Fluctuating Temperature gauge

[mboucher70 at hotmail.com]

Thanks for the suggestions.  The reason why I discounted the temperature
sender is that I figure if it was bad it wouldn't just fluctuate on the
first warmup, it would continue to fluctuate on the rest of the drive. 
i.e.,
it drops from warm to zero 10 minutes after i start a drive, 10 minutes
later its back at normal, and remains there for the rest of a 6 hour drive
(did a few long drives recently).

And as you said, a t-stat won't act that quickly and drop the temp all the 
way to cold when the car is already warmed up.

I was wondering about other possibilities...such as ...is it possible for 
air to get into the cooling system such that when the t-stat starts to open, 
the temperature sender (which I believe is at the top) is in an air-gap thus 
explaining the drop...and then the air-gap fills in a minute or so and the 
temperature returns to normal.

Another trend that I've seen with this...it seems to only happen when the 
car hasn't been driven for a week or so.

There's been no loss of coolant btw.

thanks
MC




--------------------------------------------------
From: "Grant" <glenahan at vfemail.net>
Sent: Monday, December 13, 2010 11:08 AM
To: <quattro at audifans.com>
Subject: Re: Fluctuating Temperature gauge

> A t-stat wont act that quickly.
>
> Grant
> On 12/12/2010 10:55 PM, Steve Buchholz wrote:
>> While it may be a temp sender for the gauge, don't neglect the 
>> thermostat.  I don't think a faulty gauge sender would act so 
>> predictably.
>>
>> Steven Buchholz (mobile)
>>
>> ----- Reply message -----
>> From: "mboucher70 at hotmail.com"<mboucher70 at hotmail.com>
>> Date: Sun, Dec 12, 2010 12:15 pm
>> Subject: Fluctuating Temperature gauge
>> To:<quattro at audifans.com>
>>
>> Any ideas on this?  Start the car from cold, coolant temperature rises 
>> steadily over a 10-15 minute period.  Just as its approaching normal 
>> temperature, it reverses direction rapidly and goes from needle almost 
>> straight up to fully cold in about 30 seconds.  Warm air continues to 
>> blow out the heater core so I know its not actually cold.  Fan hasn't 
>> tripped on yet, so thats not an issue.
>>
>> Then, a few minutes later, temperature gauge restarts its rise from fully 
>> cold, and is back to warmed-up (needle straight up).  From this point on, 
>> everything functions normally.  I could drive the rest of the day and its 
>> not going to repeat the pattern.  If I get stuck in traffic, the fan 
>> eventually kicks in and cycles normally.
>>
>> Next day, if I start the car from cold, it does the same thing.
>>
>> I'd suspect an electrical fault or grounding problem if this was 
>> repeating during a long drive.  But since it only happens once, on the 
>> first warm-up cycle of the day, I'm thinking it must be something else.
>>
>> Thanks for any ideas
>>
>> MC
>> Audi 100 1990, 2.3L I-5 NF engine
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