Two issues, probably not related

Hey guys! Been a while.

So today, I noticed that my HVAC will go to defrost only under medium to high load (heavy throttle or going up a long hill). I just replaced the o-rings in the auto climate control. Worked find since then. When there is good vacuum, like coasting, idling, or low throttle, everything switches and works as it should. Where should I start? There are no apparent vacuum leaks or codes when scanned and idle is good and smooth. Maybe the vacuum check valve on the firewall? I pulled the vacuum line off the vacuum reservoir about 20 min after the car was off and it was still holding vacuum there.

Issue #2:
While I was poking around under the hood looking for vacuum leaks and such, I noticed that my coolant was pretty low, about 2 inches below the "cold" line. I revved the engine by hand and it went down. Another inch of two then back up to it's previous position immediately when back at idle. I shut the engine off and the coolant level rose to the full hot line. I immediately thought it could be the intake manifold, but remembered I put a new water pump on the engine in December. Maybe I didn't fill it with enough coolant and an air pocket finally worked it's way to the coolant tank???

I don't think it is related to my HVAC issue, but thought I'd bring them both up together just in case.


Thanks for any input!

Sig. Is not up to date btw. Car has 220k on it, about 75k on the engine now.

so when does the problem occur then . I am bit lost here

The problem with the HVAC switching to defrost only happens under medium to high engine load, like when I am driving up a long hill, or heavy acceleration, like getting onto the highway or something. Coasting, idling, and cruising steady (like on a flat highway), the issue does not present itself. It only just started doing this the other day.

I don't think it would be the O-Rings in the auto climate control unit. Last year it was not switching between vent functions at all in any conditions. I replaced the O-Rings in December, and it Has not had any issues since then with switching.

Like I said, It's not completely refusing to switch vent modes, only under high load situations.

My s10/sonoma once had a Vacuum reservoir and when it cracked and no replacement could be found it was just bypassed and caused similar issues that your having in its glory days of having a fully functional HVAC system. I have manual controls so i know i don't have a vacuum reservoir im not sure about EATC version, but there has to be a vac leak somewhere or a wornout/leaky vaccumm actuator somewhere

See my reply in your other thread.

Mine does it over bumps, need to fix that eventually as well