Winter weather has arrived here. 4x4 is working fine, it engages and disengages well. But yesterday my cruise control wouldn’t work. Then about a mile after attempting to set it, it finally grabbed. After slowing down to go through a town and then reaching highway speed, again it took a mile or so for c/c to work. I had the heat on “vent” and it wouldn’t switch to defrost under load, but just before arriving home I noticed it did switch to defrost under acceleration. It only did this once. So I have a minor vacuum leak? Where do I start?

 

Vents switching to defrost under load is pretty much normal. Defrost is the default position if the system loses vacuum. These trucks were notorious for problems there. There are a couple vacuum check valves that should be there to help reduce the problem.... they fail over time, and that's what you end up with. As for the cruise control.... The vacuum servo hides under the battery, and are frequently victims of outgassing batteries perforating the servo...... They are also seriously vacuum dependent, and need a good source of vacuum to operate properly. Make sure your check valves are there, and actually work..... (there is a TSB for just such an occasion.... don't recall the number, but, I think there is a thread about "wild vents" in the faq/diy section that talks about 'em.) And also go lookin' for vacuum leaks....

 

I checked the hose under the battery and it seems to hold pressure when I blow into it. I’ll check the check valves next. I know I replaced them about 15 years ago *original owner of this ‘97)

Edit- Went out and tested the check valves. They are working.

Ugh, I hate vacuum leaks. Might need to go to a shop with a smoke machine. Any other common things to check?

 

Start it up, let it warm completely up, then unplug the IAC motor. (so it can't control idle.) Then spray something around, doesn't need to be flammable... (preferably not, really.....) If spraying somewhere alters idle speed, you found a leak.

 

Good info, thanks.

I did find that my PCV valve isn’t holding, so I’ll replace that Monday and report back

 

PCV valve is just a flow regulator. The aftermarket fellers aren't always correct.... and may flow more, or less... than what Ma Mopar had in mind. They are cheap at the dealer, so, I buy one there. Less than 10 bucks last time I got one.

 

Oh ****. I thought they were supposed to act like a check valve. So some flow-thru on both directions is ok? Then perhaps mine isn’t faulty…

 

The idea is to keep a bit of a vacuum on the crank case. Burns the stuff that makes it past the piston rings, keeps it out of your oil, and actually helps the rings seal better. You WANT that system working properly.

 

At low speed they draw oil pan gasses into the base of the carnuretor/throttle body. At highway speeds, that would make the gasses too rich so it closes and the gasses back up and go into the air filter assembly where they are diluted.

 

Sometimes the pcv valves will stick open and cause a vac leak. But if that was the case your idle speed would go up. The check valve tsb (if it's the one i have in mind) is for an update on the older trucks. They only have one check valve at the intake. The tsb adds another up by where it goes in the cab. Check the vac line that goes to the CC it is notorious for leaks.