heater problem?

[onehot951]

ive been working on keeping the car running cool, and after it almost overheated i decided to do the basics. ive already replaced the radiator cap, and have been eyeing the thermostat. the reason im looking at the thermostat is that there is no change in air temp when i use the heater settings in the car. it runs hot all the time, which means to me that the thermostat is either stuck closed (also helps with the overheating idea) or my control module for the heater is defective. any ideas on whether or no the thermostat could cause the heater to be stuck on full heat?

thank you

[S4hounddog]

Most modern thermostats have by-pass the doesn't allow it to stick closed. when they fil they usually stick open. I'd suspect it's the control module or maybe a bad vacuum hose that isn't closing a heater flap. I had the same problem. Hope this helps

[N2O-SHARK]

The thermostat would not cause the heater to stick on hot. There is a heater control valve just behind the passenger side valve cover and the airbox. Follow the coolant line coming off the back of the passenger side head. It is vacuum operated and often fails. It's cheap replacement part, however if the valve is good and it's the vacuum line that's bad things get a little more complicated.

[Danglerb]

Failure mode on the heater valve is open. Vacuum makes it close and shut off coolant to the heater core. So bad valve or no vacuum, and its full time heat.

On my car the thermostat looked fine, but the sealing ring needed replacing.

[onehot951]

thanks guys that does help give me an area to start looking in. my 380 page porsche tech book doesnt break it down that simple and has me tearing half the car apart. ill check out what you mentioned

thanks again

[surfdog4]

I have two porsches with the same issue: the vacuum actuator (that supplies vacuum to the heater valve) is not working..therefore no vacuum to the heater valve, which allows the hot coolant to run into the cabin at all times...

Another possibility (and you may have both, my 928 did) is that the heater valve diaphram is bad or the valve itself is stuck or leaks, any of which will also allow the hot coolant into tha cabin. So you have to check for source vacuum and also check the valve.

The heater valve is relatively cheap (~$15 on ebay) and easy to replace..the vacuum actuator not so..so on both cars now i have a "T" on the brake booster vacuum line which i connect up in summer and disconnect in winter..until i get the courage to rip into the dash. Some folks have mechanically altered the valve to manually set it one way or the other

It's possible you also have a flap stuck open also, but if you get the heater valve closed you at least wont have the heat source

[onehot951]

isnt the heater valve up against the firewall of the car under the hood..

thank you

[surfdog4]

yes, on the right side..easy to get to once you remove the air cleaner

[onehot951]

awesome thank you very much surfdog

[N2O-SHARK]

You can zip tie the valve closed as a temp fix if needed, should the vacuum source be the problem.. Just unzip it when things get a little colder.

[Danglerb]

Its a good time to replace both of the hoses connected to the heater valve as well.

[Landseer]

So, you have an engine overheating issue also?

Am wondering, based on the questions, if you understand how the 928 thermostat arrangement is designed? Thermostat is associated with 2 seals. One is an o-ring that keeps the housing from leaking. The other is an unusual rubber/metal bonded seal that is force-fit to a passageway behind the thermostat --- lots of people don't know its even there. When the thermostat opens, the back part of it is designed to bump into the seal and close that passageway. If this piece is bad, the car will run hot. Has little / nothing to do with the cockpit heater being stuck in the on postion (which was well-covered, above).

posts #15 and #23 are illustrative.

http://forums.rennlist.com/rennforums/showthread.php?t=400824&highlight=thermostat+seal