Maybe someone could help. When I get on the throttle whether to pass someone or coming up a hill, my heat goes from the floor the the dash vents until i let off. Any ideas? Thanks

 

The heater box in your blazer uses vacum to control the mode doors. so if you had a vacum leak the mode door wil go to default and blow out the defrost. but it sounds like it works fine until your on the throttle. when you lay on the throttle to pass someone the engine does not have enough vacum to control the mode door. so it relies on a vacum resivor, witch holds vacum during heavy assceleration. so it sound like the resivor it not hooked up or missing. that was kinda how it was explained to me before. the vacum resivor on my 1997 blazer is mounted to the hood on the drivers side. hope this helps

 

*Moved to 2nd Gen Tech* - Technical questions belong in the appropriate tech section.

On the 98+, the vacuum reservoir is located inside the driver side inner fender near the brake booster.

 

Could the problem be with the very small check vavle. I checked and it seems all hoses are good and connected.

 

Replaced the small check valvle and it did not fix the problem. any other suggestions. All other functions work fine, just when i am accelerating does it switch to the dash vents, please help.

 

Is the reservoir on the HVAC side of the check valve? You could have a leak in the lines...

 

I installed the check valve just as I had removed it. All of the lines that I couls see seem to be ok.

 

A vacuum leak on the HVAC/4wd side of the check valve or a malfunctioning check valve could cause the symptoms you are experiencing as well.