Question of cooling system design theory,I-5 engine

[urq]

The interior temperature control "philosophy" between the 4K and 5K are
completely different.  The 4K has an old style system where you are either
heating OR cooling, while the 5K has a "climate control" system that can
heat AND cool at the same time.  Cooling is flowing most of the time in a
climate control system ... not as some part of the engine temp control
though.  On the climate control system you may have both A/C and heat
running at the same time with doors to direct the air for the passenger
compartment past one or both heat exchangers.  This is most useful in
defrost mode where the A/C removes humidity from the air, which is then
heated to evaporate any condensation on the window.  

On the 4K you should expect the heater valve to close completely.  Sometimes
the heater valve slips on the spindle and no longer closes completely.
Another thing that can happen is the heater cable slips at a mount and then
does not fully move the valve from stop to stop.  If you have a car with an
original plastic valve you should replace it on general principles ... it
WILL become brittle and break some day.  

I think the recirc flap is a corollary to the heat OR cool theory.  The
capacity of the A/C is insufficient to cool flow through air from outside,
so the system uses recirculation to cool the air in the cabin ...
eventually.  

To finish the thought on the 5K system ... it does include a vacuum
controlled coolant valve that turns flow through the heat exchanger on or
off.  If you run the system in MAX A/C or turn the climate control OFF
coolant flow to the heater is stopped.  

Steve Buchholz

-----Original Message-----
From: quattro-bounces at audifans.com [mailto:quattro-bounces at audifans.com] On
Behalf Of Andrew Dickinson
Sent: Tuesday, April 12, 2011 10:20 AM
To: Tony Hoffman; quattro at audifans.com
Subject: Re: Question of cooling system design theory,I-5 engine

Hi Tony:

I have experienced similar issues on the 5k/2c cars.  I have thought of 
installing a set of valves to (A) Isolate the heater core, and (B) connect a

by-pass when the core is shut off.

I have also pondered the constant flow.  I can't see a German engineer
allowing 
hot to flow when it should not UNLESS the idea is to keep some small amount 
flowing to prevent corrosion.  I discounted that idea and actually plan to 
install the multi-valve set up.  I have even thought of ddraining the core 
during summer months but I am not sure that would be more or less
beneficial.

That's my $0.02.

cheers;

Andrew



________________________________
From: Tony Hoffman <auditony at gmail.com>
To: quattro at audifans.com
Sent: Tue, April 12, 2011 1:02:26 PM
Subject: Question of cooling system design theory,I-5 engine

Fellow Q-heads,

I have a question of theory as I'm working on the A/C system of my '86 4000
Quattro.

Firts, a little background. I live in Houston, where we already are seeing
90 degree highs :( So, since my A/C compressor dies at the end of last
summer, I've decided to upgrade while I'm in there. I have installed the
rotary compressor from a 90Q, which also requires the lines, brackets, and
Oil filter adapter. All said and done, the system is putting out 44 degrees
from the vents at 80 degree outside temps. Pretty good, for what I've seen
so far. I've never been able to get it below 40 with the old setup. If I had
the old setup just as it is now(I'll go over those details in a minute) it
would be putting out somewhere around 52-55 degrees. So, I'm pretty pleased.

However, in every 4000Q I've owned or driven recently, even with the heater
all the way off (verified under the hood against the stop) the heater never
quits giving out slightly heated air. I've replaced the heater valve on
several of them as well. This current one will be replaced, as I have a new
one here, but hasn't yet.

Looking at the coolant routing, it appears that the heater core runs are
sort of a "bypass" of sorts for the thremostat. So, if that's the case,
maybe it isn't designed to ever shut off completely?

So, here's the question, if I completely shut off that coolant flow, does
that create hot spots in the cooling system or engine. I really can't see
how it could, but I want to make sure.

With the old system, I did have a set of pinch pliers designed to stop flow
through rubber lines when you are working on them. Examples would be when
you take a fuel hose off the pump, but don't want to drain the tank. I did
that all last year, as that was the only way to get the temp from the vents
below about 55degrees on hot days. I did not experience any sort of
overheating, nor was anything damaged that I know of. But, I've got a valve
setup that I'll be adding to permenantly shut off the heat during the
summers, and I just want to make sure I'm not going to stop the flow that
may be engineered into the cooling system.

As always, TIA,
Tony Hoffman
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