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
-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