[Xcellence]

No heat in my car today . Its done this for about the last week. Before this it was working fine . Any possible reasons the heat would just stop workin ?

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[flash]

check the vacuum line on the heater control valve - with the engine running, have someone engage and disengage the heat - you should see the valve armature open and close the valve

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[Greimann]

The engine compartment heater control valve is open by default. Apply vacuum to close it, so if you see the valve arm move, it is closing.



Most likely there is a broken or loose flapper control arm in in the heater unit under the dash. This is common. There are threads somewhere that speak of this.

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[flash]

if it was open, would that not mean that water is continually going through the heater core? open valve water flows - close valve water no flow - am i missing something?

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[Greimann]

In most operating modes the heater core is hot. Temperature is controlled by flapper doors, so no heat is most likely a flapper door problem.



There is only one or two operating modes where the heater valve closes. AC Recirc mode I think.

heater_flaps.pdf Size: 59.39 KB  Downloads: 33

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[flash]

wow - that is the opposite of any other car i've had - the valve always opened to let water into the heater core, which was always a help when having a hot running engine



so, when you turn on the heater, you apply vacuum and close the valve thereby cutting off the water to the core? that just seems wrong

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[Greimann]

[quote name='flash' date='Feb 28 2006, 08:54 PM'] 

so, when you turn on the heater, you apply vacuum and close the valve thereby cutting off the water to the core?  that just seems wrong 

[right][post="16747"]<{POST_SNAPBACK}>[/post][/right] [/quote]

No, the core is hot all the time. See #8 in the PDF diagram. Flap door #3 controls the temperature. In that picture, the door is down, directing all the air through the heater core - full hot. If moderate heat is needed, the door raises part way - warm. In zero heat mode or AC, flap door #3 is full up and bypasses the core altogether, but if you want moderated AC cooling, you can blend in a little heat since it is available all the time.



If your engine is overheating, you can still turn the heat to full to cool the engine.



Not sure what mode actually closes the valve.

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[flash]

so the heat temp is controlled by airflow? then how does the fan play in? wouldn't that make it cooler when you turn up the fan but leave the temp alone, if all you are doing is adding airflow? that seems counter-productive



in a standard setup, the valve controls the amount of hot water, which seems simple, but leave it to the germans to reinvent the wheel



now i have to go look at my other valve - i am having a hard time getting my head around this - seems like a pretty lame design

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[Greimann]

It really functions the same as any other system. You have a hot source and cold source and flap doors to combine the two streams of air to control the temp. The fan is there to force the air through the system. Heater control valves in any system are either open or closed, they do not throttle flow to regulate temperature. That is the job of the flapper doors.



In the 968 the automatic system constantly adjusts the door for proper temperature so fan speed is accounted for in the middle temp temp range. Full hot or full cold sttings give you as much as the system can deliver.



Suffice to say it works just fine when everything is operating.



So Mr. Xcellence's problem is going to be most likely a flap door issue.

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[flash]

um - well, i've had a lot of cars with cable operated valves that regulate the amount of water going through the core - turn a knob and adjust how much - the british cars, and all of my dodges were that way - heck in some of the cars, there are no flapper doors at all, and the only door-like thing was a diverter to send it either down low, in your face, or to the windshield



i'm gonna go look at my jeep, but i think it's that way too



i just went out and took a look at the 968 - the valve is indeed normally open when cold or engine off



in my car, once it got warm, the valve closed as it then it applies vacuum - as soon as you turn the temp knob up to about 18, the valve opens again to let hot water into the core



so, it operates pretty much like the older cars did - it only lets water into the core when it is in "heat" mode, and bypasses it the rest of the time



so, i'd still check the valve too - if it's somehow stuck closed, you won't ever get heat



but, the liklihood is that it is something inside

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[Greimann]

Ha, well I figured out why I couldn't tell which operating mode closed the valve - my vacuum diaphragm on the valve leaks. The thing is dry but leaks air! A spin on the leaky valve problem. <img src="/forum/images/smilies/968/mad.gif" class="smilie" alt="" />



Spare goes in tomorrow.

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[flash]

that sucks - or doesn't as the case may be



it will be interesting to see if yours works the same as mine - always nice to know that something works right

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[link517]

Can someone be more specific on this? "Most likely there is a broken or loose flapper control arm in in the heater unit under the dash. This is common. There are threads somewhere that speak of this. "

My heat completely stopped working yesterday!

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[DaveN]

Just as a precaution make sure your water pump is functioning. Worst case is there is no water flow therefore no heat; you might not have noticed your temp gauge rise if it was a short trip.

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[link517]

No, coolant system acting normally. New radiator and associated hoses installed in december. HCV valve replaced from metal audi unit back to a new plastic porsche one in november 2009

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