Intermittent misfire on cylinder 3 on ‘02 Liberty

[JCBroman]

The ECU throws a code of P0303 and P0353. To better observe the situation with the coil and the injector, I rigged up LED’s to flash with the firing of the coil and the activation of the injector. I taped these to the outside of the windshield with black duct tape and voila…instant in-car monitors.

When the car is running well, the LEDs for cylinders 1 and 3 and flashing perfectly. But then sometimes the check engine light will come on and an injector LED stops flashing on. Pulling over and restarting the engine clears the problem until it decides to misfire again. The ECU supposedly shuts off any injector of a cylinder that is misfiring to save the cat from damaged due to unburned fuel.

So what is causing the intermittent misfire? Here is what I have done already:
1) Swapped coils between 3 and 5…problem stays with 3
2) Swapped spark plug between 3 and 5…problem stays with 3
3) Installed new injector for cylinder 3. Still misfires.
4) Swapped injector between 3 and 5…problem stays with 3
5) Check engine compression. Excellent and within 10% of each other.
6) Changed the crank sensor, since that is supposedly how the ECU detects a misfire.

Other hints:
1) The problem sometimes happens to cylinder 1, too.
2) I sometimes get a whiff of fuel when this behavior is happening
3) Both times I pulled the fuel rail to replace/swap injectors, the problem disappeared for 100+ miles before it returned. (No I don’t want to make a fuel rail pull a 75 mile maintenance routine!)

I hesitate to take it to the local mechanic or dealer because most just go the route of simply replacing stuff (at my expense) without having a real clue of the root cause.

Ideas of what to look at next?

 

[JasonJ]

Sounds like you've been thorough (right down to your own noid lights). I'd suspect perhaps the intake or exhaust valves are not closing or opening properly. Maybe take a look at the possibility of faulty cam lifters in the affected cylinders?

 

[renegade 04]

It could also be bad knock sensors as well. also when you replace the crankshaft position sensor you need to have a crankshaft position sensor relearn procedure preformed.

 

[flytonehundred]

It could also be bad knock sensors as well. also when you replace the crankshaft position sensor you need to have a crankshaft position sensor relearn procedure preformed.

I have not seen a reset as a requirement with the KJ, and can't find any mention of it in the service manual. Might you be thinking of another model?

 

[MattsJeep]

old thread, but people read these for information

I'm doing valve covers and plugs on 04 Libery with 3.7. 150k miles and it doesn't look like anyone has done this to engine.
Just in removing coil packs and injector plugs for overall job.
The front two cylinders, (don't make me look up the numbers), front of engine, left and right- upon taking out the injector loom plugs-
both were in the same condition:
instead of yellow plastic inside, they were 'baked' kinda orange. They had oil in them? Both disassembled upon removal. The front facia stayed in the injector half of the plug. I removed, cleaned and put them back together. One seems to have stuck in its base and seems fine. One of them is wobbly and doesn't seem to lock in. If everything goes back together and runs (like it did previous to my work) I will get to Jeep dealership and order 2 new ones.
Heat and maybe spilled oil got on/in them?
The wire looms and protection is all fubar on this machine. I'm gonna need a day to wrap and protect them from laying directly on the valve covers.
So, what am I saying?
You checked out a lot of the electronics, but what about the connectors and the plugs?? That could be very intermittent and those things don't last forever. Especially with handling and on/off. I find loom connectors very tricky and find myself twisting turning and pushing pulling and grabbing those little rascals.
So, there you go......