Reader Question: Dear Mr. Austin
I recently purchased a 2004 Jeep Certified Grand Cherokee SE 4.0L with 6 cylinders and 30,000 miles. I have only had this vehicle for 1 month and it has been back to the dealer 3 times for the same problem. In fact, it is back there today.
The idle usually runs at about 600 RPM while in Park. However this changes when driving and I come to a complete stop (still in Drive) the vehicle starts to vibrate. When looking at the tach it still shows 600 RPM.
This goes on and on, on every stop. Then sometimes at a stop, the RPM will go down to 500 and the vibrations go away. This sometimes happens when at stops, but mostly the fist scenario is always present.
The last thing that happen is when it’s at a stop and the RPM are at 500 (no vibrations) then after about 15 seconds the RPM jump back to 600 and the vehicle starts to vibrate again.
The dealer tells me that all Grand’s run this way and it is running and set to factory specs. I know this cannot be right because the vehicle has the ability to run smooth. I even had the service manager with me when all of this was happening.
There is a tech bulletin out for this same problem, but it only for the 4.7 liter engines not the 4.0.
Any thoughts would be appreciated.
I do kinda want to agree. Somewhat that the Grand Cherokee does idle a little rough, at least the ones I see in my shop. If you were at my shop, I would:
1. Make sure this is not a mechanical vibration due to weak or broken motor and transmission mounts or a harmonic balancer issue
2. Clean out the throttle body and the idle control motor
3. Check the base idle speed setting, it sounds like the base idle is set too low….they should be able to manually increase the idle speed 100 RPM
4. Make sure there are no vacuum leaks under the hood like from a broken rubber vacuum hose or hose connection
I really suspect you might have a MECHANICAL vibration, like a motor mount that is worn and allowing the engine to move a little at idle, just enough for you to feel it in the cab.
The idle speed jumping from 500 to 600 could be due to the AC compressor coming on, the alternator kicking in, or you moving the steering wheel and the P/S pump increasing the idle speed for added power.
The dealership should be able to inspect the mounts and use a block of wood and wedge it between the mount and the frame to prove that in fact this is mechanical and not engine performance related vibrations.
Please keep me posted, will you?