My 98 xj is running rough, pulled a p203 code, it is misfiring on cyl 3, so I checked the plug for the #3 cylinder injector with a noid light and the light stays permanently on instead of rapidly flashing like it should, I was thinking the ecm, any other probabilities? Thanks in advance.

 

Did the light come on after parking the Jeep for about 10-15 minutes and then restarting?

 

No, it started running rough going down the road, got home today and pulled the spark plug on that cyl, completely fouled, replaced plug and wire and checked for spark which was good, but still misfiring on that cyl, plugged in the noid light which should have flashed rapidly and dimly as it did on the other cyl's, instead it stays lit constantly and is extremely bright, this along with the fouled plug tells me that injector is constantly dumping fuel in that cyl, I'm just not sure if there is a switch or sensor that is inline from the ecm to the injectors that could be bad, but even so it would seem weird that it only happens on #3 cyl

 

Later model XJs have a TSB for a heat soak misfire issue. After driving the Jeep and stopping for 10-15 minutes, the heat from the block migrates up through the injectors and boils the fuel in the # 3 injector. Thus causing it to vapor lock that injector and trigger a misfire and injector circuit CEL code on that injector.

 

I did not know that, however....that shouldnt cause the noid light to act differently on that one cyl.....or at least I wouldnt think so

 

I have no idea on the noid light, Check your inbox in a minute or two.

 

Thanks for the link to the tsb, this does not sound like the problem as it describes a momentarily rough idle for 20-30 seconds, mine idles rough constantly, and also surges violently going down the road due to the problem, I do appreciate the info though

 

Unhook your ECM and the injector itself. Locate the wire for that injector to your ECM. Set DVM to resistance. Check said wire for short to ground. . Also, Check for voltage at OTHER wire with ECU plugged in and ignition ON ( NOT RUNNING ).

Injectors typically run on negative trigger. IE, the have a constant voltage going to them, but fire when a ground connection is made in the ECU in the injector driver. INJECTOR DRIVER circuits DO GO BAD. And will often short totally to ground, causing the injector tohang open, DUMPING gas in the cylinder constantly, and setting a code such as you mentioned.

 

Either u have a short to ground on that injector, or the injector driver in ur PCM is shot, which means ull need a new PCM, ECU, etc etc...

Send ur vehicle specs, and I can check on which wire ull be back probing at the comp.

 

Assuming u have a 98 XJ 4.0...

At your ECU/PCM U should have three connectors. A-black B-White C-grey. You will want to unhook ALL. The connector you need mostly is B, the WHITE one. Here, look for pin 5, which should be a yellow/white wire. This is the #3 injector driver.

With the injector UNPLUGGED...see if you have a short to ground by turning ur dvm to resistance measuring...and placing one lead on this wire, and the other to a known good ground...if u have resistance, you have a short in the wire somewhere.

Next, check resistance between this pin, and the yellow/white wire at injector 3. Good resistace HERE, is a good thing.

AGAIN, what ur looking for is NO RESISTANCE at this yellow/white wire with the ECU unplugged, and injector unplugged.

On a side note, the other wire at ur injector should be a "DG/OR" wire...which I'm assuming is dark green/orange...and with the key on, should show voltage.

Good luck.

 

Assuming u have a 98 XJ 4.0...

At your ECU/PCM U should have three connectors. A-black B-White C-grey. You will want to unhook ALL. The connector you need mostly is B, the WHITE one. Here, look for pin 5, which should be a yellow/white wire. This is the #3 injector driver.

With the injector UNPLUGGED...see if you have a short to ground by turning ur dvm to resistance measuring...and placing one lead on this wire, and the other to a known good ground...if u have resistance, you have a short in the wire somewhere.

Next, check resistance between this pin, and the yellow/white wire at injector 3. Good resistace HERE, is a good thing.

AGAIN, what ur looking for is NO RESISTANCE at this yellow/white wire with the ECU unplugged, and injector unplugged.

On a side note, the other wire at ur injector should be a "DG/OR" wire...which I'm assuming is dark green/orange...and with the key on, should show voltage.

Good luck.

 

Problem solved, short in the harness, thanks for the info on which wires to check and their color codes, this REALLY helped

 

you beat me to it...i was gonna say a short...was it in the engine harness some where around the valve cover?

 

Np, glad I could help!

 

Hey, I am having this exact problem on cylinder 1... I'm curious if you have any info on which wires or colors I should check for cylinder 1?? Same Jeep 98 XJ 4.0...