1999 Cherokee XJ, 4.0L:

So I've been having an issue with a cylinder missfire. First things first, I did a full tune up (plugs, rotor, cap, etc.) and it did not help. All fluids are where they are supposed to be, no mixtures of gas/oil. I replaced the TPS and same issue.

I finally took in to a shop and he diagnosed #5 had a missfire and it was a dead injector. I replaced it and it was the same issue. Returned it to the shop and he says the new injector is good and he checked all the wiring and that was good. There was no signal to the injector, sounds like a bad computer. Service bulletin says when injector #5 goes out it can sometimes burn out the computer.

All dealerships are on backorder for that computer and possibly will not be made again. Located one at a junkyard and took it to another shop in the area that flashes them. Just got a call from the shop, they flashed the new computer, and the missfire did not go away.

What is your guess? I have not checked compression since the problem was focused on the injector not working. Possibility of a bad connection that the shop did not pick up?

I might try to swap the injector with another one and take it to autozone to see if it still says injector #5. Anything else I can try?

 

Wasn't there an issue with carbon getting under the exhaust valve causing a misfire?

Can you feel it at idle or at speed?

 

It's felt at both, real bad missfire stalls the motor in drive. I had read about the carbon and followed the service bulletin to fix. Forgot to mention that, used the mopar combustion chamber cleaner, nothing.

 

I think it is a very good possibility that you have a wiring issue to the #5 injector......that wasn't caught by the shop. Not unusual for that to happen.

If you are certain you have no signal to the #5 injector, (a simple noid light will tell that story) concentrate on WIRING and the connector for that injector. There isn't much left as you have already eliminated the injector itself and the computer.

 

Two things to check:

1. There have been numerous posts on this forum that weak valve springs can cause a misfire.
2. For me, my misfire was caused by the rear wiring harness, between the engine and firewall, was cut through by the fuel manifold. Very easy to inspect, if the wiring harness is touching any part of the engine, look very close. That wiring harness contains all the injector wires.

 

Which service bulletin are you referring to?

I've read that an injector with very low impedance can smoke a PCM injector driver. Impedance (resistance) of an injector used in a '99 + is supposed to be 12 Ohms +/- 1.2 Ohms at room temp (68*F). I don't know how low the resistance has get to damage the PCM driver. I've never heard about a single position injector smoking a PCM, but anything is possible I suppose. Each injector has its own driver in the PCM

Are you using original fuel injectors or so called "upgrade" injectors.

You may want to measure the resistance of each injector you have installed to make sure they are within the above specs. Measure pin to pin in the injector with an Ohmmeter when they have cooled to around 68*F.

Edit: Find the injector you replaced and measure its resistance.

Let us know what you find.

 

It's certainly possible, but I would double-check the wiring.

Others have mentioned the valve spring TSB. That was the case on my 98. If by chance you find that the shop was wrong, and the injector IS working, the TSB may apply to you. The TSB applies to 96-99 4.0s. It says that if you have a misfire, and no other cause can be found, then you're supposed to replace all the valve springs and run Mopar Combustion Chamber Cleaner according to the TSB instructions. I ran the cleaner and replaced thesprings on just the affected cylinder, and it cured my problem.

 

Thanks for the help everyone! I just picked up the jeep and I'm reluctant to take it back to that shop, don't know what tsb he was talking about exactly, just what I was told.

I'm going to check the wiring and possibly rent a tool from auto zone to check compression. Hope it's not the springs or a valve.

 

I had the exact same thing happen in my 98 XJ.

I couldnt find anything wrong with the wiring, or any of the injectors after swapping them around. I ended up replacing my catalytic converter and my exhaust, reset the computer, and the issue was gone.

It my not be the solution to your problem, but this is what worked for me.

 

So what led you to suspect it was the exhaust system?

 

Well my catalytic converter looked like swiss cheese, and my muffler had more leaks than the Titanic...

It's worth a look. When I was researching my issue every thread about the injector #5 issue was a dead end with no solid outcome.

 

Why is a simple misfire so complicated?
Back to basics - compression, fuel, air, spark - it has to run.

Take it to a shop where they use a scope - not a code reader, oscillosope.
It will instantly check the ignition, including wires, plugs, cap, rotor, mixture, valve action and carbon deposits.
ALL GOOD THERE?
Use a noid light to see the injector pulse - checks wiring, computer, etc. They are cheap.
ALL GOOD?
Do an intake smoke test for a vacuum leak.
10 minutes of diagnosing!

 

I thought I'd give a follow up on this if anyone else is having a similar issue.

So I ended up taking it to a different (competent) shop and he called me right away and asked for permission to take off the valve cover gasket, give him the thumbs up. Called me back 30 minutes later, one of the rockers is not really moving. I'm pretty tired of not being able to drive it so I just said, fix it!

Turns out one of the lobes for #5 on camshaft is worn pretty bad. 4.0 gets a new camshaft, timing chain, lifters, head gasket, etc.

I looked at the parts after they were removed and the bottom of the lifter that sat on that lobe is no longer flat, it was worn down to probably a 25 degree angle. How does that happen?

Pretty major work was needed on mine, hopefully your missfire issues are simpler. Wish I was a little better at fixing stuff like this, I'm ok with replacing parts but I get nervous tearing into the internals of the motor.

Now that it runs and idles amazing, I plan to change all the fluids and look at some upgrades for this stock Daily Driver.

 

Thinking about that camshaft, I have a question for you tech guys. Could a stiff spring have caused that excessive wear on the camshaft lobe and lifter. Not sure if he replaced springs, I'll check my reciept. What do you think?

 

I'm sure one of the very knowledgeable other gents in this thread will have a real answer, but I wouldn't hesitate to suggest lack of maintenance could cause this. Seems to me never changing the oil would let it get worn down