PCM? Valves? Sensor? Any other guesses?

[bigtexasthriller]

Hi Everyone --

We have a 2006 Jeep Liberty 4x4.

Check engine light came on last month. Vehicle starts, but doesn't always move at first. Sometimes you need to restart it to go. It does run rough for a bit, but after you get driving, it gets warmed up and seems to run well. I took it to AutoZone and the code read P0300. They printed me a few pages and said the likely cause was a PCM sensor.

We took it next door to a well known, what I would call reputable auto repair chain (not an oil change, brake change, but engine repair). I told them what AutoZone said, and they said they really wanted to do the $99 diagnosis to be sure. After their diagnosis, they said our options were to either replace the engine (which really surprised me) or replace the valves.

I remembered we had a couple of recalls, so I took it to the dealer. They did the recall work and their diagnosis for the other issue was we needed a new battery, spark plugs, and some sort of three step engine cleaning process. We did all that, and still had the same issues.

So we brought it back to the dealer, and they now say the problem is the PCM module. They say we can drive it (I need it this weekend), but say the real issue is the PCM module -- not the sensor. They also say the problem only happens in the first 15-30 seconds after turning the vehicle on. That's similar to our experience, although my wife said it did stall on her while going up a hill.

Any thoughts/experience? We're confused by the different answers from techs.

Not sure if this would be related or would help with a guess -- there's also a whirring sound we've always had when we press the accelerator while the AC is on. We've owned this vehicle for a little over 2 years. Alternator checked out okay when we had it checked a couple years ago. The mileage is at 130K in case that helps.

Thanks everyone,
MJ

 

[ltd02]

What's the service history on this thing? Heck the wrong plugs or bad gas will make it run like crap and throw a P0300. What did the one mechanic say (specifically) is wrong that you need a new engine or heads. Assuming they actually did change the plugs, I'd try a couple bottles of Techron and a good long highway run first. May be injectors or coils but they'd likely throw a specific cylinder code. It is a Chrysler so a bad PCM is always possible but I'd try the Techron first. Really need to get some live data which I would assume the dealer would have done.

BTW, whirring noise could be the AC clutch slipping when AC is on. Might need an adjustment to a smaller shim or no shim.

 

[Billwill]

I doubt it is the valves or PCM.

This P0300 code is a multiple misfire error...either plugs or injectors.
Fit only NGK plugs gapped correctly and try some injector cleaner in the fuel tank.

Download the 2006 Jeep KJ Service Manual here.
The last section in the index is the DTC index where you look up the P0300 code and go to the page shown to trace through several possible issues.
Section 8w is the Wiring Diagrams.

KJ Service Manuals: Index of /manuals/Jeep/KJ

 

[JasonJ]

What's the service history on this thing? Heck the wrong plugs or bad gas will make it run like crap and throw a P0300. What did the one mechanic say (specifically) is wrong that you need a new engine or heads. Assuming they actually did change the plugs, I'd try a couple bottles of Techron and a good long highway run first. May be injectors or coils but they'd likely throw a specific cylinder code. It is a Chrysler so a bad PCM is always possible but I'd try the Techron first. Really need to get some live data which I would assume the dealer would have done.

BTW, whirring noise could be the AC clutch slipping when AC is on. Might need an adjustment to a smaller shim or no shim.

You'd like to think so, but really for $99 the tech is probably getting paid for a flat time of 1.0 hours to do his diag. That's not a lot of time really... it takes 15-20 minutes just to collect the data in the PCM and run through basic checks of things under the hood. Then a road test with the scan tool connected... that diag time doesn't always give the tech enough time to pull plugs either (which usually pays them an hour or more by itself), so he/she isn't always going to go that far on the basic minimum of diag time.

And if it only happens in the first 15-30 mins before it warms up, they may need to diag it and test drive it with the scan tool collecting live data after it's sat for a night to cool off.

A lot of if's and maybe's here.

Basic info is correct though, NGK plugs at the proper gap, no corrosion or water intrusion to the coils present, runs some strong cleaner through the injectors and run the hell out of it on the highway for a bit. Clear codes and see what pops back.

If it were the valves, I'd tend to think that warm or cold would not matter. If a valve spring is weak, or broken, or a lifter collapsed, it is what it is, it doesn't change half an hour later.

 

[Joenavy85]

As off the wall as it may sound, check your wiring harness leading to your coils, you might have an intruder crawling around chewing on things. My buddy had a misfire issue on his wrangler and found some damaged wiring go to the coils. Based on what was around it the only conclusion we came to was that something chewed the wires, since there was nothing immediately around them that could have rubbed against them to wear through the wrap and insulation. Like was said proper plugs are big in this situation, the first time I changed them on mine I had this code pop up. Since it was right after I changed them it was pretty easy to diagnose it.

 

[bigtexasthriller]

Thanks everyone!

Forgot to mention -- the dealer did change the spark plugs when they changed the battery. They almost certainly would be the right plugs....and they did clean the injectors -- your mentioning that reminded me that's one of the three steps in that cleaning process.....

The wiring is also a good guess....the dealer did say that they tested the wiring after guessing it might be the PCM....so they apparently looked into it...

The manual link was super helpful, PCM is on the list, as is ignition coils and a couple other things you all mentioned.

I'll put some Techron in the tank (which I'm guessing filling at Chevron will work or should I buy something at the parts store?)....and then drive it for an extended period of time this weekend....I think your answers confirmed my suspicion that at least the first place we took it was off in their assessment....if driving it doesn't straighten things out, I think we're about ready to trade it in rather than toss more money into what might be the problem....

Thanks again everyone.....if you have any more thoughts, please keep them coming....

 

[bigtexasthriller]

Oh, and to answer your question about what the first mechanic said specifically was wrong -- they weren't too specific other than the valves.....they said the main reason to replace the whole engine would be that it was then under warranty.....they also said, and the dealer confirmed, that we have a small radiator crack....my thought on that is that it must be tiny because there aren't spots in the driveway and we don't seem to be low on coolant.

We're at 130K on miles...the service history seemed pretty good from the carfax if I remember correctly -- and the dealer had just done a few things before we bought it, and then added a couple of extra repairs we asked for after it was inspected....in the two years we've owned it, we've kept up with oil changes, replaced the gas cap, other than now, it's been pretty dependable.....

 

[BillyG]

Check the O2 sensors as well- when one of my upstream O2 sensors went bad it threw the same code.

I also suggest running some Techron or Berryman B12 through it (Gumout makes my Jeep stall) and drive it on a highway for a while.

I bought a Bluetooth OBD2 connector from Amazon and use it with the Torque app. It has a diagnosis setting that runs while you drive and checks several sensors for trouble.

 

[Joenavy85]

the dealer did change the spark plugs when they changed the battery. They almost certainly would be the right plugs

You'd be surprised. I had the dealer do a coolant flush last fall. The first good hard freeze we got froze up my coolant. Temp kept going up. I parked it when it got to about 240. Then went under to hood and unplugged the electric fan. Took about 10 minutes but it finally thawed enough to flow through the radiator. Plugged the fan back in and went to work. I dumped the coolant myself and refilled with the Zerex G-05 and distilled water. Haven't had a problem since. I suspect the tech that filled it up put 50/50 in it and then added water, thinking the coolant he added wasn't pre-mixed. Thankfully it didn't rupture my radiator.

 

[bigtexasthriller]

Ah, thank you!

We had an O2 sensor changed -- that was one of the recalls.....is there more than one?

Also - I probably should have said this earlier -- the dealer said that the codes that came up were "stored codes" when we brought it back in.....that included the p0300, the O2 sensor, and two others that I don't remember....the dealer said those problems should have been solved with the cleaner and new battery and plugs....

 

[bigtexasthriller]

You'd be surprised. I had the dealer do a coolant flush last fall. The first good hard freeze we got froze up my coolant. Temp kept going up. I parked it when it got to about 240. Then went under to hood and unplugged the electric fan. Took about 10 minutes but it finally thawed enough to flow through the radiator. Plugged the fan back in and went to work. I dumped the coolant myself and refilled with the Zerex G-05 and distilled water. Haven't had a problem since. I suspect the tech that filled it up put 50/50 in it and then added water, thinking the coolant he added wasn't pre-mixed. Thankfully it didn't rupture my radiator.

Whoa! Wow....lucky that you knew enough to do that.....I'll double check and ask about the spark plugs....thanks....

 

[bigtexasthriller]

Filled with gas and Techron and drove it quite a bit....doesn't stall anymore....check engine light is still on, but it runs much smoother....