1989 4.0. Hard to track this one down. I'm getting exhaust in the cab but only at high temp above 200 deg. So highway downhill, flat or trails is no issue, but running up the mountain I get to enjoy exhaust. I previously lost a nut on the manifold flange which leaked A LOT of exhaust all the time and I fixed it. Could the high temp leak be an indication of cracked exhaust manifold?

 

It could be a cracked manifold, it seems to happen alot probably because there aren't any flex sections in the exhaust. It's strange that it doesn't leak all the time though, unless you have a coolant leak somewhere that's landing on the exhaust. Usually the exhaust gas exiting the head is anywhere from 700 to 1200+ degrees, regardless of how hot or cold your engine is.

 

Just to clarify, you put a nut on without changing the flange gasket inside flange ? I had loose nuts at the flange once and it was leaking at one spot and burnt a hole on the side of the gasket. Sure sounds like you are still leaking exhaust somewhere and a manifold crack would have to pretty big to get inside cab. A crazy theory to the uphill drive is you are producing a more rich exhaust hence you smell the leak more ? Thats all I got.

 

I did replace the nut without the gasket. Cool call. I'll get on that.