Hey everyone. I keep having problems with my HVAC system and I am positive it is a vacuum issue. I would have the fan on full blast with the vents on either extreme position (dash vents or defrost) on the **** and what would happen is that when the engine is idle and the **** is on the dash vents, the airflow would be fine. But when I accelerate, the airflow seems to decrease and I can still hear the fan clearly. Then when I idle the engine the airflow would increase. The opposite would sometimes seem true with the defrost. I understand the HVAC system airflow redirect features are vacuum actuated and shares the same system with the 4x4 front axle and that transmission fluid contaminating the system destroying stuff is common. Well that didn't happen here and that isn't even possible on this truck anymore. What I mean is that I had the Posi-Lok installed last year and a few months ago I replaced all of the vacuum lines except for the 4x4 system (I disconnected and capped them at the firewall) in response to this same issue. The transfer case wasn't leaking anyway. I just had it checked and I changed the fluid and at no time was the fluid overfilled. I also replaced the check valve and it did seem to fix it at the time, but after a while I didn't notice a change. I found out the old check valve was backwards anyway and I am going to put it back in the correct way since I still have it. The only other vacuum related item I can think of that I put in recently (but didn't seem to affect anything) is the throttle body spacer, but I put in new studs and hardware and it is all tight and sealed. My brake booster still seems to work fine and there is nothing obvious wrong with the hose there. I do have a CAI, but everything is tight and I just replaced the filter on it. Where else could I check for vacuum leaks? I don't know if it is relevant, but the intake manifold is full of carbon. When I installed the TBS, I noticed it. Otherwise the truck runs fine. Maybe I should invest in a mini vacuum pump for the HVAC controls? http://www.sparkfun.com/products/10398

EDIT: Not sure if this is related to the vacuum system, but it seems that with the temperature, anything other than the extreme left of the temperature **** (cool) is hot and as you go toward the right it is boiling hot and 70F in this truck is hot air. Is the temperature vac powered also? Maybe the problems have a common cause.

 

Just an update I put the old check valve back in where the vacuum lines intersect near the ABS system at the firewall. No change. Where are the HVAC actuators in this truck? I would like to examine them for any signs of leaks or damage. Where could other vacuum leak sites be? My intake gasket was replaced in 2009 and there was no oil contamination or coolant leaks last time I checked. I really really really would like to find this vacuum leak because my gas mileage sucks and gas is rapidly approaching $4 a gallon again. Thanks

 

Well IMO when the engine is revved up the vacuum increases - more vacuum = more leaks.
In my '02 there is a PITA of a Female Dog In Heat box in the DS fenderwell. That thing is a bitch to get to. You either remove the fenderwell liner or have arms like Houdini to get to the tubes. If you have vacuum lines going into the DS fenderwell in the engine compartment I'd look into that because that is where they transfer to the cabin controls - on mine anyway.
Maybe check this thread out?
https://blazerforum.com/forum/2nd-ge...or-heat-17808/