agramovot

Hey all,

I have a 92 XJ with just under 200k on it and have been doing a tune up since I bought it.

So far I have-

New plugs/wires/distributor cap
New battery
Cleaned injectors, fuel rail and throttle body today

BUT, it is still misfiring. We tracked it down to cylinder 4 tonight, it is getting spark and fuel. The only other part of that equation I believe is the valves, but I may be wrong. I know they cant be adjusted, but does anyone have any advice on what I can do tomorrow?

I am going to borrow a compression tester from a friend of mine and test it in the afternoon, but Im just curious as to what it might be.

The engine is not throwing any codes either.

Any help would be greatly appreciated!

Nicks90xj

Start off with the compression test and see what happens. you can have a burnt valve, bent push rod, injector not working right. see what the compression is first and let me know. if it seems low do a wet test also

agramovot

Ok, so I did the compression test and cylinder 4 was 0 psi..cylinders 1,2,3,5 and 6 were all between 130-135..so it wasnt the tester. I dont know where to go from here except for taking off the valve cover and seeing if anything looks screwed up.

Thoughts?

jmac108

U either have a hole in the piston head or block

If your lucky though could be a bent valve failing to seat or broken valve spring, u probably hear the valve spring though
First thing u should do I recheck compression if u haven't already to confirmed your results
Best thing to do like said is pull the valve cover and check. What u want to do while your looking to check for a bent valve is turn the motor over by hand and measure the height of the valve spring when the pressure from the rocker is off it and then compare to one of the good cylinders. If you don't find anything that way it's probably time to pull the head and take a look inside

RTorrez1

^^ This would be about the only thing to give you 0 psi.^^
Diffidently recheck the compression to be on the safe side.

agramovot

Thanks guys.

I rechecked the compression 3 times each per cylinder to make sure, because I thought 0 psi was absurd, and its definitely 0. Ill pull the valve cover tomorrow and see what it looks like under there.

If it is a bent valve, can I just replace the valves for the cylinder and call it good? Im trying to keep my cost to a minimum..Im a graduate student and money is tight, per usual.

good_ole_xj

Zero compression is definatley a mechanical issue. If you are sure the reading is right you will totally need to pull the valve cover and check the valve train... Maybe a broken valve spring??

good_ole_xj

To repllace the valve you gotta pull the head... pull the cover and see what you find

agramovot

If the valve is bent, and the walls of the cylinder are not scratched (I hope), any idea of what im looking at monetarily speaking in parts to do this?

good_ole_xj

Biggest problem about a bent valve is that it can damage the valve guide in the head. Which would require a machine shop repair. Gotta pull the valve cover for inspection!

agramovot

Alrightey, Ill have to get that done tomorrow and see what it looks like under there. Ill post pictures when I get it done.

Worst case scenario, there is an XJ out behind our shop that was rolled but the engine was untouched. I might be able to swap some stuff out of that one if need be. The only problem...a girl died when the jeep rolled. Not too sure about the mojo on that one..

67 GMC

I had a similar problem but compression was fine on all 6. Spark and Fuel good but no fire. Changed plugs, rotor, cap, wires, swapped injectors etc. Took the valve cover off and a push rod had come out from under the rocker arm on the intake valve (intake valve closed all the time). It would back fire out the intake and basically run real crappy. Easy fix and it runs like a champ now. Hopefully, yours could be as easy as a bent pushrod staying up and keeping a valve open.

agramovot

Thanks for the input. Unfortunately, its not going to be that simple. I took off the valve cover earlier and everything looks pretty much as it should, except for the discoloration on all of the rocker arms/springs/pushrods, which im sure is just a product of nearly 200k. Here is a pic of what I was looking at..

Now, all I can think of is either rings, a jacked up valve, or...im not sure what else it could be at this point.

good_ole_xj

Know anybody in a shop or that has an air compressor? I would suggest a cylinder leakage test. Put the cylinder at top dead compression stroke and apply compressed air into the cylinder. Then you can hear if air is escaping from the intake, exhaust, or into the crankcase. Either way the head may have to come off. I would guess a really badly burned exhaust valve or a piston/ring issue.

agramovot

Yup, my buddy's shop where the jeep is has just about every tool imaginable. Ill be up there tomorrow helping to assemble a paint booth, so I might be able to sneak that test in.

I just really dont have the money to rebuild the entire top end of this motor, so I want to isolate and just fix the problem.