I bought a 97 Cherokee Sport a month or 2 ago with the 4. O liter inline 6 around 116K mileage but come to find out that happens to have low compression on cylinder number 3. I drove it around back and forth from work about 60 miles a day for about a month and then parked it for the last 3 to 4 weeks behind the house to work on it, that and because I didn't need it. I just got around the running a compression test about a week ago and the scores are as follows: cylinder 1 120 pounds. Cylinder 2 118 pounds. Cylinder 3 85 pounds. Cylinder 4 140 pounds. Cylinder 5 121 pounds cylinder 6 115 pounds. With only 116,000 miles I'm not too sure where my problem most likely is. I'm hoping I have a sticking ring in cyl numbers 3 but it doesn't smoke at all and burns virtually no oil whatsoever. so wonder if I might be able to get it to unclench with a little Marvel mystery oil. I am treating it now with a cooking Baster via the spark plug holes. Any ideas? Will post pics soon! Thanks!

 

Does anyone know how to pin point a head gasket failure? am searching for a thread on it now... radiator and antifreeze look clean tho, same with the oil. seems water free. now if i can just keep it out of my buddys pond! haha its just so tempting.

 

Add a tablespoon of oil into cylinder #3 and re-run the compression test.

If the compression comes UP after introducing oil, you likely have a piston ring issue. (oil seals the rings)

If the compression stays the same, the problem is likely in the cylinder head.

Another extremely valuable test for diagnosing internal problems is the "leakdown test".

 

Yeah that was my next step! thanks for the info man, but do you think marvel mystery will be thick enough to work? out of oil and was gonna grab more and do a change when i get her back together

 

And how do my ranges look? i think from 140 to 115 is a pretty big range... but these 4.0L's are TANKS! and its still firing great on #3 with 80 odd psi!!!

 

MMO will be fine.

It might be carboned up inside the combustion chamber/valve area. If it were mine, I would buy a can of BG 44K and pour it into a full tank of gas and run the pi$$ out of it for the tankful.

 

thanks man! will do. just wondering where i can get some "BG 44K"? i will try local today. Do you or anyone else endorse or know of any other quality products worth trying? even just for power and fuel stability. i like running lucas oil additive in my v8's and will do lightly in 4.0 after fresh oil change. also Lucas "Tune uP In a Bottle" seems to do nicely every 3 or 4 tanks

 

NOTHING compares to 44K. High end repair shops, especially foreign, and dealerships sell it. It's $20 for one can and worth every penny.

 

nice. thanks again!

 

Assumimg your compression gauge is working properly and you performed the test correctly - engine at operating temp, all spark plugs removed, throttle body wide open, I would say five out of six cylinders are marginal at best. Hopefully, with only 116K miles, you can get those numbers up to the 150+ psi range with some engine cleaner stuff.

 

About the same results mine had. Going to seafoam it next week and add some cleaner to the gas for my trip tomorrow.

 

Do a leakdown test, find out why.

In a leakdown test, you put a known pressure into the cylinder (usually 100psig) with a regulated gage and a "leakdown" gage, which tells you how much of the pressure you're putting in the cylinder actually holds.

Conventional piston rings, at operating temperature, are typically good for 90-95% sealing.

The advantage of doing a leakdown test on a suspect cylinder? You've got a steady pressure going into the cylinder, so you can then look/listen for the leaks (a compression leak may be a worn/failed/failing head gasket, worn/failed/failing piston compression rings, worn/failed/failing valve(s), or possibly a tappet sticking badly enough that the valve is not allowed to close - even .001" is enough to lose compression!)

Once you pressurise the cylinder:
- Bubbles in the coolant indicate a failed/failing head gasket.
- Hearing the leak when you remove the oil fill cap and listen indicate worn rings
- Hearing the air escaping at the tailpipe means exhaust valve
- Hearing the air escaping through the throttle body means intake valve.

Just because you hear the leak in one place, don't stop checking! You'll feel really stupid if you fix one leak, and discover another missed before...

 

I thought the spec was 120-150. ??

 

Changes as the engine wears.

Ideally, you take an average of the cylinder readings, disregarding flyers, and all cylinders should fall withing 90-110% of that.

The "spec" is for a new engine, but it does drop with age. I wouldn't expect to see anything under 100psig on on OHV/OHC engine, tho - maybe an F-head or flathead.

 

Really good info man! ive never thought about this before... ive just got to find a regulated gauge or what not to perform this. hopefully can get one at harbor freight. been working on stuff alot and treated the rings with a teaspoon of marvel one more time for a total of three treatments now. And thanks for all the posts guys! also ive used seafoam before and had good results but ive heard if you run in your oil you should change it out after it runs through a couple days. supposedly it eats gaskets if not. but it worked well in my fuel and i used it in an 80's model 302 ford and you wouldnt believe the crap that broke loose out of the valve covers!!! it clogged the oil filter! but that was an old dirty nasty son of a gun