2002 ZR2 I am having the usual issue with fluid in my vacuum lines, I just cant decide where to go next. Here is where I am so far.
#1 Vacuum lines are soft and oily at PCV, check valve by brake booster and reservoir.
#2 Vacuum lines are dry and pliable at 4WD actuator and transfer case vacuum switch (no fluid when pulling lines off).
4WD works fine as does the HVAC. The main issue is when the engine is under a heavy load (passing, hill ascent. etc) HVAC always goes to dash vents and reverts back to floor at cruising speeds. I am concerned I will have an issue with 4WD in the event I need it.
I am thinking at this point it must be oil and not ATF. Have replaced the PCV and effected vacuum lines except right up to the reservoir that I cant get too.
The reservoir holds a vacuum but doesn't take much to pull a vacume (full of oil?). On the other hand I can't pull a vacuum at all at the vacuum line going to the HVAC.....Idea's?.......Anyone?

 

I played with all of these and this system a bit to try to figure it out so I'll chime in with some thoughts.

I'd go ahead and replace that last vacuum line to the vacuum reservoir. I know it is difficult to do but it might fix the problem with your HVAC. In combination with the one-way vacuum valve, the reservoir allows the HVAC system to maintain the proper vacuum when the engine is under load and drawing less vacuum. If it cannot hold a vacuum then the air delivery would be redirected when the engine is not supplying enough vacuum to actuate the HVAC door controllers.

You will likely need an extendable inspection mirror and long need-nose pliers or forceps to reattach the vacuum line to the reservoir. Oh, and a small flashlight. It took a couple minutes just to find the right angle to put the mirror and flashlight to allow me to see the nipple for the vacuum reservoir.

For extra insurance to insure a snug fit on the nipples, I used narrow zip ties as mini-hose clamps on the vacuum lines.

Regarding the ability to pull a vacuum at the line that supplies the HVAC, I have had it seem to work and not work. I suspect it was due to the vacuum control solenoid that was in different settings being responsible for that result. It is also possible there is a cut in the HVAC supply line that you will have look for.

If the air is no longer redirected when the engine is under load after replacing the line to the vacuum reservoir then there is likely no issue with the solenoid.

Also, did you test the one-way vacuum valve? It is really cheap to replace.

Even after replacing the PCV valve, are you still getting oil in the vacuum line from the engine? You might have to replace the PCV valve again or consider another reason why it is happening. I have had little to no oil in that line on mine.

 

I put the ole Might Vac on the check valve and tested good. I think I will go to the local pull it yard and grab a reservoir and test that way. Why is there fluid in these lines any way, doesn't make since to me, seems as if it were oil from the engine it's traveling in the opposite direction of the vacuum flow

 

Is the vacuum valve oriented correctly? Oil would travel from the engine into the hose if there was more vacuum on the HVAC/4wd control side than the engine side which would happen when the engine is under load and if the valve was either not working or oriented backwards.

A mis-oriented valve might explain the HVAC issue too.

I suggest this because I found myself double and triple checking the valve orientation when I was doing my vacuum lines.

 

Guess I should find a diagram of vacuum routing and explanation of how that check valve should be oriented. Would be great if it were that easy!!!
I will search the forum for the diagrams and orientation, any ideas?

 

Search the forum and Google for "Blazer vacuum hoses diagram" or some permutation and you should get some hits.

Be sure to report back with your results of your repairs.

 

Bringing this one back to life. Moved on to other more important issues i.e. lower intake gasket, new stereo, banging tire carrier and so on. While doing the Intake I replaced the PCV valve and accompanying hoses along with the HVAC control. Thinking all was good I moved on. Now six months later I saw oil dripping from the fender well (oil from check valve migrating it's way down). I have verified that it is oil and not transmission fluid. The only thing that seems out of the ordinary is the PCV valve rattles constantly. I am fairly sure I have the check valve oriented correctly according to all of the diagrams I have looked at. All that being said the only other thing I can think to add is the hose from the PCV to the check valve has oil in it as do the hoses to the HVAC & 4WD. The hose going to the reservoir is dry. Any help or ideas would be appreciated.

 

Replaced the check valve.....no change.

 

Since the engine pulls air out of the vacuum lines, I would highly doubt that it is engine oil in the vacuum system. There could be a path for ATF to get into the vac system though, but to diagnose further, we need some more information. Specifically, whether you have the 3-button or 4-button (with Auto-4wd) 4wd option.

 

Three button.....I know it doesn't seem possible for it to be engine oil. Pulled the lines off the transfer case and they are dry as can be, and a different color than my trans fluid, same color as my oil.