I've searched the forum for two different probs that developed after the plenum job, 4wd light no longer comes on and switch for HVAC has no effect (defaults to defrost). Both of these seem to be associated with a vac leak. I've checked the lines and don't see anything obviously disconnected. the truck seems to run much better than before, but is it possible that the intake manifold didn't seat well and I have a large enough gap to prevent any vac build up? Would the truck even run if this were the case? This seems unlikely to me as I made every effort to follow advice given here and I very liberaly applied the rtv on the gaskets, but I need some help. Is there a likely cause for both of these systems to fail that I'm missing.

I very clearly remember the suction when I cracked the vac line on the passenger side and both of these systems were flawless before the plenum job. I appreciate your thoughts.

 

If you had a big vacuum leak you'd know it. The pcm would have a hard time holding idle and when you rev'd it it would take a while to come back down to idle. There shouldbe a vacuum diagram on the front cross member in front of the radiator. trace the lines and check for leaks, primarily in the little accessory line that runs all of the items that you mentioned. If you don't own a vacuum gauge, get a length of tubing or hose, stick one end to your ear, and, with the motor running, run the other end along the vacuum lines. Listen for hissing. If you have a vacuum gauge; take the accessory line loose and get a vacuum reading off the manifold. Then plug the line back into the manifold, and check vacuum at various tees and fittings. If you take a line loose, and get the same reading as you did off the manifold, then the problem does not lie within any of the lines still hooked up. The problem could be either the check valve, or that you didn't hook the cruise control vacuum line back in correctly. That is the small line that sweeps all of the way from the manifold tee fitting, across the top of the radiator hose, and terminates below the battery. Just a guess.

 

congrats on plenum fix. i think you just have a vac line off. without vac to the htr/ac, it defaults to defrost. without vac to the tcase, it can't send vac to the cad, which doesn't slide the fork, which doesn't trip the 4wd sw.

no, its not caused by intake manifold or plenum leak. if you had that, it wouldn't run good.

while engine is running, pull off the vac hose from the passenger side intake manifold. it should be pulling a strong vac. put it back on and then work you way down that line, checking all your tee's and other lines. you should find the one mentioned above that goes back around to cruise on drivers side. one or more into firewall. one to t-case. others ?

 

Thanks DHV. I've been having a problem with my defroster staying on all the time. Happened after a tune-up. Never got into checking it out until I read your post this morning. Sure enough I must have broken the vacuum line on the passenger side leading to the vent/defroster selector. Hope JR is as successful finding his problem.