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 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 .
@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.
@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.
im working on a jeep liberty with the same issue. jeep came in with a miss complaint and code p0204 after a test drive i realized it wasnt a miss it was a cutout issue like you were hitting the rev limiter at 2000 rpm . long story short i disconnected the alternator wiring and the cut out went away. so i sold them a alternator. this next part is strange. so i got the new alt and pulled the jeep in and it actually started miss firing so i checked the scan data and it had codes p0202 and p0204 so i checked ground signal from ecm and it wasnt sending one to #2 and #4 injectors. so i sold them a pcm. after pcm was replaced code p0202 went away and so did the miss fire . p0204 and cutout issue still remained so i installed a new alternator and the cutout issue was fixed… so i thought about 5 minutes of running the cut out came back. so i installed a different alternator problem accured immediatly on start up. but if you disconnect the alt the problem goes away. still have code p0204 also. i did read that these jeeps are supposed to have so kind of noise capacitor on them to elliminate rf noise but i cant find it on this jeep
With P0204 fault there would likely be both a misfire and a rev limiter. The PCM will engage limp mode/2000 rev limit with the injector fault.
With the alternator disconnected, the rev limiter does not engage. Perhaps the engineers thought low battery voltage is a bigger concern than the injector fault. The rev limiter isn’t the problem, correct the injector circuit problem.
The injector driver inside the PCM is the only thing left, PCM failure seems to be common for the Liberty.
Does a noid-light test show injector number 4 is being pulsed? Any difference in noid-light testing 1-3 vs 4 ? I’m guess the alternator relationship is a red-herring. The car may be just on the edge of an electronic noise issue, and for some reason shows up first by affecting injector 4’s circuit or ground. Electronic noise problems could be caused by a faulty noise suppression capacitor, but for this vintage, more likely it is a grounding problem. Make sure all the grounds are solidly connected to their specified grounding location with clean, non-corroded connections, usually to the chassis, especially check battery negative, alternator negative, engine, cylinder head, & sensor grounds. My Corolla has a ground connection from the cylinder head to the chassis, which has a connector in the wire. Convenient if that wire is in the way of something, easily temporarily disconnected. But it is easy to forget to reconnect.
Nevada_545 you are correct although i couldnt feel a miss p0204 would keep coming back. replaced #4 injector and all is well. that alternater thing was really trowing me off Thanks for the help
I can see why it was perplexing. Faulty fuel injectors are a pretty uncommon report here. You are welcome to look through the forum search file to check for yourself. Link upper right this page.
My driveway diy’er basic battery/alternator test
Before the first start of the day the battery should measure about 12.6 volts, then immediately after startingthe engine 13.5-15.5 volts. If that test passes, any remaining alternator problem would likely require an o’scope test to check for excessive voltage ripple in the alternator’s output. It’s also important the battery has a low resistance connection to the alternator’s output for the same reason, b/c the battery in the circuit very much helps reduce voltage ripple. And that both the alternator and the battery have low resistance connections to the chassis. I expect the filter capacitor is mainly to reduce radio interference. Does the AM radio seem to have an unusual amount of interference?
BTW, the faulty fuel injector might be a clue to needing a replacement fuel filter.
I’ll check that . Thanks for the heads up
Thanks for the help