Took a 1K road trip and have an issue that seems to be vacuum related. As long as I am on level or downhill surface all is good. Going uphill trying to hold a steady speed the HVAC changes from floor to dash vents followed by skipping or stutter from the engine with a fluctuation in RPM and power. Once back to cruising speed with no load everything goes back to normal. When I go WOT from a roll everything is fine until the HVAC begins to change.

I have replaced every vacuum hose under the hood, to the T-Case and 4WD actuator. Replaced vacuum check valve, T-Case vacuum switch, 4WD actuator and HVAC controls. Pulled vacuum at the reservoir, HVAC, actuator and a variety of other places, they all hold vacuum.

What am missing……

 

Yes, it sounds like a vacuum problem.

Here are some places to check in case you have not done so already:

• at the hose that supplies vacuum to the HVAC system that comes through the firewall into the engine compartment
• at the hose end for the vacuum supply for the 4wd and HVAC where it attaches to the intake manifold

Might you have installed the check-valve incorrectly? Your symptoms indicate to me that there is enough vacuum when the engine can supply it but that when the engine vacuum is less (i.e. intake manifold pressure is almost as high as atmosphere) that the vacuum reservoir is not able to do its intended job of supplying vacuum under those conditions. The vacuum tee-valve is supposed to take care of this by isolating the engine vacuum supply when it is insufficient.

I don't know if this might help or not but I installed zip-ties as a kind of hose-clamp to add another level of assurance that the hoses would not leak where they attach to the various nipples. You might try that.

 

Christine, I am pretty sure the tee is correct and I did as you by using nylon clamps on the hose.Will double check that check valve tonight.....gives me something to look for, thanks

 

Yeah, the description of what you did sounded pretty thorough.

But then again, we've all forgotten things sometimes. After did my first tune-up on an old Dodge Dart I started getting more and more scared I messed something up as I cranked and cranks with the engine not starting. Then after I got out I noticed the rotor sitting just inside the fender. LOL

 

Also suggest checking the reservoir is not cracked; holds vacuum. And the vacuum line/hose to the reservoir is an easy one to miss since it is so difficult to see. Why Chevy put it inside the fender, I have no idea......

 

If you have a vacuum leak you will see fuel trims that indicate a lean condition at idle that gets better at 2500 rpm.

George

 

Is this correct???

 

Looks right to me. The vacuum reservoir should be directly connected to the rest of the vacuum "circuit" at all times with the vacuum supply from the intake manifold being isolated from the rest of the circuit when the vacuum there is less (i.e. closer to atmosphere).

Have you checked the check valve? Could it have been a bad one?

You said you replaced all the hoses in the vacuum system too. Right? Assuming you did, I'd suggest putting on more of the zip-ties on all the nipples only because it is so easy to do. I even was able to install one on hose that goes to the nipple on the reservoir. It took a bit of effort and a mirror to make sure it was on but I was able to do it. One of the reasons I installed the zip-tie "clamps" was peace of mind but also because I suspected that the original hoses might have been specified in metric not inches with the potential of some looseness at the fittings as a result.

If putting on more zip-ties doesn't help, then you should check the check-valve. (Or check the valve first as it is not that hard to access.)

Your symptoms are definitely of a vacuum leak but I'm running out of suggestions. (I only feel somewhat qualified because I was in your same situation a couple years ago.)

If the symptoms are still there, then I could only guess there is a leak that is only present when the system has been able to get sucked down first. I'm wondering if there is something in the vacuum circuit that leaks a bit, but not too much to stop operation when there is a lot of vacuum present at the intake, but when it has leaked down and everything is in is "no-vacuum" default position, the leak is isolated so that when you pull a vacuum with your pump when the engine is off you would not be able to detect it. I'm feeling I'm grasping at straws now! LOL

 

I am on my third check valve, first thing I replaced when I started messing with the vac lines when I got the blazer. Then replaced that one after I replaced all lines when that didn't work. I have a couple of the clamps off as I am going to plug some of the lines in an attempt to isolate were the leak might be. This is also an issue when loading up in 4WD pulls. Reservoir holds a good vacuum.

 

Have you checked the check-valve with your vacuum pump?