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Good Leakdown with Bad Compression

Ima Doit

NAXJA Forum User
Location
Atlanta
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.
 
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What did the spark plugs look like? Picture? Intake valve sticking? Collapsed lifter? I would pull the valve cover and whack the valve stems with a hammer. Inspect for bent push rods, etc.
 
Had a honda that wouldn't start some times after sitting, figured it was valves sticking.
If a valve is sticking you'll get crap for compression.



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Here is a picture of the spark plugs. Cylinder 1 is on the right. They follow order from there. I'm about to pull valve cover off to see what I see.
 
Plugs are out and about the same price as a valve cover gasket.New champion plugs set the gap and try again :dunno:
 
Maybe. Most likely at least. Hard to say for sure without digging. What would cause that? A lifter is pretty basic. I'm gonna toss a new pushrods in and see what happens. Isn't the first time I've torn down a 4.0L. I have another set of lifters in the garage.
 
Ouchie!! Was it like that when you took the vc off?
 
Yup. Took a picture before I pulled it out. It's bad when you don't need to loosen the rocker to get the pushrods out. There are a couple other rockers loose. I'm going to pull all rockers and pushrods out tomorrow to inspect. I'll run a straightedge across the valves and see if I can manually push down the valves. Then I'll go from there.

If all seems well, I'm going to replace the pushrods and carry on. The rockers all have oil on them still, so, they have been getting oil on the top end.
 
That would certainly explain your good leak down test and no compression -- On cylinder 2 that should be an intake valve -- If it doesn't open, no air gets in to compress
 
Ahh. Good info. What would cause it though. Assuming it did hydrolock. As I stated, it would rough start quite often but settle out. This last time, the rough start never went away.
 
#2 plug was darker than the rest indicating a problem, now obvious in that cylinder.

No mention of gas smell was reported, so my arm chair quarterback guess is a bad valve or lifter.

In some cases fuel injectors don't shut off so they spray continuously wide open even and then you have liquid gasoline everywhere... causing a hydrolock.

An engine can run surprisingly well with one bad cylinder.

The trick of whacking the valve stem with a hammer is meant to shock them and free them up if they are stuck. This can be done during a compression test to see if compression improves. If so, it points to a sticking valve.

On the 4.0L you have to pull the head to change the lifters, so I would try to isolate valve or lifter first.
 
Yup. Took a picture before I pulled it out. It's bad when you don't need to loosen the rocker to get the pushrods out. There are a couple other rockers loose. I'm going to pull all rockers and pushrods out tomorrow to inspect. I'll run a straightedge across the valves and see if I can manually push down the valves. Then I'll go from there.

If all seems well, I'm going to replace the pushrods and carry on. The rockers all have oil on them still, so, they have been getting oil on the top end.
Good work!

Excessive lash can bend a push rod. Once that push rod leaves it's seat, bad things happen.

Excessive pressure can bend a push rod. Maybe you have a problem with clearances, maybe your exhaust valve does not open like it should (Nevermind how I learned that!).

Leave your plugs out, crank the engine and watch how far the rockers move.
 
Yeah, it's pretty obvious now why your compression test failed. Perhaps the plunger spring collapsed/broke, or some debris/sludge prevents the lifter from preloading.

Excessive gap can cause the pushrod to unseat from the rocker. When the cam pushes it upward, it rides up along the outside of the rocker, bending it.

Before, it probably freed up once it pumped up and got warm. The rod end stayed in the rocker. The last time it slipped out.
 
I will say this...there was a fuel smell coming from the #2 cylinder. It was actually wet. I'll investigate that injector as well. Pretty sure I have some spare injectors sitting around. I'll swap it to be safe.

Also, it has seemed like the jeep has been running rich in general. It doesn't get driven much. When it's started, it smells strong of fuel. No idea what the MPG's are.
 
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