2000 XJ 4.0 COLD misfire

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 when 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 from work. Thanks

Sounds like a leaking #3 injector, try swapping the injectors around and see if the issue follows the injector.

sidewaysstarion said:

Sounds like a leaking #3 injector, try swapping the injectors around and see if the issue follows the injector.

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Interesting you mention a leaking injector. I had an issue a month or so back where it would sometimes take longer to crank on a cold start. I thought it was the classic '97+ fuel check valve issue since it displayed the exact same symptoms. Right when I was about to order a new fuel pump assembly the issue went away, and a week later this misfire started.

Could the leaking injector cause the extended crank time also?

I'm wondering if releasing the fuel pressure makes the injector work for a little while longer until it starts leaking again. My jeep ran flawless yesterday, and today its misfiring. Same thing happened a week ago. It ran flawless one day and the next it was misfiring.

Also, say the #3 injector was the issue. Could I replace only one injector or do they all need to be replaced to have the same flow?

Update:

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.

You mention that you have an Alabama cylinder head. Assume that you had a cracked 0331 that brought you to that?

If that is the case, it's not impossible that it damaged that cylinder as the crack on the 0331 almost always happens between cylinder 3-4, where the casting defect is. I'd start with a compression test of all cylinders and see where cylinder 3 fits in comparison to the others.

birchlakeXJ said:

You mention that you have an Alabama cylinder head. Assume that you had a cracked 0331 that brought you to that?

If that is the case, it's not impossible that it damaged that cylinder as the crack on the 0331 almost always happens between cylinder 3-4, where the casting defect is. I'd start with a compression test of all cylinders and see where cylinder 3 fits in comparison to the others.

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My head never cracked. I had the time and money, so I put on an Alabama head as a preventative measure since I am often hours from home. It has been on there for 3 years and has run great since.

My buddy also has a 2000 XJ, so I'm going to swap his coil pack with mine tonight to see if that changes anything.

After swapping numerous parts and sensors with my buddy's 2000 XJ, 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.

I feel your pain! Just spent the last 30 days going through similar misfire issues. I did the same thing in changing out a shopping list of parts with no improvment. Finally had a leakdown test performed and found a bad head gasket leak between cylinders 2 & 3. Pulled the head off to change the gasket and have the head and valves done. When the head was checked, it proved to be cracked. Now with the head replaced, no more mistery misfire or check engine lights! Thank God! If you can do a leakdown test yourself great! If not, it won't cost(1 hour charge) that much to have done and will eliminate a lot of guesswork. Maybe save some money too. Best of luck! Vaya con Dios.

jeepkid03 said:

.... had a broken #3 intake valve spring.

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I read another thread about misfires, that finally proved to be a broken valve spring.

Tim_MN said:

I read another thread about misfires, that finally proved to be a broken valve spring.

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Yup, mine was a broken #3 intake valve spring. Not something I would have suspected. The head is an Alabama head, and according to my buddy who runs the shop, he called Alabama and they get their springs from Chrysler.