1990 Cherokee 4.0 without cruise.

Not just another vacuum leak. At 2300 RPM and above the system turns from whatever to defrost. Drop below 2300 RPM and the system returns to whatever it was set.


I tested the vacuum reservoir, climate control line, power booster and all hoses and they hold at -0.8 BAR for 15 minutes.
A smoke test revealed smoke coming from the butterfly valve on the spring side of the outside of the throttle body. No where else is smoke being emitted. This is NOT my picture but is does show exactly where the smoke is coming from. What are your thoughts?

 

If you have cruise control this is what your vacuum reservoir should look like (see pic). There's a check valve in the vacuum source connection of the reservoir (see diagram). It may be defective.


A leaky TB shaft won't cause your problem.

 

For testing the cruise is disconnected. The reservoir is a newer single port unit. I could put the old one back as it does not leak.

My initial thoughts were to discount the throttle body leak as the vacuum is created after the throttle body.

 

The newer style single port vacuum reservoir would require a check valve at the intake manifold.


This is the later (pre-97 - 96 shown) check valve set up.

 

That makes total sense. I wish retailers were as knowledgeable. I will make the switch and report back.