cwiert

hi again...
so i just installed my brand new heater valve (the plastic one, i know i know). the vacuum to it is working nicely, i can see the valve closing when i start the car.
i have also checked that the door above the passenger foot well does in fact open when i start the car. so the vacuum system seems to be working properly.

i have no AC belt installed at the moment, but when i have the temp control on my dash all the way to the left, i still get fairly warm air. if i move the temp control to the right, the air gets WAY hotter. today's a cool day in CT (65deg F), so i was hoping i'd draw in some of the nice cool outside air, but its not. All I get is warm air.

What else can i check and troubleshoot?

JPTL

Go to this thread. Even though it's about AC, there's considerable discussion about troubleshooting the system that would likely apply to your situation.
Good luck!

WallyP

Make certain that the rubber seal on top of the firewall (the one that holds down the fragile black plastic cover at the base of the windshield) is properly installed. If it is missing, hot engine compartment air goes straight into the HVAC inlet.

Make sure that you installed the new valve with the black side toward the engine.

jpitman2

If it goes full hot as soon as you crack off full left on the slide, suspect the temp sensors. The usual culprit is the one in the left front fender. Itsmounted in a steel tube ~2" dia, fitted in the alternator cooling hose. If there is a break or poor contact in the wiring it will cause this condition. Mine did it, and it was caused by corrosion in the connector under the fender. Less commonly the interior temp sensor, or a problem in the selector, can cause the issue. Elsewhere here there are steps to diagnose the sensors involving checking resistances at the control head inside the console...
jp 83 Euro S AT 52k BTDT

cwiert

Hey Wally...
That entire fragile plastic cover is missing on my car. Is that my (or part of) my problem? Here's a pic. The rubber connector thing (technical term) seen in the pic is in good shape.

pjg

Looks like you are missing the plastic cowl and the gasket,

check this thread: https://rennlist.com/forums/928-foru...ng-this-2.html

and download Pet7, this will give you all of the part numbers and exploded views of any Porsche.

cwiert

so is that plastic cowl pretty essential? i still have warm/kinda hot air coming from the vents at the coldest setting, and super hot air coming when the temp slide is all the way right. it doesn't seem like that thin little plastic cowl should make that much of a difference, but i could be wrong...and often am.

jcorenman

The plastic cowl cover is not essential, but the rubber seal is. It seals the hot air (and fumes, steam, flames, etc) from the blower intake, as well as securing the plastic cover to the lip. Without the extra thickness of the cover, the seal may not fit tightly enough to stay in place.

Part# for the seal is 928 511 141 03, and is not expensive. (I am not finding the cowl cover in PET...).
EDIT: Found the cover part# used in 92-on, 928 574 157 00. I believe that they are all the same.

Mongo

Isn't there a vacuum actuator behind the center console that can fail causing this?

Imo000

If all else fails, get a small pair of wise grips, wrap the ends/teeth with electrical tape and clamp teh heater hose shut. This will for sure stop the hot coolant from going into the heater core.

cwiert

yeah, but I don't have a cowl OR a rubber seal. So, according to Jim, even though hot water isn't going to the core, I'll STILL get hot air because the fan is drawing in all the hot air from the engine bay, right?

dr bob

Right.

That cover keeps rain and carwash water from a few electrical bits in that tray, like the wiper motor and the AC freeze switch. When you install the plastic cover, make darn sure that the rear edge is held up high against the cowl by the metal tabs underneath. If the cover is allowed to sag, the wiper mechanism will chew up the cover pretty good, and very quickly. The covers aren't cheap, and two covers are twice as not cheap.

WallyP

Check the output temp within one minute of cranking the engine.
Check the output temp after the engine is warm with the hood up.
Check the output temp after the engine is warm with the hood closed.

The temps will tell you whether your problem is with coolant flow thru the heater valve or hot air coming into the HVAC inlet, which is the hole at the base of the windshield. Also check the oval rubber flap about a foot down the firewall on the passenger side. It needs the leaves and crap cleaned out occasionally, and the flap should be nearly air-tight.

JPTL

Since I was getting 20º warmer air coming out of my center vent than the rear unit, I pulled I my original heater valve and tested it by running water through the inlet. With the valve held closed, the water dribbled through pretty quickly. I assume that's not normal, so I replaced the valve with a metal one.
Prior to installing it, I tested the metal one just for the hell of it, and water dribbles through it as well. Not as quickly as the original valve, but still goes through. After install, I'm still getting warmer air up front. Although the center vent appears to be an AC/warm air mix, I'm noticing warm - almost hot air coming from the d.s. footwell area.
I've now completely blocked off the hose prior to the valve so that I can isolate the heater valve from the equation. Haven't had a chance to recheck to see if it's the heater valve though.
If blocking off the hose makes a difference, then I'll probably block it off every summer.
UPDATE:
Gave it the acid test yesterday: 100º w/high humidity. Heater valve blocked off at the inlet. Warm engine, 2nd fan speed on recirc. Temp at the center vent was 49º or thereabouts. Consistent with the rear AC temps. Also no warm air blowing around the footwell. So this new heater valve is either defective, or a little dribble is tolerable.
Until someone can vouch for a replacement valve that's guaranteed not to dribble - even the tiniest bit, I'm going to block off my heater valve every summer. Nice cold AC is worth the effort IMO.

Imo000

If it makes a difference then the new valve is faulty too.