How bad is my compression?

So I checked the compression tonight on my '88 4.0 with about 200k on the clock. From front to back, 150, 152, 142, 117, 135, 148.

pretty low on #4, but is it that bad? also, the plugs looked pretty good and very consistent.

Isn't the guideline to be within %20 on neighboring cylinders? If so, I'm just within that between 3 and 4, and just over %20 when you relate 2 to 4.

It's rediculous how much I know about suspension, axles, etc, yet how comparitively clueless I am about engines. The 4.0 is just too good, it has never given me any problems (besided rear main) so I've never really needed to learn about it.

My FSM says compression pressure should be 155 to 185, with a max variation of 30 psi between cylinders. So ... you're a bit tired across the board, but the only really bad one is #4.

Where there's only one bad cylinder, it's often a valve problem rather than rings (which wear evenly). Try squrting some heavy oil into #4, test it again, and see if it comes up significantly. If so, it's a ring problem. If a shot of heavy oil won't seal it, it's a valve problem. Could be burned, or there could be a speck of carbon stuck to the valve face that won't allow it to seal. A treatment with GM Top Engine Cleaner might take care of that.

i did do a treatment of AutoRX which seems to be one of the few non-snakeoil additives. however they recommend 2 treatments for engines with mileage like mine and i only did 1. Is GM Top Engine Cleaner the same type of stuff?

Eagle, are you sure those compression numbers you gave are for the 4.0L?

Those sound more like the 2.5L compression specs.

I think I recall the 4.0L having a compression between 120psi and 150psi with the same 30 psi allowable variation between cylinders.

185 psi seems awfuly high for an engine running an 8.8:1 compression ratio.

hmmm, any more input out there?

Bender said:

Eagle, are you sure those compression numbers you gave are for the 4.0L?

Those sound more like the 2.5L compression specs.

I think I recall the 4.0L having a compression between 120psi and 150psi with the same 30 psi allowable variation between cylinders.

185 psi seems awfuly high for an engine running an 8.8:1 compression ratio.

Click to expand...

Oops.

Yes, my bad. Those were the figures for the 2.5L. Sorry.

so is 120-150 the right numbers for the 4.0?

if so, that's pretty schweeet

FSM "MOT. I-6"

120 to 150 psi, max variation 30 psi

Mad MaXJ, with regular maintenance intervals you will have many happy miles left in your 4.0L. Just be sure your oil pressure remains at a nice level so you don't lose your bottom end.

I swapped in a full guage cluster (from dummy lights) last summer with the appropriate sending units, and the oil pressure has always been right around 60. I think the sending unit is probably bad.

Go to a Jeep dealer and buy some Mopar decarbonizer, and spray it in the throttle body as per instructions on the can. Then check the compresion again, I bet all the cyld's will be within spec. Jeep tech at my small town dealer swears by this stuff, about $10 a can, and recomends using it about avery 50,000mi.

Bender said:

Eagle, are you sure those compression numbers you gave are for the 4.0L?

Those sound more like the 2.5L compression specs.

I think I recall the 4.0L having a compression between 120psi and 150psi with the same 30 psi allowable variation between cylinders.

185 psi seems awfuly high for an engine running an 8.8:1 compression ratio.

Click to expand...

I have a 95 jeep wrangler and i am starting to have problems with the engine. It is a 2.5 4 cylinder with 188k on it. Its starting to get oil in the airfilter, maxium speed of 50mph, blows no oil out of the exhaust, gas milage is only about 10mpg lifters are noisiy.
We took a compression test 125psi dry on 3 cylinders 135psi on the last one
wet- increased to 135 on 3 and the last was 140psi

Why doi start any suggestions?
or please email me at [email protected]

It's not that bad, just run it.

What Eagle said was right on, sounds like it may a valve. Squirt some oil into the cylinders and read it again.
Make sure your doing the compression test correctly. Blow out the spark plug wells/area with air, remove *all* the plugs, open up the TB (so it isn't sucking air from/through the air filter). Prop the throttle open all the way (or tie it open). Make sure you have a fresh battery. Turn the motor over enough times to get a good reading. I unplug the CPS and unhook the coil wire.
Funny, mine had middle low compression on the two middle cylinders 115-120. I replaced the O2 sensor, mine was running lean at mid to upper RPM's. And a year later, did another compression test, the motor cured itself, my compression is pretty much flat and consistant in all cylinders and right around 140-150. I don't know exactly what the problem was, but it cured itself. I suspect a couple of exhaust valves were a touch crispy or a couple of intakes had some tiny pieces of junk stuck in the seats.
I also routinely, use a second compression tester on a cylinder or two, just as a cross check/reference. I almost tore into a motor once, because of an out of spec. compression tester. Check the "O" ring on your tester for cuts.

Eagle said:

FSM "MOT. I-6"

120 to 150 psi, max variation 30 psi

Click to expand...

Hi Eagle ... I just put in a new 4.0 HO motor, and the compression is 160 on all 6 cylinders.

Is it possible that quote above is for the non-HO motors?

Thanks for the info.

montanaman said:

Hi Eagle ... I just put in a new 4.0 HO motor, and the compression is 160 on all 6 cylinders.

Is it possible that quote above is for the non-HO motors?

Thanks for the info.

Click to expand...

It could be but that shouldn't make any difference. Both the non-HO and the '91-'95 HO have an 8.8:1 CR and share the same camshaft.
I wouldn't concern myself so much about the absolute compression numbers (unless they're very low) but rather how each cylinder compares to one another. Maximum variation should ideally be no more than 10%. In BrettM's case, only cylinder no.4 is out of spec and that could, as Eagle said, be due to a sticking or burnt valve.
A good treatment with Mopar Combustion Chamber Cleaner might free the valve if it's sticking and restore compression. If two treatments don't make any difference, nothing else will in which case, the no.4 exhaust valve could be mildly burnt instead.

ok ... thanks for the help.