88 90 Cooling System question

[Dennis Ruffing <altia!dennisr@uucp-1.csn.net>]

Friends,

My 88 90 is asking for attention again and is manifesting that need in 
the form of coolins system problems.

The behaviour is perplexing, and shall be described shortly.

My question is how the thermostat on an I5 should funtion to keep the
engine both warm and cool, at the right times.  Given that the 
thermostat's job is to open and close over a temperature range to let 
varying amounts of fresh, cooled coolant into the engine so it can 
conduct the nasty heat away, does the following "I think my thermostat 
is stuck open" theory make sense?

I'm curising down the highway at 75 (ooh, I love colorado!) without a 
care in the world, and the engine is running cool.  "Well," I think to 
myself, "it is 20 degrees outside- it should run cool like this."  In 
fact, given my postulate, I'm assuming the thermostat is open and 
letting that lovely, expensive, green stuff in to keep the engine 
operating at a nice temperature range, while the 75 mph breeze through 
my front grille is making the radiator's job that much easier.  Life is 
ducky and Herr Audi is happy.

I pull off the highway and come to a brief rest at the stoplight.  And I 
watch my temperature gauge drop slowly and steadily down to zilch.  
"Hmm... I think, this is not good.  It's not overheating... but 
underheating."  drive down the neighborhood roads at 40mph for a mile or 
two and it slowly climbs back to the highway level of warm- but not hot.

If, when I pulled off the highway, my thermostat was stuck open, would 
it be sending through the engine block all sorts of cold coolant that it 
really didn't want?

Mr. Bentley's picture book (I'm sorry, but I know X programmers that 
document things better...) does not describe the flow or behaviour of my 
cooling system, perhaps leaving it as a homework exercise for the 
reader.

My previous experiences with cooling systems have all been pretty simple 
and dealing with leaky hoses and leaky water pumps.  And the symptoms 
have been pretty obvious, including running hot or puddles under the 
car...  This is strange to me.  My other theories are a bad temp gauge 
sender- unlikely, given the gauges consistent behaviour, a dead water 
pump, which would cause over-heating, and just plain old mean and 
vengeful Audi gods.

Why in the world must a water pump go ~ every 100k?  You know, Mazda's 
have an anecdotally supported propensity for blowing their alternators 
every 80k, but that's all that ever goes wrong with the things...  Of 
course, I'm not going to own a Mazda...

Ah, well...

Any help or suggestions that could be discussed on the list and/or sent 
to me at dennisr@altia.com would be greatly appreciated.  Bryan Lally's 
recent pressure cap solution will be tried as soon as possible (which 
would be as soon as I can swap the cap with the one on my wife's 90).

Audieux,

Dennis

-- 
Dennis M. Ruffing, Altia, Inc.  
719-598-4299
dennisr@altia.com,  http://www.altia.com
~~~~~ Put a smile on your face.  Drive a Quattro. ~~~~~