Rough Idle / WARM engine WJ 4.0

KaiTTown · 01-25-2010, 06:20 PM

Hello,

today I took my 2004 Grand Cherokee to the local dealership to diagnose the following problem:

My Jeep stumbles / shakes when it comes to a stop (i.e. red light)

I know, there are about a million threads on this problem. But unlike all the threads I read on this forum so far, this happens ONLY when the engine has warmed up.

The dealership told me, my PCM "is on it's way out" and I need a new one for $650. And this does not guarantee that this is the cause of the problem. It will help them identify what causes the stumbling.
Now my question: Is this normal????? I am supposed to pay all that money, just so the dealership can tell me what they need to repair???

After reading all kinds of PCM related threads on this forum, I am really starting to doubt that the PCM is the problem.
The vehicle runs fine, everything works (i.e. intrument clusters, cruise control, radio, etc....)
My RPM is always steady at around 650-700 RPM in idle, EVEN when the car stumbles.

Does anyone have any ideas what could be the problem? How should I proceed, as I feel the dealership is trying to compensate for their low new-car sales, by selling existing customers uneccesary parts?

Thanks a lot,

Kai

yasika116 · 01-25-2010, 06:29 PM

what did they tell you your engine codes are or do you even have a check engine light at the moment?

KaiTTown · 01-25-2010, 07:43 PM

the malfunction light was on at times. It went off a day before I took it to the dealership.

No problem codes were told. Could it be that the PCM was already unreadable to the scanner?

steveheibel · 01-25-2010, 07:56 PM

Unplug your oxygen sensors for a couple days and see if it still does it. sounds funny, but I have been chasing this same issue for a long time and just figured it out. I had mine to my mechanic, and the dealership, I replaced my tps, aic, map, battery, and spark plugs. Nothing worked, and I spent allot of money for nothing. This is a free test and I am willing to bet it works for you too. I will never go to the dealer again. $100.00 for nothing is BS

JoshuaDHarp · 01-25-2010, 08:12 PM

Vacuum problem? I was reading a thread yesterday about the brake system sucking all your vacuum and starving the engine. All your lines tight?

yasika116 · 01-25-2010, 10:38 PM

possible but if u were to put it in park and let off the brake it should pick ur idle up. and did they check ur fuel pressure? has it been raining with this has been happening? find out what they have ruled out before replacing the pcm its possible and happens but everything should be ruled out first. a cam sensor or crank sensor can fail or give weak signal when warm

yasika116 · 01-25-2010, 10:40 PM

and remember every problem is different just bc a shop or dealership had problems with one diagnosis doesnt mean they dont know anything i find it funny that dr. practice and make mistake and its well they cant know everything but if a mechanic cant figure somthing out its the end of the world at least dr.s patients talk lol

yasika116 · 01-25-2010, 10:42 PM

and also with a vacuum leak you should hear an unsual whistle or hissing noise

SnowLeopard · 01-25-2010, 10:54 PM

Sometimes finding a vacuum leak isn't so simple. Tracing it can be a pain.

My 4.0 idles at around 500rpm when warm and in park, under that when stopped at a light or something in drive. I am not quite sure why that is. There's a slight vibration when under load in drive like that, at a stop, but I really don't know what the cause is. One more thing to track down eventually. I think it's hard to figure out what the status quo for these things is.

yasika116 · 01-26-2010, 09:00 PM

use brake clean and spray lines and around the intake manifold if the engine bogs that means it sucked the brake clean in through a vacuum leak and that gives you an area to focus on but what ur explain can be anything from tps, fuel pump, filter, or in need of a tune up dont forget cap and rotor

^^ snow leopard (figured i should make clear who im speaking to)

Hexonut · 01-26-2010, 09:08 PM

coolant temp sensor for the computer... or possibly an IAT since computer is quite dependent on these two sensors for fuel trim ect...

just a guess since you have no codes present or stored in memory.

ZeeJay1997 · 01-26-2010, 09:29 PM

I wouldn't come out of the gate with PCM.

Try different things like putting it in neutral as you are stopping and turning off anything loading the alternator and see if the problem gets any better. Check all your gauges while it's happening and watch for drops.

These engines are very sensitive to the charging system. Be sure all main cable connections are cleaned on both ends. It may be a matter of methodically cleaning all fuses and connections.

Steves suggestion of disconnecting the O2 sensors is a good one. It will make it throw a code, but will go away after reconnecting them or disconnecting the battery. Just be sure to zip tie your connectors so the harness doesnt get toasted by the exhaust or wrapped up in the drive train.

KaiTTown · 02-09-2010, 02:02 PM

Hi all,
thanks for the plenty feedback.
I finally retrieved the error codes using the key method.
I get P0456 and P0601
Both of these do point to PCM issue. I did disconnect the o2 sensors and it helped, but how do I know it's the sensors and not the pcm misinterpreting the signal from the sensor?

On top of that I now have a grinding / scratching sound at idle, but it's the loudest at 1500 rpm.
Any ideas?

KaiTTown · 02-09-2010, 02:03 PM

Hi all,
thanks for the plenty feedback.
I finally retrieved the error codes using the key method.
I get P0456 and P0601
Both of these do point to PCM issue. I did disconnect the o2 sensors and it helped, but how do I know it's the sensors and not the pcm misinterpreting the signal from the sensor?

On top of that I now have a grinding / scratching sound at idle, but it's the loudest at 1500 rpm.
Any ideas?

4Slow9erHybrid · 02-09-2010, 02:56 PM

Quote:

Originally Posted by KaiTTown

Hi all,
thanks for the plenty feedback.
I finally retrieved the error codes using the key method.
I get P0456 and P0601
Both of these do point to PCM issue. I did disconnect the o2 sensors and it helped, but how do I know it's the sensors and not the pcm misinterpreting the signal from the sensor?

On top of that I now have a grinding / scratching sound at idle, but it's the loudest at 1500 rpm.
Any ideas?

How many miles on those O2 sensors? If it's 50k+ miles, they are due anyway for replacement.