I've read a failed seal between the transmission and transfer case will cause fluid to fill transfer case, which causes fluid to get sucked into line. I've checked the transfer case and there's no extra fluid. I've just been running with the line going to the transfer case unattached with a cap on the tee to prevent fluid going into my HVAC. Today I pulled off the 3 nipple vacuum line off of the transfer case vacuum switch and it's covered with fluid/oil. I replaced the old rotted vent line and I blew into the transfer case vent to make sure it wasn't blocked-it filled with pressure and released as soon as I quit blowing into it...so doubt that was problem.

I'm wondering if this is connected to my p1870 code that goes on and off along with my diaphragm actuator not working. It's frustrating because I'm not totally convinced it's a failed seal as my transfer case hasn't been overfilled when I check. I double checked the lines on the vacuum switch to make sure they were in the correct spots-all good. I do know that whenever I blow into the vacuum supply line going to the TCS I can hear a gurgling noise from fluid in the line. I replaced all the lines a couple of years ago trying to get my 4x4 to work. (It completely failed me last winter when I needed it.) I'm at wits end trying to figure this out.

 

If you have fluid in those 3 vacuum lines on the transfer case, I would suspect the selector switch that those lines go to may be leaking. That switch is what controls the vacuum to the front diaphragm to engage the front axle disconnect. I believe that switch is only about $30, so it may be worth a shot to change it.

Also if you have any suspicions of the seal between the trans and transfer case failing please keep and eye on the transfer case fluid level. My dad had this seal fail while driving down the highway in his 1997 blazer. it pushed all fluid into the transfer case and seized the transmission which in turn locked up the rear wheels at about 50mph. Luckily he was able to slide it off the side of the road, but definitely not an experience that he will forget.

 

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

 

I've already replace that switch once 2 years ago-and then I replaced it again just to be sure I didn't get a dud the first time. I've checked the transfer case and there's no extra fluid. If anything I've had to add a little bit every now and than. I did some investigating to today and I found engine oil in the main vacuum source line. I'd already replaced the PCV valve and I replaced the rubber 90 degree boot and straight piece that goes on the plastic deal linking the PCV valve and the 'plenum' vacuum thingy sitting on top of the valve cover. Is that orange O-ring on the plenum that you give a 1/4 turn to open supposed to be fairly hard? It looked like oil was getting past it. Unless I have bad blow by the only thing I can think of is a leak somewhere which is just enough to work the HVAC, but not enough to pull the diaphragm in for the axle. My HVAC works fine-no hissing and all the functions work. If the darned kit wasn't so expensive for a cable ($150 from LMC) I would have gotten rid of the entire vacuum 4x4 and just put a manual cable pull on. However, I do really need to figure out why it's sucking engine oil. My problem seems to be something not common that'll just take an afternoon of troubleshooting to figure out. I love my Blazer-but I hate all the little problems she loves throwing at me. (Getting ready to do the TCC valve upgrade to fix my p1870 code.)