Heater stops at 2500rpm....

[Mr Goodfahrt]

I DID do a search first... Heater works great up to around 2500 rpm or around 50mph, but then blows cold air. When I pull over for a moment heat comes back. Blower continues to work. HELP
Monovalve appears ok. Vacuum?

What does your water gauge show? My heater blows 80*f air when my water temp is 95*c+. It just means your thermostat is WIDE open. When it cools down the heater quickly goes back to 140*f air.

Thermostat stuck open, I had this same problem in my car when I was going down hills...

[Brad123D]

Vacuum, I do believe.

The Vac pump pulls higher vacuum at higher RPM.

There should be a flap to divert air flow either through or around your heater core. If the servo that operates the flap has a leak/bad connection etc. Then you have intermittent operation depending on RPM. That's what I suspect the problem is.

Originally posted by Brad123D
The Vac pump pulls higher vacuum at higher RPM.

It pulls contant vacuum, it only increases in pumps per second as rpms go up.

[Brad123D]

Haynes 123 series manual, page 173 -

"...connect a hand operated vacuum pump which incorporates a gauge. Start the engine and make sure the vacuum reading increases along with engine speed when the throttle is opened, indicating the pump is operating."

I stand corrected... :p :p