Heater Controls Question

[GeneP]

Wife's 97 Jimmy heat is only luke warm. I've replaced the thermostat with the correct 195 thermostat. Turns out the original was OK. Before I go for a clogged heater core I'm thinking something is screwy with the controls & would appreciate some input. When stepping on the gas as when pulling a hill I can hear vacuum operated vents, etc opening / closing under the dash. There is no vacuum leaks under the hood & motor runs fine. I'm thinkin the control switches might be the cause. Any opinions would be appreciated before I rip it apart.

Thanks,
Gene

[djtroz]

I just bought a '94 a few weeks ago and (as I see in the posts), many are having the same problem including myself. From what I've read, the heater core blocks up commonly. As we speak, I am doing a backflush on the system. Hopefully it resolves the problem. Good luck.

[old_master]

If you're hearing doors open/close in the dash while accelerating, you have a vacuum leak.

There is a check valve in the vacuum line going from the intake manifold to the HVAC mode control switch. The check valve prevents the blend doors from changing position during low engine vacuum conditions. The valve is either malfunctioning or missing, or there is a vacuum leak after the valve, (toward the interior) i.e. vacuum line from the valve to the heater control, heater control, vacuum lines to a blend door, or a diaphragm in a blend door itself. If the HVAC system incorporates a vacuum controlled water valve, (mainly used with Automatic Temperature Control), a vacuum leak in the system may cause the temperature output from the heater to vary also.

[rlith]

The other thing is that your hvac may be functioning properly but your heater core is clogged. For giggles, before you start tearing the dash apart to get to the actuators, back flush the heater core and see if that solves your problem.