yoramw

I got my 1986 951 a few months ago.
Since I have it, I notice a strange behavior:
The heat seems to turn on start up by itself and stay on for 5 minutes.
After the engine is running for 5 minutes the heat turn off by itself and stay off (HAVC temp is set to minimum, AC is off)
If at this point, I turn the engine off for sometime and start it back while engine is still warm, I do not get the 5 minutes of heat anymore.

Next morning, same thing...

Its getting very warm these days where I am at, so what was a feature in the winter is becoming an big annoyance in the summer...

I have check the plastic clips which people say break sometime, but they seem fine.

Ideas are very welcomed..

951Dreams

errrr.... You've pissed off the HVAC spirits?


Sorry, I got nothin for ya, I've tried to run scenarios that could explain that behavior, but can't come up with anything. Hope someone with a better imagination can think of something.

The only long shot I could think of was something with the CPU for the temp control. That when cold it got false readings or was sending bad signals, and once the board warms up it corrects itself......

Northern_aqs

A vacuum leak will allow the doors/flaps to open and allow full heat. As your car runs, it seems vacuum builds to the point where the doors/flaps close. A vacuum gauge placed at the vacuum line connection under the dash will confirm this (if you T it inline) while the phenomenon is occuring. All those rubber bits dry out, crack and shrink over time.

yoramw

Thanks.
I have already replaced most of the vacuum lines already.
I plan to do the rest soon, so hopefully it will help.

yoramw

I looked need the HVAC blower and I saw an empty bracket which I understand suppose to host an external sensor.

Mine does not seem to have it.
Can this be the issue?

arthropraxis

Mine did that and it was a broken heater clip.

951Dreams

Beyond the clips I had to replace, due to a heat all the time issue, and no floor heat, I haven't messed with mine too much. However, all my doors were motor controlled. Including the blend door. Mines an 87, so I can only speak to mine and that year.

Someone who's much more experienced (or more well read) on the HVAC on these can chime in, but about the only vacuum part I know of is the heater control valve, but honestly on these cars I haven't personally looked into what exactly it does. Opens coolant to the heater core? I'm just speculating, some cars I've owned had a valve to keep cold water from circulating through the HC till it had warmed up some, to help it warm up faster. (and turn it off when the heater was off, to help with residual heat) My Acura won't even turn on the heat (blower) till it's warmed up some. I'm not sure what it does for these cars however.

But anyway, my point was, HOT/COLD is controlled by the blend door. If you have A/C on, the HOT side should be closed. HOT is the default "fail safe" setting, maybe yours sticks for a bit till the motor can get around to breaking it free?? My floor vent and blend door clips were broken, so I had perma heat and top vents/defrost only till I fixed the clips. It was 16 F outside when I bought the car, and my feet were getting chilly!!! LOL

I THINK there are three temp sensors on these, if I understand correctly. One to test outside temp, one to test inside temp, and one to test airflow temp. I know on my Suburban and Acura, the outside temp sensor is in the front bumper area. The inside one on the Acura is in the control unit itself in the dash. I think the Porsche in is in the dash as well, but somewhere in the clock area?? (going from memory, maybe wrong) I had assumed the 3rd one was after the blend gate, so it could test how much to adjust the blend, but I really don't know.

These MAY use a sensor, where your showing, for outside temp. Would be a logical place to be testing, if the computer is smart enough to be looking at how much it needs to heat/cool the actual air coming into the system. My Acura isn't that smart, it just test ambient outside temp.

If no one post up with REAL answers (I'm just trying to share the little I know) I'll peek at mine tomorrow and see if I have a sensor there or not. And I'll let you know.

If I'm wrong on any of the above, someone will for sure want to correct me, so at least we'll get some answers. And we'll both be smarter for it. :P

yoramw

I had a chance to spend a few hours yesterday on the car.

I found the sensor I thought was missing, it was lying hidden not far from the mount. I have placed it back in the mount.

I replaced the vacuum line to the heat valve (which was in decent condition as far as I can tell).
I have already done a couple of weeks ago all the vacuum lines under the intake.

I replaced the heat control value with a new one. I inspected the old one and it seems to be ok.

Question about the heat value, when does it suppose to get vacuum and close?
I started the engine cold and ran for 5 minutes int he garage.
I see the heat valve does not move when I play with the temp. dial.
When the engine warms up a bit and warm air get to the cabin, I tried to plan with the temp dial and I feel warmer air gets inside when I turn it, but even at min position, the heat value stay wide open.

yoramw

951Dreams - Thanks for the help

arthropraxis - are you talking about the plastic clips near the driver feet? I have checked them and they seems fine as far as I can tell. Check out the photo:

951Dreams

Those are the famous clips, yes. You are however missing the metal retaining clips that go over the plastic clips. It will work fine without them, but they have a tendency to pop out of those white plastic clips when missing. Google the clips, you'll see what I'm talking about. It would be worth ordering some new clips to get the metal retainers. I'd throw the retainers on the clips you have, and throw the new plastic ones somewhere cool and safe for when yours break. They will some day.... They all do.

yoramw

Thanks 951Dreams,
I will be ordering them right away.

In addition, looking for someone who can explain me when the heater valve suppose to close and when does it suppose to open.
Is it based on the HVAC temp sensors and the temp dial?
When I start the engine cold, does it suppose to be in the open position or close position?

Jfrahm

The water valve for the heater core defaults to open (heat) and closes when you do not have heat selected. It might be closed or open with the car shut down depending on the condition of your vacuum system I believe, if the stored vacuum bleeds down the valve opens. if the valve does open when the car is off it should close right away when the car is started and has vacuum (a second or two).

Have you checked for vacuum at the heater valve? How about the vacuum line and check valve that supplies vacuum to the canister by the battery (LHD)?

yoramw

Well, mine does not close what I start the car up even with temp dial set to minimum.
I have replaced the vacuum line from the canister to the heat valve.

I have not checked the check valve next to the brake booster.
How can I tell if it is bad?

From reading in the last few days I understand the vacuum to the heat value is controlled by a solenoid. Do you know how I can access it and check if it is working too?

I am away from home this week so I will be able to update findings when I am back.

Jfrahm

If I have the climate control set to cool, the valve on my 1992 968 is open with the car shut off and closes when I start the car. This car has an OEM heater valve and it appears that the rod gets sucked into the vacuum pod to rotate the water valve closed (to stop hot water flow to cabin). Vacuum to the heater valve closes off water flow.

When you replaced the vacuum line to the vacuum canister/HVAC did you check for suck? If so, how?

The check valve is before the canister. It allows suck to the vacuum canister and prevents blow. If it's not working it will do either or neither. If it is backwards the HVAC will not see vacuum and won't work properly. If you have no suck to the HVAC system you get heat/defrost all the time.

I do not recall for sure but I think the vacuum solenoids are on the heater box with all the flaps and servos and whatnot.

Mach944

I have this EXACT SAME issue when I cold start my 1991 S2.
I get heat for about 5-10 minutes whether I'm asking for it or not,
regardless of the outside temperature. Let's see if we can figure this out.