I have a98 2500 with a 5.9 gas engine. Under a load (accelerating), the air conditioning vents start diverting the air to the floor level. The air conditioner works fine otherwise. The vacuum check valve and the vacuum reservoir have been replaced. The mechanic "checked" the vacuum lines including under the dash (heater core replacement). The truck is stock except for a 50 mm throttle body (Fast Eddie) and a Super Chip reprogrammer. Is it possible that the motor could be lacking enough vacuum? As usually, thanks in advance for your help!

 

no. you still have vac leak. there are 2 or 3 check valves around the passenger side intake/firewall. make sure you replace all of them, and make sure they are turned in the correct direction.

 

Check along the passenger side firewall and that side of the intske. . Towards the back. Lots of thin frail plastic lines that like to break.

 

Inspect the vacuum line where it goes through the firewall (with the A/C lines)... mine chaffed a hole on the line on the firewall... I was lucky to retrieve the line so I could join it back up... if it falls inside, you have to drop the dash to get it.

The CAD and Cruise Control are other leak sources (if you have them).

 

What a sense of accomplishment you will feel when you find that vacuum leak 0-o Such a fun task *) Don't lose faith in your mechanic they are tricky to find at times-