911addict

My car is RHD.
Regardless of whether I set the heat at cold or hot, the big plastic tube that goes from under the dash (bulkhead) driver side to the front right side (driver) footwell is very very hot.

Where does this tube go?
And should it always be hot? If not, what has failed?

Please help, I'm roasting!

Rocket Rob

I suspect one of your heater servos has failed. There is a good write up to diagnosing them in Adrian's book and on John Miles' 964 web

I have two servos fail. One I was able to repair by cleaning its wiper contacts internally. The other was too far worn and I had to replace it. Overall, the diagnostic process and removal/replacement is pretty easy.

Good luck

911addict

I've seen the articles and also read Adrian's book. They don't really go into any detail as to the separate servers and nothing on the rear ones. Also no mention of what the big pipe I refer to is, ie. where it goes to and from relating to the airflow.
So far, I have spent hundreds of £'s and 18 months, with at least 4 garages and even an aircon rebuild and still no luck. Maybe another £1000 will do it...
Thanks anyway.

MisterRisky

Addict, Let me point you to some posts that might help:

Great heating system diagrams:
https://rennlist.com/forums/showthre...=259853&page=2

Other heater info worth reading:
https://rennlist.com/forums/964-forum/360080-964-heater.html
https://rennlist.com/forums/964-forum/310146-hot-all-the-time.html


I am suspicious that your issues with your AC may really be heat leaking into the system. When I bought my car it had signs of the PO trying to fix an AC issue by addressing only the AC system. In reality a loose servo was allowing the mixing chambers to let hot air leak into the system and, thus, causing the AC system to not work worth a fiddlers damn. In addition the PO had then blown a fuse and over charged the AC. I was able to get it all working by FIRST addressing the heater issue and then the AC issues.

When you have your AC off and just have the vent blowing, do you get hot air blowing out after your engine temp gets up to normal? If so, start with that.

Let me know if I can help. JasonAndreas and dfinnegan were both of great help to me in diagnosing my issues. Our HVAC systems are not really that complex, but they can be hard to troubleshoot without some guidance.

911addict

JD,
thanks, and you are right. Its hot air all the time. The aircon needed rebuilding but that was separate and works fine (except it has to fight the hot air!).
I am giving the car to my garage now to work thru the heating system for once and for all.
PS: I'll take a look at the diagrams later.

MisterRisky

I'll give you the same advice that JasonAndreas gave me many months ago: If there is hot air coming out of the vents then it HAS to be coming through the mixing chambers. This means that

a) servo(s) and/or linkage out of adjustment
b) servo(s) not working - broken servo, fuse, wiring issue
c) Mixing chamber issue - bad seals inside, broken linkage inside, cracked plastic, etc.

Your shop should be able to close in on this really fast.

911addict

Here's the latest from the garage. These guys are thorough so hope we'll get to the bottom of it now:
The big pipe fixing I mentioned, is broken - ordered new one.
At least one server is faulty (needless to say its not an easy one to get at)- ordered new one.
Have to wait a few days for parts now, then we'll see if fixed or more faults.

MisterRisky

If one of the heater feeder tubes was broken, you probably had hot feet. That will be fixed for sure. If the servo they were replacing is hard to get to, then its probably not a mixing servo, they are easy to get too.

Good luck!

amfp

I would check the heater tubes or just replace them first. Make sure when replacing you don't kink the ends them when installing. For the servers, I think there are 4 of them, you can check operation when you remove the top shield near the firewall. For me, it was the heater tube(s).

Ag02M5

So should the tubes in the footwells always be hot? Or with the servos working properly, should they remain relatively cool?

Mine get fire hot right now.

demonfish

always hot, as air is always entering them...

hot air travels from the engine up the side pipes, most common failure is here, constant heat from broken pipes. replace these and it should all be good. they will get very hot to the touch though. any excess air, ie when the hot mixers are shut, is forced out of the wheel well by the differential flapper valve behind the rear wheels.

if you still have heat, as said its the servos. one of mine has failed, i only get recirculated air, which on a hot day is rubbish, so i disconnected the servo, and pushed it back manually for now, all sorted...

MikeA

Original (foot well) large heater tubes dry out after 10 years. If they are original, just replace them. This can easily be done by you.

99% of time when foot well is hot, this is the culprit.

If your front vents are always blowing hot, then that is a servo motor problem.

Ag02M5

But why would I want the tubes in the footwell hot at all times even if it isn't blowing hot. It is still going to heat the cabin if those tubes are hot even when venting the hot air out back.

When it's 95 deg, last thing I want is any extra heat.

demonfish

I guess if there is even the slightest leak, it will allow air thru. My pipes get hot, even with no hot air coming into the cabin.