98 Grand Cherokee vibration problem

[Manic Mechanic]

Hi Guys:

98 ZJ 4.0 select-trac

I have replaced all 3 engine mounts and the transfer case shifter linkage kit (the one with the new bronze bushing and rubber bellows) just recently, replaced the exhaust manifold 1500 miles ago, and a new muffler 2 months ago. Both exhaust hangers appear to be in great shape.

Now I have a vibration at idle or when stopped (in gear or out doesn't matter).

The engine's computer does not have any codes stored in it nor is the MIL light ever been on.

I have checked and the cleaned the throttle body, TPS, MAP, and the IAC. They all seem to be in working order.

Here's the strange part - she runs smooth once warmed up. Or at least a whole lot smoother once she's at operating temperature.

Has anyone else experienced a similar situation?

Can anyone offer me a suggestion as to what to look into next?


Thanks
Len

[tgivensjr]

Suggest removing the drive belt when cold to eliminate all the things driven by the belt.

[Manic Mechanic]

I tried the suggestion of removing the belt. The engine ran with the same vibration and smoothed out when warmed up.

I drove it around, with the belt reinstalled, and noticed that when the engine is initially started the engine idles just over 1000 rpm and the vibration is minimal. The real problem starts when the engine is warming up - the vibration is felt through the steering wheel and the seats and the rpm's are around 800. But like I say, when the engine is @ operating temperature is quite a different story with minimal vibes.

Still no code has been thrown.

It definitely feels like an idle control issue.

Any suggestions??