I have a strange vac leak issue and am not getting very far with it. It's starting to drive me a bit crazy. When the engine is under load on the highway the vents kick to defrost indicating loss of vacuum.. as soon as I let off the gas they will go back to vent mode. No CEL is triggered.

These are all the things I have tried so far.

-I have replaced every single rubber vacuum line under the hood including a whole new PCV hose assembly. Most of the lines were deteriorated but did not have any holes.
-Cleaned out IAC / battery reset
-Inspected vacuum reservoir('s) and vac lines behind battery. No issues.
-Inspected / function checked (blew threw them) the 2 check valves for the vac lines feeding the 4WD solenoids + HVAC line going through firewall. both were good.


Because I wasn't getting anywhere with the basic stuff, I bought a mightyvac hand pump and also a vacuum gauge.

-Pulled a vacuum on firewall vac line feeding HVAC and selected different modes (vent, panel, defrost, max ac). Vacuum holds consistently for over 1 minute on all modes- everything behind the dash is good!
-Pulled vacuum on both vac reservoirs, they both hold vacuum.
-Pulled vacuum on main black vacuum hose supplying 4WD, reservoirs, and HVAC. Vacuum holds!

-Measured vacuum with engine warmed up @ idle and am getting 17" steady. Is this low for this engine? I figured 20 is ideal.

I have also tried plugging various manifold ports with the gauge hooked up to the brake booster vac line to see if vacuum increases, but it does not make much difference.


This is a fairly fresh rebuild.. New pistons, rings, valves, crankshaft, and all new gaskets in the top end. Anyone have any ideas what else to try?

Starting to wonder if it might be a small crack in the lower plastic portion of the intake.. might try spraying some propane around to see if idle changes.

 

What does the vacuum at the manifold port do when the the RPMs go up? At the output of the vacuum reservoir? At the feed through the firewall?

Manual or automatic climate control?

 

I think you helped me figure out the HVAC issue, thanks!! I'm still not sure if the overall engine vacuum at idle is good or not though. Maybe I'm just overthinking things now.

It's manual climate control.

At manifold, the vacuum drops slightly (1" for a brief moment) if I rev the engine to about 2000RPM, then it goes to 18-19" steady if I maintain 2000. If I snap the throttle closed from 2000RPM, the vacuum shoots up 4-5" and then floats back down to 17".

After running a few more tests to get more vac readings, I found the one check valve with the 2 ports on the downstream side only works 50% of the time. I killed the engine and instantly lost vacuum to the firewall vacuum line feed. the 4x4 feed held vacuum still. Blowing / sucking on the check valve it only seemed to seal randomly. Shaking it seems like the flapper might be sticking inside