2000 Chevy Blazer LT. When accelerating, usually anything over 2k rpms, the AC fan slows and blows warmer air. Also seems to shift a bit erratically at highway speeds. After letting off of the gas, fan and temperature return to normal. AC blows cold and will freeze you out when parked. Replaced catalytic converter, downstream O2 sensor, fuel and air filter, pcv valve, vacuum lines on driver side by brake reservoir. Any help would be appreciated.

 

Manual HVAC or Automatic Climate Control?

 

Computer climate control. BTW it runs good with the AC off, kinda crappy with it on.

 

Bump. Anyone?

 

You have a vacuum leak causing the vacuum operated doors in the HVAC system to return to their default state. Find the leak and I bet your HVAC performance will return.

 

My leak causing same issue was at the blend door inside the dash. replaced the blend door and all is well

 

Just to be clear...Did you replace the blend door actuator?

 

Sorry yes it was the blend door actuator that was replaced, but be careful as the hoses get pinched easily and if they are it will prevent it from shifting correctly and in my case had no heat for a day or two till I got back in there lol.

 

Seems that it was the blend door actuator. At some point the 4wd stopped working too. So after replacing all of the vacuum lines, the blend door actuator, the 4wd actuator solenoid, the 4wd switch, and cleaning the TCCM contacts... I have 4wd again, and the AC seems to be working much better. It also somehow seems to be running smother with the AC on, so that's a plus even though I'm not clear on how the blend door actuator had anything to do with how the engine was running. Thanks for the help Whisper6691 and swartlkk, much appreciated.

 

So, I spoke too soon. When I test drove it after replacing the blend door actuator I didn't get on the highway, and although the problem has improved somewhat, it's still a problem. Runs a little better with AC on, the fan still kicks down to where it's barely blowing when accelerating and more often at highway speeds. Don't really know where to look for the problem now.