1999 Dodge ram van 3500. When I accelerate hard like climbing a hill the air will change from blowing on me to blowing defrost. Once I let off the accelerator and rpm comes down I can feel it switch back to feet then to blowing on me. I'm thinking a vacuum leak?? Any help. Also I am getting an intermittent misfire on cylinder 8. This is accompanied by an intermittent cold rough idle. Doesn't happen all the time and could go weeks in between. Thanks!

 

I had the same problem today I found that there's a vacuum line coming from my passenger side of my intake manifold that leads up and over into the firewall on my driver side engine compartment mine was broke so once I re connected the line together my air worked fine and my temp gauge went back to normal my engine is the 5.9

 

Funny, I have the exact same van but 2001. I have the exact same problem that I've been chasing for quite a while. Many suggestions are to replace your vacuum check valves. If you remove your dog house, you will have 1 valve under the intake on the passenger side and another one under you blower controls on your dash looking up. I replaced both mine and thought it helped, but I think it was in my head. Have same symptoms still. I've has the same p308 code on and off. I've changed plugs, wires, distributor, rotor, etc. All help for short while and it comes right back. Most recently I get p300, p304 and p308 that just happened 2 days ago.

 

If you are getting misfire codes, you need to have a look down the throttle body, and see if oil is pooling in there. The intake is a two-piece design, and is known for the gasket blowing out, which gives you a vacuum leak. (so, low vacuum for your vents means they don't work as well as they should)

Check valves are generally the first step. Another option is to add another vacuum reservoir for the HVAC controls.

 

I did just have that gasket replaced about 3 months ago. Happened before that. Guess I'll look at check valves.

 

Was it your plenum gasket that was replaced or intake? That's a common forum answer as well. I'm the 2nd owner so not sure if mine was ever replaced and my plenum plate is steel/not aluminum but all tests I've done show that my plenum isnt leaking. I've also read that there could be a vacuum hose crack near the cruise control servo. That's the next one I need to check. I've put 30k miles in 3 years since the random misfires and the wild hvac. I've read cracked heads etc could also be it, and quite a few other suggestions and hate to fix every repair suggestion. I'm thinking it's still a hose leak but they are hard to track down.

 

It was supposed to be the plenum gasket.