Hi all,

I got a couple funny problems that I need help with solving. I think they could be related but not sure.

The first one would be that when the engine is under load, say going up a hill at highway speeds, the air delivery will change from defrost to the vents. I confirmed that the intake manifold pressure is related by monitoring the pressure as reported by the MAP sensor. When under load, the pressure in the intake is only a couple PSI (not inches of Hg) below atmosphere whereas it is about 4 to 5 PSI at idle.

The second is a strange thing is a reoccurring few second long groan or moan that most often happens when the truck has been sitting for at least overnight. It is most noticeable when I first start it up. It can take up to about a minute for the noise to go away.

I have an Auto-HVAC, the one you can set the temperature.

I've replaced the under-hood vacuum hoses and checked the vacuum lines to the actuators with my Mityvac vacuum pump. They seem ok but when I pump on the supply line to the HVAC system from the engine-compartment end it can hold vacuum but also I've had it slowly leak down.

I suspect the vacuum solenoid that distributes the vacuum for the HVAC is at fault for both of the symptoms. But before I go dig back into my dash and spend money on the part, I wanted to check with you all.

Here is a link to a video of the noise:
(The noise first occurs at about 0:19)

Thoughts?

 

Update, the noise was from my AC compressor clutch engaging randomly. Will be doing AC service in the spring.