2005 3.7 Jeep Liberty # 4 injector problem

I have a 2005 3.7 Liberty & I have a code for # 4 injector . I have definitely narrowed the problem down to no ground signal from the pcm for that injector . I traced the wire from the injector to the pcm connector , bared a small spot in the wire just outside the pcm connector & there is no ground signal coming out of the pcm on that wire .
Does this definitely condemn the pcm or could something else cause the pcm to not send out a signal for one injector only ? I suppose it’s possible , but I think highly unlikely that the problem could be in the pcm connector itself . I have had it unplugged & the connector & pin both look ok .

I have also been on Jeep forums & I don’t think I’ve already asked this question here but if I have forgive me please .

I’m going to throw a wild suggestion out and accept the corrections of those who may know better.

Perhaps the crank position sensor is not telling the pcm that it’s time to fire that injector? Faulty CPS possible?

I know the { I think cam position sensor } signals the pcm to fire the injectors . Whichever is correct I don’t know if a faulty sensor would skip just one injector .

I stand corrected.
I’ve never heard of one doing so, but I thought I’d throw it in anyway.

The driver in the PCM for #4 injector has failed.

The injectors have constant voltage supplied to them. The PCM uses the drivers to ground each injector at the proper time to fire them.

Looks like a trip to auto recycler for a used replacement PCM.


@Tester , That’s what I’ve also been thinking , just trying to see if someone that knows more about it than I do could suggest something different before I buy a pcm .

Before doing that – and that may be the end solution – since you are removing the PCM anyway, once it is out and on your work bench take a look at the trace on the PCM circuit board that corresponds to that connector slot. Use a magnifying glass and good lighting. Notice any signs of the trace being cracked? Any indications of a broken solder joint on anything that trace is soldered to? Check both sides of the circuit board. If in doubt at all, re-solder all those joints and give it go. Then double check with an ohm meter you have a good low ohm connection (less than 0.05 ohm) along that entire trace. If you are feeling super-experimental, you could solder a test wire to that trace, then plug the PCM back into its connector, and use the ohm meter to verify you have a low ohm connection all the way from the PCM to the injector control signal. Who knows, might work.

Another idea to consider if the above doesn’t work, if it uses discrete power transistors as the driver, if you have a neighbor that is electronics knowledgeable, ask them to test the driver transistor. They might find it is kaput, and know how to replace it with a good one.

What specific symptom and fault codes do you have?

Have you cleared all the fault codes and started over, looking for injector signal at the start? If the PCM detects a misfire for a cylinder–for any reason–the PCM will disable injector operation for that cylinder to save the catalyst from ingesting unburned fuel.

@asemaster , Your post is something I hadn’t considered . I had a code for # 4 injector . I’m in the process of learning so instead of checking the wiring to the injector first like I should have , I replaced the injector which didn’t help . I also had codes on a couple other cylinders that had to do with coils or plugs . I installed new spark plugs & those codes went away .
Cylinder # 4 now has a different injector & a new spark plug , perhaps I should swap # 4 coil with another cylinder , clear the codes & see what happens . There was no coil or plug code for # 4 though .
I have verified power on one wire to the injector , verified continuity on the ground wire from the injector to the PCM & used a noid light on # 4 & no flash , used the noid light on other injectors & flashed .
I’ll clear codes , swap coils & see what happens . I’ll then post any codes I have .

By the way , from the looks of the spark plugs I replaced I’m reasonably sure they were original .

I had stated previously that with the alternator wires connected & the serpentine belt on , the engine wouldn’t rev above about 2000 rpm’s . With the belt on & the alternator wires disconnected it rev’s fine & with the belt off & alternator wires connected it rev’s fine . Obviously something about the alternator spinning causes problems . I have also tried another alternator with the same results . I still have that problem .
I swapped # 4 & # 2 coils . I cleared the codes . With the alternator wires connected & the belt off I get no codes but disconnecting # 4 coil makes no difference in how the engine runs . With the belt on the P0204 code comes back . Even though there is a constant problem with # 4 cylinder I get no codes unless the belt is on .
I have swapped injectors & coils & installed new spark plugs . I have verified proper voltage to the injector , I have verified continuity on the ground wire from the injector to the PCM . Why the alternator spinning or not spinning makes such a difference I have no idea . I found a thread on another forum that another person was having the same problem but his problem didn’t get solved in the thread . I emailed him several days ago & have received no response .

Have you verified a good healthy spark at # 4’s spark plug? If not, might be worth the time to do the experiment. If you don’t know how, ask, someone here does.

The only proper way is to use a spark tester, available at any parts store for just a few bucks

Anything less than a very bright blue spark is unacceptable

@Sloepoke, one test you can do to check the PCM circuit for injector 4 is check the resistance at that pin with the connector removed to the PCM. Using a digital meter place the common probe of the meter on the ground pin of the PCM and the other meter probe on the faulty injector pin of the PCM. Check for a high resistance, you might also use the diode check mode also. Then compare the readings to a working injector pin and see what kind of difference you have. If there is a significant difference you can pretty safely say the trouble is within the PCM. I think you will need to either have yours fixed or replaced. There are places you can send yours in for repair.