heater fan issues
 

folks ive got a issue with my 70 gmc jimmy . when I bought it , all of the ac system was gone, evaporator , heat and ac controls , etc. I rounded up all of the missing components and installed them, well when I turn on the heat and cut on the fan , it blows strong, but when I rev the engine all of the fan air goes away, you can barely feel it blow, when I go back to idle it get strong again . can anybody tell me whats wrong ?

Re: heater fan issues
 

I would start with looking under the dash at your heater wires. Your truck should have the hard link type for the throttle, not cable like 71-72. They could be rubbing something under there under acceleration.

Re: heater fan issues
 

Check all your vacuum system. There are flappers that are controlled by vacuum. The round ball on the AC housing is supposed to store vacuum for when it gets low (I believe its been a while).

There is also a blend door in the under dash box that tends to break, maybe the flapper is closing upon increased RPMs?

Re: heater fan issues
 

yes sir ! every single one of the vacuum hoses were stopped up ! even the 3 way tee ! I should have replaced them when I was putting it together, you fellows are smarter than the average bear ! thank yall very much !