compression in one cylinder from 80psi back to 150psi - same as the other 5 cylinders.

But I consider myself lucky.

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post #4 of 7 Old 04-08-2009, 03:56 PM Thread Starter
Lone Star XJ
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Sure won't hurt to try the PB Blaster. Unfortunately my luck always seems to deal me low compression engines. But, this thing has been sitting for a long time. Any suggestions on what to use to clean the top end?
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post #5 of 7 Old 04-09-2009, 06:42 PM
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give two cans of sea foam a try for the top end. do a search for the most common proceedure. its kind of the reason jeep guys swear by sea foam...

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post #6 of 7 Old 04-22-2009, 10:14 PM Thread Starter
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Well, I put PB Blaster in all they cylinders on Sunday. I let it sit until today when I could finish putting new plugs, cap, rotor and wires in. She fired up and still seemed a little rough. I let it warm up then ran 1/3 pint of Sea Foam through the brake boostger vacume hose. Let it sit five or ten minutes and fired it up. There was very little smoke at all. I then took it out and ran it hard for fifteen minutes or so. After the first few hard take offs, the engine seemed to smooth out.

I brought it home and checked compression on number one. It was up from 90 to 120! I checked a couple of the other cylinders and they were at 150 like my buddy had found. Anyway, I pulled the negative battery cable to clear the code and will see if the check engine light will stay out. Looks like I am on the right track. Thanks CJ7 and colt.
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post #7 of 7 Old 09-28-2010, 10:15 AM
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Just read this post. I have a 91 wrangler with low compression in #2. #s 1, 3, and 4 read 135 on the nose, and #2 is at only 100. Gonna try this method and see if i can clean some of the gunk out. It has been sitting for a while too.
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