I have a vacuum leak in the dash at the HVAC controls. A P0420 code popped yesterday. First day I needed heat since buying. POOR heat. Perfect motor temp according to gauge. Somewhere just south 200. Vehicle runs just like it did before code. Every time I move throttle plate at all under load you can hear the vacuum leak until it levels back out. So with cruise on it is going on and out on a regular basis. Could vacuum leak cause heater valve to not open all the way? Or is that a mechanical connection?

 

What year is yours? '97 up don't have one.

 

99 classic. When you say it doesn't have one, what are you talking about?

 

It doesn't have a heater control valve (HCV). Jeep eliminated them on the 97 and up. They are a maintenance nightmare, and really don't do much.

Your heat is controlled by a "blend door", which allows cabin air to circulate over your heater core. If the blend door isn't opening properly, you wont get much heat. Surprise! It's controlled by vacuum. So, that's probably your problem.

Either that or your heater core is partially plugged.

 

One "correction". For 99+, the blend doors are electric.

Can you describe your problem in more detail?

Poor heat?

Does that mean you can switch the vents but the air blowing out is not hot? (this could be a blend door issue).

Or does it mean the heat mostly/only blows out the defrost and you can't select any other vent options or won't stay there under load? (This would be a vacuum issue).

 

poor heat, vacuum leak at the HVAC controls. Every time I adjust throttle I hear it the leak. If I mash on it, the air sounds trapped in the plenum until vacuum normalizes. Sounding more like heater core, huh? 1999 is year model