This is normal. The blend doors are vacuum operated. So is Cruise Control. Then the engine is under hard load/acceleration, you loose vacuum to those systems. There is a vacuum reservoir under the battery, that keeps vacuum for a bit once the engine starts to work, but will run out. When it does, the blend doors default to defrost, their at rest position. Once engine load is reduced, vacuum is restored, and the blend doors go back to where you had them set.

 

It's expected, but not actually normal. The jeep should have plenty of vacuum to be able to run both systems, and should have check valves in the vacuum lines to keep vacuum on the cruise/hvac side of the system when the manifold doesn't have any.

So yes, vacuum leak.

 

My leak was in the tiny line that jumps through the firewall and feeds the hvac control. Instead of tearing the whole dash out for that line, I ran a new line through the antenna grommet and spliced it about 6 inches away from the control connections. Cost about 30 minutes and a 8 ft piece of vacuum line.

 

I think I'll be doing the same thing as I work on replacing my heater and evap cores...

 

My 94 does the same thing but I dont need to be using cruise. Just long hills will cause it to happen.

I've not taken the time to find the vacuum leak.

 

Thanks for the replies. Ntrenn, how does that section of the lower dash come out?