The_Remora

I did a search for this topic but the key words are so vague I couldn't find anything. I recall reading about this a while back but didn't pay attention cause it didn't apply to me at the time.

Anyway, how do you stop the air vents from constantly blowing warm air into the cabin? Even with the setting on 'cold' and the fan 'off', the vents lightly blow warm. I recall reading that they can be blocked off somehow. Since my A/C is crapped out right now, I need to fix this asap, or my car will become a convection oven by the time I get down to SITM. Any advice or directions to a good right-up? Thanks

cold_beer839

I think you can remove the center vent and close the comb flap and tape it down with some ductape or ac-heating ducting type tape.

This would stop a lot of hot air that passes through the heater core from coming through the center vent.

But first look at your comb flap and see if it closes all the way.

ZEUS+

The hot water valve can be wire tied closed so it doesnt flow. Disconnect and plug vacuum hose, close valve and secure. If coolant isnt leaking past valve internally, you'll be cooler.

dprantl

Also, if air is still blowing while driving even though you turn the system off, your fresh air flap diaphragm is probably faulty.

Dan
'91 928GT S/C

chaadster

Okay, so the heater valve defaults to "open" when there's no vacuum, e.g. at shutoff, but, if the car is shutdown, the waterpump aint pumpin', so we shouldn't get any hot coolant into the heater core at that time, right? Same goes for hot engine startup, right? If the vacuum is functioning, shouldn't the heater valve close before any coolant gets pumped into the heater core?

If the above is true, if, like me, one gets hot hair upon hot engine startup, does that signify fresh air flap failure?

My MO has been to run the A/C for a minute upon startup to cool the air from the vents quickly; it works. When running, I don't get a continuous supply of hot air. The hot engine startup hot air from vents issue is perplexing, though.

dr bob

Convection. It's a dirty word for 'heat rises', and he same heat differences cause coolant to migrate slowly around the system even after the engine is shut off. Heater core is high up in the car relative to a lot of the engine, so a stopped engine, open heater valve equals a recipe for heat in the heater core on restart of a warm engine. invest in a couple plastic tie-warps and use them to clamp a NEW heater valve closed. Heater valves give up after a couple years of service, and often allow hot coolant to bleed through even when closed. A good new valve is relatively inexpensive. With a replacement stub of hose to the head and a couple clamps you are good to go.

chaadster

So Doc,

The hot air from the vents on hot engine startup is a normal, stock performance phenomenon?

I understand the whole tying the valve closed thing, but for regular, normal heater valve operation, the hot air is to be expected (given a hot engine)?

The_Remora

Thanks for the info guys.

Found a good write up on this mod on Tony's site: http://members.rennlist.com/v1uhoh/summer.htm

Alan

For a little while until the AC gets cold and cools the main cooling airflow path (<1minute). It will still be a little warmer until the heater block cools off - this may take >15 mins since there won't be any airflow through it. However with the blower at max its residual heat convection isn't going to change the vent temperature very much.

Alan

RyanPerrella

Where is this diaphragm located?

I thought its the comb flap which regulated the temperature to a degree by mixing the cool air with some other air source? Is that correct?

dprantl

Yes, it is the comb flap that mixes air coming from the evaporator with air coming from the heater core. I was just saying that if air still blows out of the vents while driving when the climate control system is turned off then the recirc actuator is faulty. You should know, you just replaced that one.

Dan
'91 928GT S/C

dr bob

Hmmm. The comb flap provides a partial flow restriction to air blowing through the center vent when you've selected defrost or floor vent with the dash slider. It's needed to force more air out those other vents. Temperature control is by the mixing flap, a motor-driven deflector that blends air from the heater core with fresh and/or AC airflow (determined separately by the fresh air flap). The mixing or blend door motor position, along with temp slider switches, determines whether the heater valve is open or closed while engine is running. Blend door motor position is a function of temp slider position, inside air temp measured at that little sensor in the face of the dash, and outside temp as measured by the sensor in the alternator cooling duct in the LF wheelhouse below the headlight bucket.

Got it?


Tying the heater valve closed blocks one of the two possible paths of heat migration to the heater core with engine off, but more importantly it prevents actual coolant circulation. Heat may still move by conduction through the still-open side, but convection won't cause actual coolant circulation. Blocking the flow completely with the tie-wraps cuts down on that first hot blast when you restart after a heat-soak period. The AC and air circulating still need to shed al that heat that's accumulated from the sun though, so it will still take a minute to get cool again. On hot restart, you can turn the AC on but leave the fan off for a minute. That lets the AC stsem pump the accumulated dash and heater core heat out through the condenser, instead of blowing it all into the cabin. Saves you a facefull of superheated air.

dprantl

Yes, dr. bob is right. I just got my flap names mixed up. The mixing motor mixes the air, the comb flap just diverts the air to different vents. Fun stuff.

Dan
'91 928GT S/C

The_Remora

Still haven't made it out to the garage today to look... but do you access the heater valve up under the dash or under the airbox underhood? Anyone have a good pic of this handy? The pics in the WSM are real hard to see. Sorry for the newb questions...

dprantl

Open the hood, remove air intake tubes, remove top airbox cover, remove air filter, remove bottom airbox cover, heater valve is under there near the center of the car towards the passenger side.

Dan
'91 928GT S/C