Yes yes I know, been discussed to death. What is the conclusion .... I remember on a previous S4, the heater was always on, and replacing the heater valve I think stopped this irritating bug. So, am I right? Does a new heater valve hold the vac so as to keep that hot water out of the core after driving?
All the best,


The 928 hot water valve is a poor design - - it can lose vacuum, and the flapper part can warp, either way, heat will get in the car. Someone had a write-up a couple of years ago on replacing that one with a better unit from NAPA, I think. (No, not the wine country...) Just can't think of who it was right now. As a temporary fix, you can tie-wrap it so the hot water won't flow through.

Whoever did the NAPA replacement, front and center!




Hot water valve explained (I hope):

The heater valve is electrically controlled by a three-way solenoid valve. When the ignition is switched off, the solenoid valve allows air to enter the heater valve vacuum diaphragm, thus allowing the heater valve to open.

Because the heater core is located higher than the engine, warm coolant migrates to the heater core through natural convection when the engine is warm and the ignition is switched off.

A simple cold water trap would have prevented this minor annoyance.

90'S4, Slate Metallic


Front and center !

See HVAC Heater valve.

I did not note the part number because I bought a 5/8" valve which is too small for our 928's - we need the 3/4". Be sure the valve you buy defaults to open (no vacuum = open coolant flow, vacuum = shut off). What I did was ask the NAPA guy for a heater valve for a 1994 Dodge Caravan (but that has the 5/8" fitting). So you could start with that. If someone figures out then good part # let me know and I'll update my page.

I also had searched the internet and our VW and Audi friends were already on top of this with alternative valves. So we're not alone....

John Pirtle
87 a/t 147k


When you shut the engine off, the heater valve opens and hot water from the engine migrates into the heater core. If you restart the car after a short time, you have to cool that hot water before you get cool air from the vents. If you want to circumvent this problem. See the write-up by Gary Knox on my website.

It takes less than half an hour and is reversible to stock operation in less than a minute.


The hot-water valve in my '88 928s4 leaks.
It closes, but does not close completely. That's what's causing my cooling system to perform poorly.. i.e. it works fine, but is unable to provide the right temp. of cool air. The valve has a sort of seal-ring inside that makes the valve close tightly (under pressure of the water that the pump provides). The seal is the problem maker. It slips away, and app. 10% of the hot water enters the heater body, trying to un-do the work of the cooler.

I agree with a previous author to temporarily disable the flow by closing the hose. It does need a replacement valve soon however.

The valve is directional, the black side is towards the engine. If you reverse the valve it seems to work a bit but the pressure of the water makes the valve leak. Putting it in correctly, the pressure pushes the valve to close tightly.

Mercedes and Volkswagen/Audi use the same part. The Mercedes partnumber is: A0008305784 , and costs 16

Theo Jenniskens
'88 928s4
The Netherlands


The heating valve is available at Mercedes for about 20 euro, and this is the same part that Porsche sells at 70 euro.
I bought a new valve at Mercedes:

Part number: A 0008305784
Cost: 15,45 plus taxes