When my 98 GC 5.9 limited is warming up, and the automatic climate control is calling for heat, I get what smells like exhaust fumes in the cabin. I can smell the same odor under the hood in the passenger side area of the firewall. The odor goes away after the engine is warmed up and doesn't return. The odor doesn't occur in warm weather when the ACC calls for air conditioning. The dealer and my neighborhood mechanic are stumped. It's hard to diagnose because the odor goes away when the engine warms up. I thought it might be coming from the catalytic converter but if it is, the problem should be present year round.
Anyone have any ideas?

... The odor goes away after the engine is warmed up and doesn't return. ...
I once had a car with an exhaust seal that was going bad that did a similar thing. Leaked when it was cold, fine once it warmed up. Parts expanded to stop the leak when it was hot - I guess.

The seal was the "donut" looking thing between the exhaust manifold and the first exhaust pipe.

Just a thought.

Take Care,

Thanks for the input Greg! Tonight after work I started the Jeep and waited. It was 34 degrees here this afternoon so the engine was cold and the heat definitely would be called on by the ACC. I listened for any exhaust noise that might be coming from the area of the exh. manifold and head pipe- nothing unusual. I "sniffed" in anticipation of the odor coming from the engine compartment and inside... Nothing! In fact tonight the odor NEVER appeared.
If I didn't love this vehicle so much I would....
Well i'm definitely frustrated!
Thanks,
Kevin