Rough Idle 2000 Grand Cherokee

[jdbenja]

I have a 2000 Jeep Grand Cherokee 4.0L with 54,000 miles. I'm having a problem with my Jeep's Idle. It has a very rough idle when in gear, but when in neutral or park it is smooth. I had a tune-up and fuel system cleaning about 8,000 miles ago and that never helped with the problem. About 1,000 miles ago, I had it at a dealership for them to fix my door locks under warranty and asked them about the idle problem. They said that they just idle rough. I'm not buying that because I know it did not idle like that new. I'm open to suggestions. Would a new fuel filter help? I have a Factory Service Manual (1000+ page). If anyone can help, please let me know. Thanks

[oljeeptek]

Originally Posted by jdbenja

I have a 2000 Jeep Grand Cherokee 4.0L with 54,000 miles. I'm having a problem with my Jeep's Idle. It has a very rough idle when in gear, but when in neutral or park it is smooth. I had a tune-up and fuel system cleaning about 8,000 miles ago and that never helped with the problem. About 1,000 miles ago, I had it at a dealership for them to fix my door locks under warranty and asked them about the idle problem. They said that they just idle rough. I'm not buying that because I know it did not idle like that new. I'm open to suggestions. Would a new fuel filter help? I have a Factory Service Manual (1000+ page). If anyone can help, please let me know. Thanks

Fuel filter will do no good for a rough idle. Is it actually misfireing, or is it a vibration your feeling? These should idle nice, not rough.

See if there is any carbon deposits inside the throttle bore and clean it out with carb spray and a rag.

Have someone hold it in gear and give it a little spray in the throttle to see if its lean.

Slightly rough idle can also be vacuum leaks, worn plugs, wrong plugs, shifted o2 sensor not reading correctly and causing incorret fuel mixture, among others.

Vibration at idle can be a mount contacting brackets, hoses/lines resonating, too low idle speed from sticking iac valve etc.

[jdbenja]

Thanks for the reply. The idle is definitely not a misfiring. What do you mean by "see if its lean"? I'm by no means a mechanic nor a person who knows much about vehicles, but I'll check all those out. Thanks

Originally Posted by oljeeptek

Fuel filter will do no good for a rough idle. Is it actually misfireing, or is it a vibration your feeling? These should idle nice, not rough.

See if there is any carbon deposits inside the throttle bore and clean it out with carb spray and a rag.

Have someone hold it in gear and give it a little spray in the throttle to see if its lean.

Slightly rough idle can also be vacuum leaks, worn plugs, wrong plugs, shifted o2 sensor not reading correctly and causing incorret fuel mixture, among others.

Vibration at idle can be a mount contacting brackets, hoses/lines resonating, too low idle speed from sticking iac valve etc.

[Bob D.]

I can tell you with almost 100% certainty that nothing is wrong with your motor.
You had a - "tune-up and fuel system cleaning about 8,000 miles ago" - and you said it's not misfiring right?
Unless the IAC - Idle air control motor, is bad, everything is ok.
Here is an easy way to check it; first check the idle speed, it should be between 650 rpm and 750 rpm.
Turn on the AC. The RPMs should hold steady and not fall...a good IAC compensates for this.

The fact that it's ok when in neutral or park and is rough or "vibrates" in drive is two fold; What the dealership told you is partially true...because of modern emission standards and gas mileage considerations, the idle on late model vehicles has been leaned out considerably and the 4.0 L is not the most smooth running motor to begin with. Add a load to the lean running motor at idle as in putting it in gear and the vibration is exacerbated - the motor appears to run rough. Try adding a little throttle while in gear with your foot on the brake and the idle should smooth out.

What are we getting at here, and how do you fix it?

At about 50k miles the motor mounts crack and or harden, the only remedy is to replace both motor mounts and the transmission mount - the bad motor mount/s are hard on the trans mounts so you might as well change them at the same time.
ONLY USE FACTORY OEM MOUNTS. Not cheap but after market mounts will not help and may make it worse.
I can tell you this because of my experiences with the 4.0L motor going back to 1989- they all get rough eventually.

Dealerships don't like to address this because they don't want customers who just spent hundreds of dollars coming back with subjective opinions of how smooth a 4.0L should run.

Having said that, buyer beware - I know of someone who's 2004 had a idle vibration problem and after the dealership replaced the mounts the vibration was still present. The dealership basically told him to go pound sand. (ie it's normal)

Most people just live with it, and if it's any consolation, once they go, they usually don't get any worse.

[oljeeptek]

Bob's right on track with his comments.

What I meant about see if it's lean; if you add a little fuel (Carb spray, starter fliud etc.) at the condition and it smooths out, you could be running a little too lean at idle. A vac leak should be compensated for by the computer system with three exceptions coming to mind-
1. An o2 sensor that is contaminated and out of range. Simply put it is lying to the computer and affecting mixture.
2. Iac issues mentioned above. The pcm thinks the iac is responding to its command to raise idle by giving more air to the engine and adds the corresponding extra fuel, thereby actually making it run too lean.
3. egr valve (if equipped) leaking exhaust through into the intake at idle. You should see a rich limit code set if this is an issue.