Check the white vacuum line under the hood in the passenger side corner above the heater box and blower motor. Might have to gently dig into the loom a bit to cut out what is bad and just replace with vacuum hose.

 

Strangely enough mine just starting acting weird. Under heavy throttle(low vaccum), it closes doors until you ease off. The vacuum can seems clean and solid, not rusty. I poked around last night to check vacuum lines but didn't find anything bad.

 

 

 

HVAC system has it's own vacuum reservoir located on the side of the blower/evap/heater core box

 

You most likely cracked a vacuum line leading to the HVAC. The white vacuum line is a frequent failure component.

 

Thanks! I'll check into it tonight. I didn't notice it last night when I put new spark plugs in. I was tracing the red and white(?) lines from the can on the inner fender.

 

 

That should have a vacuum pump on it because a diesel does not create any vacuum itself(like a gas motor does).

 

Problems with air coming out of the defrost vents is generally caused by loss of vacuum to the vacuum motor that controls the blend door or the vacuum motor diaphragm has failed. In both cases the blend door defaults to the defrost position, and so ac comes out the defrost vents.

The HVAC system has quite a few vacuum lines and vacuum motors under the dash controlled by the HVAC selector switch. The selector switch is selecting which vacuum circuits and vacuum motors to activate. The switch is fed from one vacuum line from engine bay. One of the lines comes back out to the engine bay and feeds vacuum to the blend door vacuum motor. This line has a habit of degrading where is takes a sharp turn to attach to the motor causing intermittent problems depending on the strength of the supplied vacuum. Often the line is cracked or even has small holes but still supplies enough vacuum that you don't notice a problem until engine load increases, vacuum signal is reduced and oops ac comes out the defrost vents. The motor itself can fail as well. There are many parts that could fail, but the lines and motors under the dash aren't usually a problem.

There is a dedicated vacuum reservoir for the HVAC system as well as a check valve to minimize operational problems from normal changes in vacuum signal but they can't compensate for a large downstream leak.

 

The vacuum line is black from the manifold IIRC. A single black line to a check valve/tee. One side of the tee goes to the HVAC VRES, the other side goes to the HVAC control console inside the cab.

 

Good question.