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#2693656 - 07/24/12 09:15 AM Jeep 4.0 misfiring and running rough
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
2000 Jeep Cherokee 4.0L, Alabama cylinder head, 124k miles.
-New plugs
-Filled up at different stations with good gas
-Gets a consistent 17 mpg
-Runs at 195-210* all day long

My Jeep has had this issue a few times on a COLD start, usually after sitting overnight. The Jeep will start right up but will idle very rough. Pressing the gas does not help at all. When I revved it up, it triggered the check engine light. I get the same P0303 code each time (misfire cylinder #3).

The first time it happened, I replaced the spark plugs with the proper NGK ZFR5N plugs. It helped a little at first but the misfire was still apparent.

I released the pressure in the fuel rail using the little valve on the side. It fired right up and ran well. I even drove it to the shop with no issues.

My buddy owns the shop and he looked it over really well. It drove fine for them. They checked the coil pack, the connectors for the coil pack wiring, used dielectric grease on the connectors and verified I was getting spark.

They also tested the fuel pressure, and it was within specs. That was last week, and it has been driving fine until today where it started having the same problem.

What baffles me is how releasing the pressure in the fuel rail would cause it to run better after it had been misfiring.

Could it be the #3 injector?

This year 4.0L engine is notorious for the heat soak misfire. What happens is the cat converters under the manifold heat up the engine bay and intake manifold, specifically the #3 injector, which causes a misfire on a hot start. There is a TSB where the dealership puts a heat shield over the #3 injector, and reprograms the ECU to make the electric fan turn on everytime the jeep is started hot. I had this problem when I first bought the Jeep. I had the TSB performed and the issue has not come back over the 4 years I've owned the Jeep. Could this previous heat soaking issue cause the injector to fail sooner?

When it misfires it is from a cold start NOT a hot start. Once it is running ok it continues to do so even when at operating temp.

I'd just like some ideas or advice before I start swapping parts when I get home. Thanks thumbsup
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2693701 - 07/24/12 10:21 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
440Magnum Offline


Registered: 02/01/09
Posts: 6390
Loc: Texas
Boy that behavior sounds EXACTLY like the heat-soak misfire, right down to the throttle not having any apparent effect other than to make the code set quicker. Been there, done that, and insulating the fuel rail hasn't fixed my 2001 completely- but it only does it in the WINTER when we have crummy refinery-waste for fuel (aka "winter" blend high vapor-pressure gasoline). Its amazing to me that my '99 doesn't do this AT ALL even with no insulation. What a difference a true tubular header and single under-chassis catcon (the 99) makes compared to a cast-iron manifold with twin light-off catcons (2000 and later).


If I had to SWAG, I'd say its one of 3 things.
1) you may be correct in thinking that the heat has abused the #3 injector to the point that its starting to fail.

2) Its actually just an odd manifestation of the heat-soak issue where the fuel in the rail and #3 injector does vaporize after shutdown, and for some reason just isn't clearing out correctly when you start the car cold the next time

3) The fuel pressure is not coming up to spec enough to prevent #2 above, in which case the fuel pump would be suspect.

I'd get a fuel pressure gauge on it ASAP and see what's going on. I'm betting its #3 causing the behavior described in #2. Get ready to drop that fuel tank... thank God its easy on Cherokees.
_________________________
'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 Cherokee 4.0, '11 Grand Cherokee 3.6

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#2693703 - 07/24/12 10:22 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
Lead Shoes Offline


Registered: 03/03/10
Posts: 2390
Loc: Mn
Start with the simple, have you ran any injector cleaner with a high amount of PEA in it to see if its just an injector flow issue?
_________________________
1992 BMW 325i M50b25 w/Dinan I/E/Chip 161k
M1 0w40/Mahle

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#2693727 - 07/24/12 10:40 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
I've ran a few cleaners through it while I've had it. Marvel Mystery, Techron, and BG44K. I might even run fuel cleaners a little too often. I use MMO about once a month.

I had the fuel pressure tested at the shop because I also had a problem with long crank times when it was cold. I thought it was the classic fuel check valve failure that is common on 1997-2001 Cherokees. The problem mysteriously disappeared and then about a week later this misfire thing started happening. The fuel pressure tested within specs according to my buddy who owns the shop.

Also, when it is running rough I can shut it down, and releasing the fuel pressure with the valve will cause a small squirt of fuel to come out. So it does seem to have fuel pressure when it is misfiring like that. Once I release the fuel pressure I can start it and it will run smooth.
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2693741 - 07/24/12 10:47 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
440Magnum Offline


Registered: 02/01/09
Posts: 6390
Loc: Texas
Something is vapor locking the injectors. I still suspect something like low fuel pressure at start-up, or maybe the check valve IS bad and its letting the pressure drop while the engine is still warm and that's allowing vapor to block the fuel rail. Burping it clears the vapor, and off you go.

I'd put a gauge on it WHILE its cold. Note that most diagnostic fuel pressure gauges have a pressure release button and fuel dump tube on them so you can crank and watch the pressure while it runs rough, and then "burp" it without disconnecting the fuel gauge. If you're careful (run the gauge's fuel dump tube into a portable gas can) you can press the burp button while its running and see if it smooths out without shutting down and re-starting. On my gauge, the tube is clear so you can even see if there's vapor coming out with the fuel.
_________________________
'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 Cherokee 4.0, '11 Grand Cherokee 3.6

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#2693863 - 07/24/12 01:01 PM Re: Jeep 4.0 misfiring and running rough [Re: 440Magnum]
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
Originally Posted By: 440Magnum
Something is vapor locking the injectors. I still suspect something like low fuel pressure at start-up, or maybe the check valve IS bad and its letting the pressure drop while the engine is still warm and that's allowing vapor to block the fuel rail. Burping it clears the vapor, and off you go.

I'd put a gauge on it WHILE its cold. Note that most diagnostic fuel pressure gauges have a pressure release button and fuel dump tube on them so you can crank and watch the pressure while it runs rough, and then "burp" it without disconnecting the fuel gauge. If you're careful (run the gauge's fuel dump tube into a portable gas can) you can press the burp button while its running and see if it smooths out without shutting down and re-starting. On my gauge, the tube is clear so you can even see if there's vapor coming out with the fuel.



Is that something I could rent at the local autoparts store? That is the only reason I took it to the shop, since I don't have many of the diagnostic equipment they have.
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2693954 - 07/24/12 02:36 PM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
MNgopher Offline


Registered: 09/17/02
Posts: 2999
Loc: Lakeville, MN
Any auto parts store that loans tools will likely be able to loan a fuel pressure guage. Not a terribly expensive tool to pickup either.

If your pressure is bleeding down after shutoff beyond spec, either the check valve is bad, there is an injector leak, or fuel is leaving somewhere else (not sure on a 2000, but on my old '88 that somewhere was usually the fuel pressure regulator bleeding fuel into the vacuum hoses). The lack of pressure would certainly add to the potential for vapor lock.

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#2694454 - 07/24/12 11:22 PM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
Well I got home from work and decided to mess with it. It started and ran fine after sitting all day. I then decided to swap the injectors around to see if that isolated the issue to an injector if it was to happen again. I swapped the #2 and #3 injectors. Now it misfires and shakes every time I start it. I get the same P0303 code for a cylinder #3 misfire.

I unbolted the coil rail to check the boots that go over the spark plugs. They looked fine, so I bolted it back on. I then started the Jeep and it idled fine for a few seconds. When I pressed the gas it began to run rough again. Does this sound like a coil problem? I'm not sure if it ran normal for a few seconds because I messed with the coil packs or because it sat for a few minutes. I even released the pressure in the fuel rail again and it seemed to have no effect.

I even jiggled the wires around for both the injectors and the coil packs while it was running rough and nothing changed.


Edited by jeepman3071 (07/24/12 11:24 PM)
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2694725 - 07/25/12 10:39 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
SteveSRT8 Online   content


Registered: 10/10/08
Posts: 15279
Loc: Sunny Florida
I read this thread and with all due respect I do NOT see an injector or fuel issue.

This is a coil, plug, wire, crank trigger, cam sensor kind of thing.

Diagnosing it will be the hardest part. Swap coils around first.
_________________________
"In a democracy, dissent is an act of faith."
J. William Fulbright
Best ET-12.79 @ 111 mph
4340 pounds, Street tires
Just like we go to Publix

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#2694747 - 07/25/12 11:10 AM Re: Jeep 4.0 misfiring and running rough [Re: SteveSRT8]
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
Originally Posted By: SteveSRT8
I read this thread and with all due respect I do NOT see an injector or fuel issue.

This is a coil, plug, wire, crank trigger, cam sensor kind of thing.

Diagnosing it will be the hardest part. Swap coils around first.


I was afraid of that. The fact that it had the issue, then ran fine for two weeks, then had the issue again screams electrical. My buddy has the same year jeep as me so today after work I'm going to swap coil rails with him. The problem with this year 4.0 is it has the one piece coil rail instead of plug wires. It makes diagnosing misfires difficult.

Edit: I should also state that the cam position sensor was replaced 2 years ago by the dealership.


Edited by jeepman3071 (07/25/12 11:24 AM)
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2694828 - 07/25/12 01:05 PM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
SteveSRT8 Online   content


Registered: 10/10/08
Posts: 15279
Loc: Sunny Florida
I'd always suspect the replaced part first. But that's just me.

I have no familiarity with your platform but I think you are now barking up the RIGHT tree!
_________________________
"In a democracy, dissent is an act of faith."
J. William Fulbright
Best ET-12.79 @ 111 mph
4340 pounds, Street tires
Just like we go to Publix

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#2695867 - 07/26/12 10:38 AM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
jeepman3071 Offline


Registered: 04/15/10
Posts: 1803
Loc: Storrs, Connecticut
After swapping numerous parts and sensors with my buddy's 2000 Cherokee, I had to take it to a shop. I brought it to the local mom and pop shop. They do great work and I know the owners. They said I had a broken #3 intake valve spring. He is going to text me a picture of it today.

Apparently when they ran it they could hear it popping through the intake when it was misfiring. When I had swapped the parts and listened to it running, I had the air filter hose connected to the throttle body each time so I didn't notice the popping through the intake.

I trust these guys, and a few of them own jeeps. Does this sound on track?

When I drove it to the shop it ran perfect the whole time, but hours before it would only run rough in my driveway. I'm thinking maybe the spring still had enough pressure to close the valve but every once in awhile it would show itself and run badly.
_________________________
2000 Jeep Cherokee Sport 4.0L (148k) - 5 qts PYB 5w30 + 1 qt PP 5w30, Napa Silver 31085, Magnefine trans filter

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#2696052 - 07/26/12 12:52 PM Re: Jeep 4.0 misfiring and running rough [Re: jeepman3071]
SteveSRT8 Online   content


Registered: 10/10/08
Posts: 15279
Loc: Sunny Florida
OUTSTANDING! Yes, a broken spring or anything that could keep that intake valve from closing is a very good diagnosis.

Backfire is the key word!
_________________________
"In a democracy, dissent is an act of faith."
J. William Fulbright
Best ET-12.79 @ 111 mph
4340 pounds, Street tires
Just like we go to Publix

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