Jfrahm

1992 968. I put in a new heater valve earlier this year when I had the car apart for a water pump. Since then I often get no heat except when moving, and when moving it'll cycle from hot air to less hot.
At idle I can see the heater valve is closed (for no heat) even if the cabin is cold and full heat, defrost, etc. is set.
If I pull the vacuum line to the heater valve to open it, I get loads of heat at idle. The climate control is closing the valve.
Every now and then it works normally for a few minutes and then goes back to doing this.

Stuff I have done:
Checked for air in the system, bled: all OK
Tried a different climate control unit: Same behavior
Outside temp display is normal
Coolant temp display is normal

Car runs OK but the DME temp sensor has not been tested.

Any ideas? This seems pretty odd. Does the climate control use the DME temp sensor for something? Cabin temp sensor?

jeyjey

Vacuum leak?

Jfrahm

No obvious vacuum leak, plus the symptom is vacuum TO the heater valve when it's not supposed to be. If I release the vacuum, the heater valve opens and I get cabin heat.
Footwell airflow on/off seems to work as expected, not sure if that is vacuum.

jeyjey

968 has automatic heat control I assume? Could be bad connections between climate control unit and vacuum valve. When at idle the system voltage is likely to be lower than at part throttle....

Jfrahm

Pretty much the same as a 944/2, there might be some without automatic climate control but I have not seen one.
I have tried holding the RPMs up in case RPM is affecting the heat. Nope, still no heat when stopped.
Coasting in neutral at speed was inconclusive. Maybe less heat but I could not be sure.

I can imagine a problem with a vacuum solenoid, I suppose these are on the HVAC lump under the dash. Seems like a logic problem though, maybe related to airflow or engine load. I will check the vacuum solenoids under the dash, maybe try to rig a test lamp,

jeyjey

UK cars by and large didn't have the automatic climate control so I'm not familiar with it. The air-flow issue does sound like it's outside-temp-sensor related. When you say that reads normally, where is it displayed? By the automatic climate control, or is there some separate display for it? (Or do you mean it shows the correct resistance when tested?)

Jfrahm

968s have a digital temperature readout on the dash, where the digital clock is on a 944/2. The clock moves down to the storage cubby. I see from a schematic that the CC head looks at 3 temp sensors; inside, outside and the mixing chamber. Offhand I do not know if there is another outside temp sensor other than the one for the dash display.

I suppose if I have abundant vacuum at idle, and the vacuum solenoid for the heater valve is leaking, the valve could close at idle uncommanded and then open under engine load with a little less vacuum. But when parked with the throttle held open a bit the valve might stay shut as that will raise RPM but not at the cost of a lot of vacuum.

It could also be that the valve I installed is "too easy" to pull closed vs. the stocker. I can check that by putting the old valve on the line and seeing if it behaves normally. However I have used these metal Audi 100 valves in several Porsches over the years and they always served me well.

931guru

The 968 has a separate temp sensor for the digital remp readout. It is located in the passenger outside mirror housing.

The climate control uses the same sensors as the 944 series 2 cars does to control the heater mixer servo motor. The best test is to let the system sit overnight, then check the readings of all three thermistors at the same resting temperature and they should all be the same.

Jfrahm

Update: I think it's a leaky vacuum solenoid. I put a vacuum gauge on a tee to the heater valve and observed low vacuum, instead of none, when the valve was supposed to be open (5 in/Hg) and a normal reading when I shut the heat off (15 in/Hg). The low vacuum is enough to close the Audi valve I fitted and the old plastic Porsche valve that I had squirreled away in the hoard.
Under a little load, driving normally, the low vacuum drops even lower (around 3 in/Hg at light cruise and zero at speed) and that's probably what allows for normal heating when driving.

I do not relish swapping that valve, rather work on oily stuff than be under the dash. I wonder if kept any of these valves from my parts car?

Tiger03447

The more that you open the throttle, the less vacuum you will have. Max vacuum is at idle.

Jfrahm

Actually you can get more during engine braking.