Ok... XJ 4.0 L with a P0303. Very rough idle and misfire noticeable during acceleration. Definitely something is not right.

Initially we replaced the coil pack and plugs with a new coil pack and new Champion plugs. We emptied the gas tank and filled it with fresh fuel and added a little bit of fuel system cleaner. We measured the fuel rail pressure and all was acceptable. Started and drove the XJ, but there was no improvement. CEL came on and P0303 returned.

Removed the connector off of the #3 injector, swapped injectors around, P0303 is still the problem. Installed a new injector just for fun. Still same problem. Still same code continues to reappear... P0303

Used an LED to "look at" the injector pulses. Cylinder 1, 2, etc looked good, nice pronounced on/off pulsing of the LED. But the injector #3 output was near solid LED light with a mild flicker... definitely not a pronounced on/off pattern. #3 injector did not appear to pulse, but instead stayed illuminated and flickered quickly and lightly.

I believe the injectors have a constant 12V to one side and the computer turns on/off a ground reference (trigger) on the other side of the injector connector. All injectors have the 12V signal at the plug. We took a look at the PCM connector end (pin 5 of the middle plug in this case specifically) and didn't see any issues. With a digital volt meter there was no indication that the trigger or ground pulse wire was shorted to ground. The wire was continuous from the PCM connector to the injector.

So it appears the #3 injector is not getting pulsed on and off like the others. We had no way to see what is specifically coming out of the PCM in regard to signal aside from what is going through the harness.

Has anyone seen this specific issue?

What do you think it is?

PCM?

It appears the #3 injector pulse wire from the PCM is intact and functioning...

Thoughts?

Any help appreciated... trust me, this is killing me.

 

Sounds like the PCM driver for #3 injector is faulty, but that probably should be proved before you just replace the PCM. That is, of course, unless you can find a known good PCM to try. I haven't seen anywhere an internal circuit diagram for the PCM so I can't even tell you which chip needs replacing or maybe there's a crack in the conductor on the circuit board. Since they've started using lead free solder, there was a time when the new solder would "whisker" and short a circuit. In today's "throw-away" mindset, nobody repairs anything anymore. If you actually open up a PCM, you'll find it's "potted", meaning it's covered by epoxy that has to be removed before you can even replace a capacitor.

Sorry, rant over.

 

I was able to replace a dead injector driver in my '95 XJ PCM, see https://www.cherokeeforum.com/f2/how...cm-95-a-248325. I don't know how similar a '95 PCM is to a 2000 PCM though. I would suggest hooking up a voltmeter to the positive battery terminal and measuring the voltage to the driver pin. If it is 0 V then there is no pulse from the injector. If it is a constant 12 V then the injector driver is shorted out and the injector is stuck open, which was my case.