I'm having a couple of minor problems with my truck and trying to decide if it is a plenum problem or just a vacuum leak under the hood, which i suppose isn't simple anyway. I have no idea as to the condition of my plenum or if it has been upgraded. I've never noticed any smoke under hood, average 13mpg (about 10 to 11 in winter) and it feels like i have plenty of power

My problems are that my cruise will let off on the highway, and my HVAC system doesn't work right

If i'm applying throttle then it shifts defroster nearly all the time, but occasionally functions currectly even at WOT. However letting off the throttle it will blow out of whatever the dial is set at

As for cruise, I set it and it holds fine, however I will gradually over several miles lose speed. The cruise is still engaged however, and if i speed up manually it will hold on the speed i set it at before slowly slowing down again.


So you all being the experts on Magnum engines(5.9 if i don't have it typed out somewhere), Where should i begin my search for the problem.

 

Well, your defroster-switching problem is more that likely the 'wild vents' issue.
I'm kind of thinking that 'wild vents' can bugger up your cruise and stuff, I don't remember though.
Do a search here for wild vents, a whole car load of stuff will come up. lol

Heyyou or zman should be here in a little bit..

 

So from reading its sounding like a check valve issue?

 

Was I paged?

You probably have a couple different problems there. One is indeed the "wild vents". On the passenger side of the intake, is the vacuum source for vent control, and front axle engagement. Need to check those lines, (plastic lines.... notorious for becoming brittle, and breaking). In there somewhere, is a check valve. That probably needs replaced. (get one from the dealer...... they are the right size.)

On that particular snakes nest, one line goes into the cab, for vent control, another disappears up under the cowl, to a vacuum reservoir, and another goes down to the switch on the transfer case. From the transfer case, two lines run off to the cad. One of them will always have vacuum applied to it. Need to have a look at ALL of them, and make sure nothing is broken. (the steel lines that run along the frame like to rust.... check those too.)

Cruise has a separate vacuum source on the front of the throttle body. That line runs along with the throttle cable from the cruise servo. Another member had the same issue with cruise that you do, and found a leak in that line. (I think he simply replaced the whole thing.....) Also, the cruise servo sits UNDER the battery........ there have been occasions, where the battery has dripped acid on the cruise servo, with predictable results.... have a look, see what you find.

 

Will I need to check the transfer case lines? My 4WD is functioning propperly

 

my cruise vac was in the same "snakes nest" that the 4wd, climate, and resevoir were in. i beleive.......

if 4wd is functioning properly, see if you cant just locate a nice leak in the engine bay, otherwise youll have to check all of them, as loss in vac will result in loss of vac associated compenents i.e. CRUISE! especially at 65mph in overdrive!

 

Hey i just found a broken line on the pasenger side in the engine bay. one end led up under the cowl. and didn't look further because in following it a broke the plastic line off. Just replaced it with rubber hose. We'll see if that fixed everything