Hey all, the truck in question is a '99 4wd 5.9l half ton. I installed a vacuum gauge a while ago that feeds off the passenger side of the manifold. It always hovered steadily around the 18Hg mark at idle, and dropped like a rock at WOT, which is normal. The truck also always had the "wild vents" problem but it wasn't a huge deal so I let it go.

Last night I noticed that the gauge read 0Hg no matter what I did with the throttle. I also had no control over the vents (always on defrost), and I could not get the 4wd to engage. The 4x4 indicator light doesn't even come on. 4wd worked flawlessly up until this point.

I have NO clue how the vacuum system works on this truck, so I don't know where to start. Any suggestions? The underside of my truck is caked with snow and mud, so I'd like to know what to look for before I go crawling around underneath it. I can deal with the stupid wild vent problem, but I need my 4x4 for work, so any help is greatly appreciated!

 

My guess is it's one of the lines attached to the CAD on the front axle. Most of the vacuum setup is under the hood but on the 4WD there are two vacuum lines that run down to the CAD. I believe it's on the passenger side of the front axle. Sounds like you may have knocked one of those lines loose.

 

Probably a vacuum leak further upstream, close to the manifold. If it was just a cad issue, you would get some flavor of reading in one drive mode or the other. My guess is, one of the plastic lines broke.

 

Thanks guys, I'll dig into it later today and report back with what I find. What a time to lose 4x4, on the day we get the wettest, heaviest snow of the winter.

 

I lost my vents like you did.

It was a little vacuum line coming off a bigger line on the back left side of the intake.

 

There are also two black plastic lines that go through the fire wall, that commonly break. One up high and one next to the heater core tubes.

 

Fixed it! Turns out that one of the rubber connectors on the passenger side of the manifold dried out and broke. A few pieces of duct tape later and everything's good as new. Thanks again!

 

The new one's use duct tape? No wonder we have vac leaks all the time.