92 944 Insufficient Heat

benfro944 · Post by **benfro944** » 04 Jan 2011, 13:20

92 944, 88K mi, heat is just not very hot. Removed heater control valve and put in straight connector for full flow, flushed heater core - great flow in both directions, new thermostat, inlet and outlet hose to heater core nice and hot, plenty of coolant, good air flow from vents - although not always where it's supposed to come out, and still heat is just not that great - any ideas?

Post by **billofdurham** » 05 Jan 2011, 12:06

Welcome to Matthew's Volvo Site.

The doors inside the heater unit are vacuum controlled. If you have a vacuum leak the doors will not respond correctly, as evidenced by "good air flow from vents - although not always where it's supposed to come out, ". You need to check the vacuum lines starting at the heater and working back to the engine. Usually the fault is near the heater.

Bill.

Kmaniac in California USA · Post by **Kmaniac in California USA** » 06 Jan 2011, 17:40

Not only are the vent doors vacuum operated, so is the water valve to the heater core. If you had a leak in the vacuum source lines to the heater controls, you would experience NO hot water flow to the heater core AND vents that don't select. When this happened to my 740, I found the vacuum reservoir tank was cracked and I had a continuous leak of unmetered air into the engine. No wonder my car pinged on regular gas. Look at the vacuum tank, the vacuum line from the intake manifold to the vacuum tank and the vacuum lines that tee off to the heater controls.

benfro944 · Post by **benfro944** » 06 Jan 2011, 18:04

Good suggestions - Thank you. I will check out the vacuum situation and let you know. Before I removed the heater control valve I did notice the vacuum actuated arm appeared to move through it's full range of motion when I changed the temperature selector. I suppose the vacuum to the doors could be screwy. I also noticed for a couple of hundred bucks you can buy a new control unit - the unit in the dash with the dials - I guess that wouldn't do much good if there was a leak or two in the vacuum lines. Question, is it possible for the vacuum to the heater control valve to work properly and the vacuum to the doors not work properly - or is it all interconnected?