rdr8887

1994 Cherokee XJ 4.0L


This one has me stumped. Any input is appreciated.

Been running like a top for a while. The motor at least. Lately, after sitting a week or so it would run rough when started. Then it would clear up after a few seconds. It doesn't get driven much.

I let it warm up the other day and drive it pretty hard. A few wide open throttle runs up to about 40mph. Parked it. Next day went to start it and it ran rough. Shaking violently. No CEL. Cleared up a little and now is rough all the time. Was leaking fuel from the FPR, so, I replaced it. Didn't fix the problem.

So, I warmed the motor up about halfway and pulled the plugs. Hooked up my leakdown tester and checked for losses. Test showed about 6%-11% loss. Pretty good I think for the age. I could hear air coming from the oil filler cap in all cases. Not much in intake or exhaust and nothing in radiator that I could tell. Results posted for this.

So, after I was done I decided to do a compression test to see what was going on as well. 150psi on all cylinders but #2. That cylinder had no pressure. Maybe 20psi. Repeated text and got same results. Repeated leakdown and got similar results. Hard to tell compression stroke this time (wasn't pushing much air out) so I checked TDC of both exhaust and compression stroke. Couldn't tell which was which, but, results were either 6% or 15%. Both leaked into valve cover.

So, how does leakdown pass but compression failed so bad? Any ideas? Problem doesn't appear to be spark, fuel, or vacuum. I'm guessing a problem in the head? Maybe a blown out piston? But, leak down doesn't seem to verify this.

Scousebil

Hmmm, so you're saying that during an entire stroke the compression is bad but at top dead centre the leakdown is good? So is it pointing toward something further down than TDC, say a worn spot in the wall for example? I've never done a leakdown, I just know of the procedure, so bear with me, but is it possible to do a leakdown further down from TDC? Say quarter, half way, three quarters ... ?

The only other thing I can think is to make sure the piston is at the same height in the cylinder as the others at TDC - if it's lower then something is amiss with the rod causing poor compression but the cylinder is still sealed up quite well causing good leakdown result. Maybe.

When it's running bad is it running on 5 cylinders?

Scousebil

Of course you can't do one quarter, half way etc like I suggested as the valves would be open. My bad. I would still check for a bent rod tho...

rdr8887

Yea. It's running like it has a misfire. I can pull the valve cover and see what I see real quick. Who knows. Sounds like I'll be pulling the head anyway n

rdr8887

Well, this might explain the rough running. now I need to find out why.

DFlintstone

Stuck valve? Intake? The intake valve stem is exposed to whatever comes down the pike. (was going to suggest a sticking valve but you beat me). I'd try depressing the valve. If you CAN push it down, make sure it's up again before turning the engine. If it stuck down low enough to be hit by the piston than would be bad!

Side story probably. After finding a mouse nest in my air cleaner a second time I installed wire mesh over the intake. Just thinking...remove the boot from the air cleaner, hold the valve open, then blast air into the plug hole and see if mouse nesting blows out! (Idk)

Another story. On an old Briggs the intake valve stuck. I pulled (it's little flat head), and worked it till the valve freed up. Worked-ran, then stuck again. THEN is when I figured out it was bad old gas in it. Freed the valve again, changed the gas and that did it.

Turbo X_J

For whatever reason that valve is stuck closed, if it's stuck hard enough to bend the push rod I'd pull the head and see what's up.

Motojunky

I'm just thinking "out loud" here. As others have suggested, I'd depress the valve manually to see if it's stuck. It sure looks that way. Wouldn't it also be possible that the lifter is DOA? Not sure how to explain the bent pushrod in that scenario, but if the valve moves freely I'd be looking at the lifter.

Can the 4.0 lifters be removed with the head in place?

rdr8887

Okay. So, she's putting again. Replaced the bent pushrod and pulled all rockers. All valves appear to be sitting correct with a strait edge laid across the tops. Re-torqued rockers down. Pulled fuel rail. Injectors were nasty. Replaced with a newer set. Cranked it up with valve cover removed and waited for oil to reach the rockers. Runs great.

Problem solved for now.

No, you have to pull the head to remove the lifters. I believe it hydrolocked because an injector flooded the champer with fuel. Just a guess though. It's running good now.

Scousebil

Brilliant mate - good to hear you've got it sorted.