98 xj - intermittent misfires/weak ignition #1 cylinder

I just picked up a 98 xj 4.0L 5speed.
Engine has 250,000km on it. Runs well, has lots of power.
Starting is a bit weak, and requires a second or so to fire up. Takes a bit longer when its cold (0c).

The #1 cylinder seems to be intermittently weak or misfiring. I haven't checked codes yet, but the previous owner said its throwing a #1 misfire code. The motor shakes a bit at idle (due to the ignition not being balanced) and cleans up as the RPM's increase above 1100 RPM. At idle, I can disconnect the #1 fuel injector wire and there is no noticeable change in engine sound. When the RPM's are higher, the disconnected injector makes a noticeable difference.
But, as I said, this is intermittent. I'll stop for gas, start it back up and the ignition sounds and feels normal.
When the problem is happening, the mixture is definitely a bit rich, suggesting incomplete ignition.
I've checked the spark leads/wires and the previous owner said he's swapped injectors. Doesn't sound like a mechanical problem, and the engine generally sounds very healthy.
What should I look for next?

P.S. I've got an OBDii scanner coming in a week or so. In the mean time, I'm working with hand tools and a multimeter.

I always start with fresh tuneup hardware. Might be something else but don't want to be chasing your tail for a spark plug, plug wire, distributor cap, or rotor problem.

If tuneup hardware is fresh and injector for #1 has been swapped and you're still pulling a code for that cylinder, despite how the engine is running, you gotta run a compression test on all cylinders to determine if something is weak on #1.

Well, too late, I already compression tested. #1 reads 0psi.
I always assumed that if you had no compression, the motor would run like balls. #2 holds 118psi. Didn't check 3,4,5 cause they all made a noticeable change in engine noise when I disconnected their injectors. Tested 6 as well and its 90psi.
While I was at it, I decided to have a listen to the injectors (with a home made stethoscope), and noticed that #1 exhaust port is blowing a bit of exhaust out between the pipe and the metal clampy surface. (related?)

1 - 0psi
2 - 118psi
3
4
5
6 - 90

There is no water in the oil, or vice versa. Engine has tonnes of power and sounds good.
So what causes 0psi on only a single cylinder? Valves not seating quite right?

Also keep in mind that the problem at least SEEMS intermittent.

Googling

As it is intermittent I would suspect a sticking/carbon covered valve.

Try some Seafoam or MCCC through the intake and then retest compression.

Zero PSI on #1 cyl means that you need to pull the head and take a look. Then you may see that a valve is bent or stuck open. The valve seat could be burned. The head gasket could be leaking. Pull the head and you will see why you have the 0 compression. 90 on # 6 is also basically not functional.

What have you driven before to think that this motor has good power. Don't waste any more money on it until you get the compression up. (At a minimum, rings and valve job.)

winterbeater said:

Zero PSI on #1 cyl means that you need to pull the head and take a look. Then you may see that a valve is bent or stuck open. The valve seat could be burned. The head gasket could be leaking. Pull the head and you will see why you have the 0 compression. 90 on # 6 is also basically not functional.

What have you driven before to think that this motor has good power. Don't waste any more money on it until you get the compression up. (At a minimum, rings and valve job.)

Click to expand...

I have another 98 xj auto with 300,000km in excellent condition, a 90 xj with 500,000 that runs very well and gets about 10km/L. Comparing to these. The new 98 has quite a bit more power, accelerates about 1/5 faster, but seems to get poor mileage (compared to the other 98 and the 90). I race my 90xj against lots of newer jeeps off the line, and I usually hold my own so I assume its going pretty good.

My 90's compression is about 100psi avg. the 98's is about 118psi avg.

Would a bent valve work intermittently, and better at high rpms? It sounds to me (and I'm entirely uneducated) that the lifter is sticky, or the spring is weak. I've considering trying some kind of detergent or water injection to see if something just needs to be cleaned up.

Can anyone think of anything else to test before pulling the head?

(Also, how do you change the piston rings? can the piston come out the bottom, or does the head gotta come off?)

I'm not so sure your gauge is working correctly. Your readings on these different engines are below good ignition pressures. Try borrowing or renting a different gauge. Make sure you are taking the reading for each cylinder after three strokes. If you are using the key to crank the engine disable the injectors so you don't wash the cylinders.

If the engine has POWER, and the miss is intermittent, I sure would try some Seafoam BEFORE pulling the head.

joe_peters said:

I'm not so sure your gauge is working correctly. Your readings on these different engines are below good ignition pressures. Try borrowing or renting a different gauge. Make sure you are taking the reading for each cylinder after three strokes. If you are using the key to crank the engine disable the injectors so you don't wash the cylinders.

If the engine has POWER, and the miss is intermittent, I sure would try some Seafoam BEFORE pulling the head.

Click to expand...

Seafoam in the TB while its running?

blistovmhz said:

Seafoam in the TB while its running?

Click to expand...

Yes, put it in through the brake booster vacuum line.

There is a TSB on the 4.0 for sticking/carbon valves but I don't remember which years and I don't have access to it, you can try searching the internet.

Well, dumped some sea foam in, but it didn't smoke as much as I was hoping for. The problem still exists, but seems like it clears up at slightly lower rpms than it used to, though this may just be my imagination.
Not my imagination however, is a new knocking. sound. Don't know where its coming from. Idea's on where it came from, and what to do from here?

*sigh*. The new knocking sound is probably related to my #2 fuel injector connector not pulsing anymore. Must have bumped something the wrong way while testing the compression issues.
http://naxja.org/forum/showthread.php?t=1032975
I gotta figure this injector problem out first now.
No pulse on #2 connector. Traced back to about 3 inches from the PCM connector and still no signal. Any idea's?

blistovmhz said:

*sigh*. The new knocking sound is probably related to my #2 fuel injector connector not pulsing anymore. Must have bumped something the wrong way while testing the compression issues.
http://naxja.org/forum/showthread.php?t=1032975
I gotta figure this injector problem out first now.
No pulse on #2 connector. Traced back to about 3 inches from the PCM connector and still no signal. Any idea's?

Click to expand...

Keep on following the harness looking for the signal. Wish I had a better suggestion for you.

Good luck.

I actually already did so. I even replaced the #2 pulse wire. Still no pulse. I'm assuming the 12V positive is common, so at this point I believe my PCM is the problem.

Can anyone tell me I'm wrong before I go buy a new PCM?

After much XXXXing around, I decided the PCM was probably hooped and figured I couldn't hurt anything any worse, and I wanted to rule out the possibility of the 12V lead not being able to supply the amperage I needed. So... I back probed the #2 pulse lead to my multimeter and connected the other side to the frame, started the engine, and got 1A steady. This of course made the injector very hot and puke fuel all over the cylinder. I smelled gas and shut everything down. Disconnected the probe and jumped the #1 pulse to the #2 injector and started the engine back up. Still nothing. Shut down, removed back probes, started up again and stethoscoped again. #2 is XXXXing working now! WTF?

All I can think is that something in the PCM desoldered itself, and when I shorted it with 1A, it resoldered...?

At any rate, problem is fixed for now. Goona track down a spare PCM just in case.


I'm going to try to pull the head this weekend. Never done it before though. Anything I need to know? Can the head go right back on without going to a machine shop?

Can it go right back on--sure! Should it go right back on--not really.

I would, at the very minimum, check it for flatness--feeler gauge and a true straight edge (I have cheated in my youth and used a new level).

How about the block? Anything I need to do to it aside from clean up the mating surface?

Nothing else on the block, except look for bad cylinder gouges, etc. The valve sealing can be checked by putting head down flat with combustion chamber up. Fill combustion chambers with gas or mineral spirits. See if it leaks out.

You could do a "poor man's valve job" and lap the valves in a little with some valve grinding compound.

blistovmhz said:

How about the block? Anything I need to do to it aside from clean up the mating surface?

Click to expand...

Clean it up, you could run a tap to clean up the threads for the head bolts, and speaking of head bolts--if they have paint marked on their tops then they have already been re-used once and should be replaced.

Is there any way to tell if the valves are sealing without pulling the head? (ie: with just the valve cover off).

compression test? blow-by test with gutted plug and air? if you want a detailed procedure on the blow by check then lemme know.