Hey guys I need some advice for the heep. I am getting a cylinder six misfire code. Heres everything I have done so far.

• Replaced Coil Pack
• TUPY head from an 03 grand cherokee
• new spark plugs
• new 02 sensors (threw the code so why not)
• better ground straps
• Compression test on all cyl before new head and with new head (good)
• checked fuel pressure

When replacing the head I broke the intake manifold on accident and it ended up being cheaper to pull one from a JY but I could only find one off of a 93 so I know I lost some HP with that. Minus the code, I do have a "rough" idle but nothing terrible, it could pass for a vac leak if it didn't have the code. (I need to replace pretty much all the vac lines and ccv fittings).

I have the californina emissions so double pre-cat. Now with that the previous owner welded it or something but it looks crushed almost where the two halves meet. When I make right hand turns at a certain speed/angle it rubs the transmission and makes a terrible sound and smells like burnt wax for a little afterwords. Maybe the decreased airflow has something to do with it? A new one of those stupid things are 250 min.

I think I need to swap my fuel injectors to see if the code follows. Maybe check voltage pulses on the injector harness and a compression check with a better tool than the crappy auto zone loaner. Does anyone else have any ideas what the problem might be?

 

And what was the result? (You're looking for 49.2 psi plus or minus 5 psi.)

Yes, do that.

Auto Zone should have a noid light through their tool loan program to test your injector pulses. Keep in mind that connecting the noid light and running the motor with the injector disconnected will throw an injector code, so be ready to clear that and move on. (Don't let the code become a red herring.)

You could also try reindexing your camshaft position sensor. It's free and fairly easy, especially if you already have the coil rail off to test compression. Instructions are here:

http://www.jeepforum.com/forum/f11/h...ensor-1145301/

While you have the sensor off, check the oil pump drive to make sure the inner shaft isn't worn and wobbling. Also look in the sensor to see if the magnet has any bites taken out of it.

The only other thing that comes to mind would have to do with the head, springs and the valve rotation TSB. But if you had a cylinder 6 misfire before the head swap and still have it after the head swap, you'd have to be pretty unlucky to get two heads with the same valve or spring issue.

 

1. Definitely swap fuel injectors! And verify pulse to #6 injector with noid light

2. What brand spark plugs are you using? Are they gapped to .035?

3. As mentioned, what is your fuel pressure? 49 plus or minus 5 is your target.

 

New to the forum. I just went through cyl 1 and very occasionally a cyl 6 misfire. Did a whole lot of work which I don’t regret due to the age of the vehicle but mine ended up being a crank sensor. Can’t explain it but mopar sensor fixed it. Issue remained when I tried an auto zone sensor. Havnt has it come back in quite some time and it runs better than ever minus fuel economy but I’m a bit greedy on that.

I even ordered a new set of injector pigtails and made a new injector harness being as most of my clips were broken.

 

 

My CPS would always test OK but when it was hot, it would randomly misfire and RPM's would drop erratically. Likewise if it was really cold.

I ended up swapping it with a MOPAR crank position sensor and it has been fine ever since. Definitely check the injector resistance on the misfiring cyl, and possibly just swap them around to be 100% sure since its easier than changing the CPS. I can't remember if my misfire was limited to a single cylinder or all of them though

 

 

Fuel pressure was 45 and some change, but I’ll retest this weekend since I changed out the original fuel filter after 220k so I’m sure I’ve got a bit more now. If I get time after work I’ll swap injectors around, and hit up autozone or orilleys for the NOID light. I really don’t want to pay a shop just for a scan if I don’t have to. Anyone in the Washington Area have one? Or know somewhere that will check this thing for cheap? Because of the age, mileage, and condition of the parts I’m changing are so bad I don’t mind taking the time to replace but eventually I’m going to run out of cheap fixes haha. The little red clips on my injector harness are all broke from the previous owner. Is there like a pigtail kit or are they individual solder in jobs?

 

 

 

 

This was where I got mine from.

 

I'm not sure if you're saying you have Bosch plugs in it, or you replaced Bosch plugs with something else. So let me make it simple:

NGK ZFR5N gapped to .035

That's what should be in a coil rail ignition 4.0L. Just plain old copper NGK ZFR5N. Don't let an auto store parts monkey convince you to buy anything fancier than that. No platinum. No iridium. No E3s.

Our ignition system is designed to work best at the resistance the copper plugs provide. The resistance is actually too high in the other type plugs, and it causes the computer to do strange things. Our Jeeps have a particular disdain for Bosch plugs, and if those are still in yours, that could very well be your culprit.

In a pinch you can also use good old copper Champion RC12ECC plugs, and that's actually the recommended plug for '99 and down. But '00 and '01 liked the NGKs enough that Chrysler added it to the spec for the coil rail motors later on.

45 is pretty low. Your minimum acceptable level is 44.2.

 

I have some random auto lite plugs in there now, but I just ordered the NGKs.
I'll retest the fuel pressure and give an update on that this weekend as well as the NOID light test, and any scan results if I can get my hands on a good scanner.


Ill check that this weekend as well, this might be the issue, I poped the hood when I went to the gym and pull the harness off injector #6 and while the CEL came on (didn't bother to read the code) idle was no different then with the thing plugged in.

 

On the pre-99's what is considered the best plug? NGK or something else?