Please help newbie with 86 GMC Suburban

[Bubba86]

I'm now the proud owner of a 1986 GMC Suburban with the 6.2 Diesel engine with 122K. The truck passed through MD state inspection, but I still have a few issues.

1. Defrost doesn't work. The air blows but only out of the vents, not the windshield area. Does the HVAC use vacuum to control where the air is directed? I can imagine there are some leaking vacuum lines. Any idea of where they are located?

2. Truck seems to rev high before shifting gears. When it does shift it sort of slams into gear. Seems like this will definitely cause some problems down the road. Can't control the slam by feathering the throttle either, seems to always do it.

Thanks for your help!!

[sandjunky77]

Originally Posted by Bubba86

I'm now the proud owner of a 1986 GMC Suburban with the 6.2 Diesel engine with 122K. The truck passed through MD state inspection, but I still have a few issues.

1. Defrost doesn't work. The air blows but only out of the vents, not the windshield area. Does the HVAC use vacuum to control where the air is directed? I can imagine there are some leaking vacuum lines. Any idea of where they are located?

2. Truck seems to rev high before shifting gears. When it does shift it sort of slams into gear. Seems like this will definitely cause some problems down the road. Can't control the slam by feathering the throttle either, seems to always do it.

Thanks for your help!!

yep sounds like a vacum leak theres vacum going up to the selector and back out to the different diaphrams controling the doors, that vacum selector switch can leak ,& if its a big enough leak it may effect the way youre trannys shifting it sounds like by youre year you got a turbo 400 or 700r4 iether way they run off vacum shift modules,but im not sure how youre vacum is set up bieng a diesel hope i could be of any help

[Bubba86]

As it turns out, it was a disconnected vacuum hose. I was checking the airfilter when I noticed how easy it was to lift the entire assembly off the top of the intake. Once I did that I saw an elbow with a short vacuum line that was not connected on the other end, running along the back of the engine near the firewall. The hose had a small tear so I replaced and plugged it all back together.

Now I have defrost and the transmission shifts like butter. Hard to believe one little disconnected vacuum hose could cause so many issues. Now the AC started working, so that's an added bonus!

Thanks for your help!!