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.

Elegant solution:

(1) put an electric solenoid vacuum switch in the heater valve vacuum line.

(2) wire the solenoid valve to shut off when the ignition is off.

Heater valve stays in current position when ignition is switched off.
A more radical solution might be to fix the vacuum system so that it works as Porsche intended.

If you replace or block off any leaking vacuum actuators, repair or replace any leaking vacuum lines and fittings, and clean/replace the vacuum check valves, the system should hold vacuum for much longer than required for the engine to cool off - end of problem.
Wally Plumley

OK someone linked to this (old old old) thread and I saw this answer. Unfortunately its just wrong - even if the vacuum system works perfectly the heater valve still opens after the ignition is off. This is the way Porsche designed it. The heater valve is controlled by a solenoid valve that turns off when he ignition is off. When off the downstream side bleeds off vacuum quickly through a vent port & the water valve opens.

Barry, the flappy solenoid closes the nose entry pretty nicely. So it keeps the vacuum on the warm water valve when you shut down the ignition. The valve also has a bleed port (rear round black cap, you can see on the pic) and it can be left as is or closed. Closing is easy, as you can take the plastic cap off. If you close the bleed port like I did, it works like a charm.
Eventually, after some time, the warm water valve will loose its vacuum due to minor leaks and will open. But by then the engine has cooled down and there is no reason to keep the warm water valve closed.
Thanks Theo!

Flappy Vacuum Solenoid Valve:
P/N Nomenclature
911.618.123.01 Vacuum Valve

HVAC Vacuum Solenoid:
P/N Nomenclature
928.573.691.02 Solenoid Valve

The electrical connector to the flappy part may be hard to source, while the HVAC part uses a common type blade connector.
Barry Orlando
Theo - it isn't guaranteed to work. It will mostly work - but the hot water valve can be on in AC mode - if it is when you shutdown you won't close the water valve.

Whenever the AC has cooled the car enough - the way temperature is regulated is to open the water valve and adjust the mixing flaps to warm the AC fed air slightly. The evaporator is always full cold in AC mode and all the air goes through it... the only way to be less cold is to partially mix heated air with it (not the most efficient - but that's how it works).

Essentially the AC on just reduces the temperature of the air feed - temperature regulation works the same as it would be with ambient air of the same temp...

true, if the AC would have switched on the heater (turned off the vacuum to the heater valve) then I'm still getting hot water into the heater core. It happens sometimes, although the effect of the little mod is good. It keeps a {closed} warm water valve closed for a long time.

In the USA you use the AC probably a lot more then we do in the Netherlands. If you want to fix that AC-heat-on state also, you could connect the rear side of the valve (bleed side ) to the vacuum booster. I think that is what Barry already mentioned. There is a convenient hose connection if you take the rear-side cap off the solenoid. The new connection will put vacuum on the heater valve when you turn off the ignition. I'd need to check but I think it won't cripple the normal operation as it seals off the bleed side when 12v is on the solenoid, and connects the heater valve to the HVAC solenoid assembly.
1992 928gts Midnight Blue
1988 928s4 Cherry Red
The Netherlands
Theo - yes, exactly. The flappy solenoid valve seems to be the way to go here since it can more easily be used as a true 3 port valve (rather than to 2 port with bleed of the HVAC solenoids) - thanks for that info. Still considering all the options.

OK I've been pondering this and I think I'm going to implement the Theo vacuum route with the stock water valve using the flappy solenoid valve with the vacuum reservoir plumbed to its vent port.

(sounds a bit like Colonel Mustard in the library with the lead pipe...)

My reasons are that after consideration there are 2-3 downsides to what I proposed with the default closed hot water valve.

1) Filling the car with coolant with the ignition off would be complicated by the closed water valve and mechanics working on the car would not know this.

2) If the HVAC should loose vacuum in the winter you would be without ANY heating - I'm sure this was a Porsche consideration in choosing the default state - not just for engine off but also for vac loss.

With the stock valve and no vacuum you'd have good heating and functional but compromised AC cooling.

3) Adding an ignition controlled solenoid is far easier than adding a function reversing relay. The failure mode of the solenoid is also to default to heat - while the failure mode of the relay would be default to no heat.

For me working though this makes it clear which path to take. The flappy solenoid is already rated for engine compartment use - that's where it lives. I will install the solenoid in the bulkhead area and then the vacuum reservoir feed is also very easy. I already have used the firewall grommet near the wiper motor - so that's easy too.

This was a great discussion and changed my thinking here quite a bit - this is probably a cheaper and easier to implement solution to this issue - and easy to return to stock behavior too. I think I'd recommend it to anyone who lives somewhere really hot - however I'll implement it before saying more...

Big thanks to Borland for this idea - a very good one - and the solenoid part # and to Theo for both the same idea and implementation details...


You can see in the second photo the rubber vent has a plastic insert with screen.

There are several of these on eBay. Search “Mercedes EGR Valve” and look for the solenoids. About $16 with shipping. Make sure you ask the seller for a electrical connector and pigtail.

It functions exactly the same as the flappy solenoid valve. I did some tests with my Mity-Vac. Here’s the results:

Coil Un-energized:
1. Vacuum at actuator port (3) - draws air from vent port (1)
2. Plugged at actuator port (3) - draws vacuum from vent port (1)
3. Vacuum at vacuum port (2) – holds vacuum at vacuum port (2)
Coil Energized:
1. Vacuum at vacuum port (2) – draws vacuum at actuator port (3)
2. Vacuum at actuator port (3) – draws air from vacuum port (2)
3. Vacuum at vent port (1) – holds vacuum at vent port (1)

Do you think this will work?

I think it might be easier to install behind the center console to reduce the vacuum line lengths, but where would you find ignition 12v power available some where in the engine bay?