Randy V

Definitely the heater valve is the prime suspect.

Do a search.

It's easy to replace, but very important to not install it backwards.

Dan also gives good directions on how to access it.

The_Remora

Once again, thanks guys

Jeff928S4

Know of any good write-ups on replacing this part?

I have the air flow when the sliders are to the extreme left.....I also have the "knock, knock" in the center vent when I put the heat on the feet. Related??

Tampa 928s

My air stays at 59 Degrees after checking the pressures and adding a little 134 they are good.
I noticed today on the left lower side of my center air outlet it was blowing warm air the right top side was cool. Now is that not interesting?

dr bob

Tampa--

Fix yer vacuum problems, matey! AC is fighting the open heater valve. It's losing!

Tampa 928s

Heater valve is bypassed !

Alan

I don't think it actually does ALL that... doing it will let the evaporator get cold which will avoid a hot blast from that source - but will have a limited effect on removing the heat soak of the HVAC airbox or reduce the heater core temp much since they are seperate elements. Overall the evaporator may be the biggest initial culprit - over a length of time it accumulates engine heat, dash heat from the sun, heater core convection heat...

Of course the fan is never really fully off with the system running...

Still the heater core heats up rather faster so I still think we need a hot water valve that stays in whatever mode it was last in when the power goes off.

Alan

fraggle

Mine was doing this last week and it ended up being a vaccum leak. Fixed and everything is much nicer.

6mil928

I'm fighting the same problem now. How did you trouble shoot it?

zekgb

I put some aluminum speed tape here:



As suggested in this thread. Makes a huge difference without zip tying the heater valve closed.

medipedicman

The most efficient way to temporarly prevent hot air from ALL vents is to Zip tie the heater valve as has been suggested. In my case the heater valve was functioning properly (holding vacum) but there was a problem with the vacum line. After doing this procedure I have no hot air whatsoever.
Many thanks to Charley B for walking me through this procedure.

Tails

Excellent suggestions already given, but I would suggest that the system is first checked for its correct operation before undertaking modifications or by the "band air" fixes.

Note: There excellent write up on the HVAC System written by Wally P from 928 Specalists. A copy can be obtained if e-mail him.

Good fault finding and repair procedures are given in WSMs and also in extra document on Jim Moorehouse’s Technical CDs, and many other good descriptions given in various threads on this forum.

As a guide:
1. All vacuum hoses need to be checked for leaks and that they are connected the correct way for each MY.

2. Vacuum actuators need to be checked for correct operation as per WSM. Roger at 928SRUS has new vacuum actuator diaphragms available for most actuators.

3. Blower fan needs to be checked for correct operation. WYAIT it would be a great time to clean the evaporator inlet surface from leaves and crud.

4. A/C system should be checked for correct operation.

5. Mixing flaps control motor need to be checked for operation and actuating connecting rod to mixing flaps need to be set to the correct length. Refer to WSM

6. Mixing flaps need to be set to ensure that they fully close off. When all is operating the porportional movement of the mixing flaps can be viewed by watching the stepper motor push rod or by watching the differential movement of the control linkage to the mixing flaps. Both these are external of the air box.

7. Outside temperature sensor (located in LHS front fender behind front vertical plastic cover) need to be checked for correct operation. Temperature/Resistance figures give in WSM.

8. Cabin temperature sensor needs to be checked for correct operation. A) check that fan is actually sucking in cabin air and B) check Temperature/Resistance figures given in WSM

9. Check continuity of circuit of outside temperature and inside temperature sensor by using a Digital Multi Meter. The 2 pin plug connector fitted to the outside temperature sensor is prone to internal corrosion and could need cleaning. On reassembling use dialetic grease to protect this 2 pin plug from further corrosion in the future. The correct operation of the mixing flaps are that they are closed at the coldest setting on the slider and fully opened at the maximum setting, however, if the closed loop between the outside temperature and the inside temperature sensor is opened circuited for any reason, such as loss of continuity of wiring or failure of a temperature sensor, any movement of the temperature slider off the coldest setting will cause the out let air temperature to go to maximum heat, as the mixing flaps and the heater valve will go to their maximum open position. When the temperature slider is working correctly (that is controlling the differential between the outside temperature and the set cabin temperature) the stepper motor will operate correctly to match the slider temperature position and this will proportionally open the mixing flaps to achieve the set temperature and to maintain it. This operation will operate with either the A/C compressor operating or not, however, in hot weather you will receive no cooling effect, but in cold weather you will get the correct heating effect if the outside air temperature is sufficiently cold enough. When operating correctly with the A/C in circuit it is a balanced system and it will maintain the cabin temperature at the selected temperature regardless of the outside air temperature.

10. Check the operation of the heater valve manually by using Mity Vac or by the olde "suck" method. This will check that the diaphragm is OK. Unfortunately the valve can leak when it is in the closed position and allow hot water into the HVAC heater when not required. The system is designed so that the valve fails in the open position.

For the centre vent to blow hot air into the cabin all the time, when the engine cooling water is hot, it could be a combination of a heater valve failure, an electrical vacuum solenoid control valve failure or leakage and/or the slider operation failure.

Now to the modifications and band aid fixes.

1. Holding the heater valve in the closed position (internal gate valve closing off engine cooling water flow). This will invalidate the “Climate Control” function of the HVAC System. By locking shut the heater valve it will work in hot weather, as it will either stop or restrict flow of hot water to the heater core to a minimum and will allow the A/C cooled air to overcome any heat imparted to via the heater core, however the HVAC system is now turned into a manual operated fully maximum cooling system. This fix will not stop “Thermo-cycling” when the engine is shut off as the heater core is located at the highest part in the engine’s jacket water cooling circuit and the hot water will rise up into the heater core via the inlet pipe. When the engine is restarted hot air will be initially blown into the cabin. Note the so called “comb flap” is designed to shut when heating is required into the cabin to allow for better mixing of the hot and cool A/C air to obtain a constant temperature outlet air. As full description of the operation of the HVAC System is contained in the WSMs.
2. Placing gaffer tape or other type of tape over centre vent “comb” heater outlet (not the comb flap), as this will stop hot air being blown into the cabin via the centre vent. I have tried this method in the summer weather with a measurable effect.
3. Fit a shut off valve into the inlet piping to the heater core. I have a spherical valve fitted into the delivery circuit and during the summer I shut this valve to stop the inlet flow of hot water to the heater core, however, it does not stop the hot air being blown into the cabin when the engine is restarted, as the system still suffers from thermo-cycling via the outlet valve which goes to the open position when the engine is shut off. By shutting off this inlet isolating valve in the summer time and using the tape over the centre vent hot air comb outlet into the centre vent I have reduced the hot air flow from the vents to a minimum.

Tails 1990 928S4 Auto

Alan

This mainly affects the flow of air from the heater to the console/door vents - it will help cool these temps - but won't also cool footwell and or defogger vents - so you will still get some indirect warm air.

I think the hot water valve is still the better way to go generally... if it worked well enough for you good! BTW the hotwater valve isn't half as hard to get to as it sounds - the airbox is an easy one...

Alan

the flyin' scotsman

If you have a old/used hot water valve zip tie it closed and seal it with glue; no way you'll get hot water to the core then.

Tony has a great write up on how to set the car up for the summer heat; no that we have any up here yet

dr bob

Perhaps the search needs to be made for a heater valve that fails/relaxes to the closed position, opposite of wbat the original valve does. A little hocus-pocus with a different solenoid (like the one used for the intake flappy...) would eliminate the hot-soak problem at least as far as preventing water circulation. Once that valve is discovered, it would be a snap to install two of them and tee the vacuum to both. That would completely isolate the heater from the hot engine unless a specific demand is made for heat.