1995 Dodge Dakota 3.9L has strange HVAC behavior when under load

My neighbor has a 1995 Dodge Dakota that I have been trying to get back in good reliable condition. It has the 3.9L V6, manual transmission, and is 2WD if that matters. It is a pretty basic truck and must have about the fewest options they had that year. It has air conditioning but no radio as it came this way from the factory.

It has an issue which I suspect is vacuum related. When you accelerate or climb a hill, the HVAC acts really crazy. The vents start blowing air and then stop. It doesn’t do this on a flat surface but does it immediately when you accelerate or otherwise put the engine under greater load.

I have inspected all the vacuum lines and found nothing wrong. There have been some issues with mice building a nest in the air cleaner but I couldn’t find anything chewed.

Anyway, does anyone have suggestions as to where to look?

I just realized I listed this as a Dakota. It is a RAM and not a Dakota. Was just talking to another friend with a Dakota with the same engine from a similar year…

Every Chrysler product I have ever owned with air does this. They all cut off the a/c when you call for full power going uphill. Their engineers must figure you are better off being able to pass and pull back in front hot than to die out in the left lane cool.

They said it didn’t do this when they first got the truck. I know the blend door is controlled by engine vacuum and am suspect of something there. You don’t have to really be flooring it for this to happen either. Normal daily driving acceleration does it, not full on passing. I have seen other vehicles that turn off the AC compressor at high RPM. I don’t know if this is for economy or full on power without the AC sapping it but this isn’t unusual.

Well, when you open the throttle the intake manifold vacuum gets less (closer to atmospheric). If the doors are powered by vacuum, they’d have less power to work with. It may be the door operation is just sticky with age, and when there is less power for the vacuum motor to work with, it can’t move the door. Also if the engine compression is reduced due to piston ring wear or whatever, that can affect the vacuum level. You could try checking the compression and intake manifold vacuum readings. But I expect it is just that the venting doors are sticking.


I’m betting that your vacuum check valve is bad. It is supposed to prevent the problem you’re experiencing

Where is the vacuum check valve located? I see there is one on the brake booster but I am assuming this is a different valve. I assume this is a cheap part like a PCV. This looks to be a very common problem as several Dodge vehicles came up when I looked up this issue.

The truck seems to have decent compression and doesn’t use oil so I doubt it is a major engine problem. I guess the manifold could be leaky but I am assuming it is the check valve based on what I found researching this issue.

Is this a dealer part or will any parts store carry it?

I called a parts store as well as a dealer. The only check valve listed is on the brake booster. Is this this one I need to replace or is there another?

Follow the HVAC vacuum line from the firewall near the heater hoses to the right side of the intake manifold, you will find the check valve near the manifold connection.

It looks like there’s a black vacuum line going from the intake manifold vacuum source to the check valve then another black vacuum line going from the other side of the check valve to port 3 on the vacuum switch. Probably a good idea to check that whole path, might be the check valve is good but one of the lines is cracked at a connector.