Was towing a Uhaul cargo trailer Saturday with a few hundred pounds in it and the truck bed; maybe 1400 lbs total. I was running on I-81, Shenandoah Valley going up some pretty good hills at times.

When the truck was going uphill in 5th, the AC started cutting out. I think it was low vacuum since I had the gas pedal on the floor. When I let up some or shifted into 4th gear, the AC came back on. This happened several times. I am not sure if the blower was stopped. Very little air came out of the dash vents and I could not feel it on my feet; not sure of the defrost.

So I suspect I may have a vacuum leak somewhere; either that or the vacuum was just so low that the blend door just closed off. I'm hoping someone here knows what's going on.

Any thoughts?

 

Probably a vacuum leak, like you said. Mine used to do the same thing until I discovered a vacuum hose that had come off the reservoir under the cowl (at the base of the windshield, on the outside). I fixed that and it's been fine.

 

You're running out of vacuum either due to a leak or a bad check valve.

Near the brake booster you'll find the smaller line that feeds the HVAC system. There is a check valve in the line. If it fails you're running the HVAC on manifold vacuum and when you lose vacuum the doors change position.

Air should pass one way and not the other.

 

I will have to take a look at that! Thanks.

 

Yeah, that's the first thing I checked when mine was doing that. Turned out to be the other thing, though.

 

Any way to test the check valve? Or is that the test?

I have noticed when I run the AC a while, and then shut off the engine, I do have enough vacuum to flip the door to various settings, floor, dash level, etc. And I can do this 2 or 3 times with no problems. I do hear the hissing sound and I assume that is normal since I am using air pressure.

Given that, I think I'll have a look at the cowl tomorrow and see if there's a way to determine if there's a hose off beneath it.

 

If it's holding vacuum, engine off, then the check valve is good and you have no leaks after it. If you did have a leak, there wouldn't be any vacuum to actuate the controls. There is a reservoir under the passenger side cowl.

You can lightly blow into the valve one way, then the other. Lightly!

Actually, I don't really think you have a problem if it only happens under a heavy load. The manifold pressure will rise, especially under load. When you downshifted you would have been able to close the throttle some and increase the vacuum.

 

The AC would return to normal operation anytime that I lifted the throttle about half way or simply downshifted to 4th gear. I'm sure I was pulling it hard in 5th; these were mountain passes after all.

When I got home and got the trailer empty, of course I towed it back to Uhaul, empty. And I tried it out on a very steep hill near my home, lugging it on a 2nd to 3rd shift....could not duplicate the problem.

I think we'll just wait and see if it happens again. But I wonder if there's any sort of reducer I could put in the vacuum line to force more vacuum even if I am at WOT and lugging it.

 

You can't force more vacuum at WOT. It's normal for the manifold pressure to increase at WOT that's how an engine works. You open the throttle and the manifold pressure becomes closer to atmospheric.

I don't think it's a problem.

 

Look under the dash near the gas and break pedal, I had a disconnected vac line that cause this problem once.