The dreaded HVAC issue...

Tuesday September 1st, 2026 - 8:10am

Good morning, everyone.

Here's what I am working with:

1998 Dodge Ram 1500 5.2L Automatic

Plenum gasket replaced about 1,000 miles ago (resolved to Cylinder 8 misfire at the same time)

Check-Valve's replaced maybe two or three years ago.

Plastic line that goes through the firewall from the Vacuum "T" broke at the firewall. When the truck was in the shop awhile back, the tech ran a vacuum line through the firewall by the steering wheel, and it is attached to one of the HVAC lines. I presume the other side of the plastic hard line that broke, since it's a plastic hard line it's plugged into.

So everything works as it should. Air blows out the vents when I have it on vent, floor when I have it on floor, and I can tell when it's on Max A/C, and that works as well.

My problem is that whatever I have it on (vents, floor, etc) blows perfectly fine until the truck needs vacuum, such as going up a slight long grade, etc. At that point, it does the dreaded default to defrost vents, until the system I guess builds up the vacuum again, and then it goes right back to which setting it was on no problem.

Troubleshooting done:

Vacuum pump with gauge on each check valve with the motor off. The gauge stayed strong and didn't move.

Vacuum pump with gauge on the plastic line that the vacuum line going through the firewall connects to with the motor off. Gauge bounces all around and goes right back to nothing.

Vacuum pump with gauge on the line going to the speed control servo with the motor off. The gauge stayed strong and didn't move.

As mentioned above, the plenum gasket was replaced about 1,000 miles ago, if that, which resolved other issues I was having when going up slight long grades as well.

I try and keep the truck as well maintained as possible. When it's in the shop for an oil change, or something else, and they see something, I go ahead and fix it. Only thing I have right now, I believe is a very slight rear main seal leak. And when I say very slight, I mean like the truck can sit for weeks (or even a month or so), and the oil on the dipstick hasn't dropped whatsoever. Only way I know I have something going on, is the dried oil puddle on the driveway.

If anyone has any idea as to what else I can check, a fix from when someone ran into this exact same thing themselves, etc., I would *really* appreciate it. It is driving me crazy, and it's literally the only issue (outside of a bad clockspring) the truck has right now.

Thanks in advance!