I found a loose vacuum line yesterday that was plugged with a screw by the PO of my car. I found out that it was the vacuum line for the heater bypass valve and reconnected it. This morning the heat was ice cold, never did warm up. On the way home from work I disconnected the line, plugged it, and the heater worked, although it's not as hot as my newer car is. My question is WHY is it closing the valve with vacuum? The first idea is to replace the valve, which is easy enough. But there may be more to it than that. Could it be the car thinks the AC is on for some reason? If so, that might explain why it's sending vacuum, or is there vacuum in that line all the time? It might also explain why the heat doesn't get as hot as it should even when it's working. Don't understand. BTW, I have a brand new therm and heater core installed.

 

There's a small coolant leak at the bottom of the valve, so it probably needs to be replaced. But I want to make sure there isn't some other issue going on also.

 

Valve does not close with vacuum, it diverts hot water from passing through heater core.

Most likely a problem in the controls. No vacuum should pass through that line to the heater valve when the heat is selected.

 

Ok, I realize what it does, but the question is why. The settings at the controls were the same, the only thing that changed was the vacuum line. Sounds like, then, you are saying the actual heat/ac slider is not engaging the valve when heat is selected and the PO just removed the line rather than fix the actual problem. I hate it when people do crap like that. In other words, the controls are telling the valve the AC is on even when it's not. That's what I was afraid of. Thx.....

 

My next question is to ask if anyone knows what I need to look for if I pull the control module out?