[s-cars] Local, intermittent overheating !???!?!

Thu Dec 16 08:18:09 EST 2004

Hi Paul,

I agree with the sticking Tstat causing overheating but the temp reading
story is wirder than your explanation.

When the car is overheating, the dash temp is not only 20C below the channel
51 reading, it's perfectly stable at 90C whatever the channel 51 read, from
96to 118C. What's puzzling me is not the temperature difference but the fact
that the MFTS(dash)has a perfectly stable reading when the car is
overheating and has not when the channel 51 is stable at 89C. When the
channel 51 suddenly drop from 104C to 92C, probably due to a sudden
circulation of coolant, the dash temperature fall too, but when the channel
51 is reading 96C and climbing, the dash read 90C and stay there even when
the channel 51 reach 118C.

I've suspected an air bubble to cause my low dash reading before and a
larger air bubble could also cause the overheating - I'll change my Tstat
anyway, don't want to take a chance - but I have no explanation on why the
dash temp is stable at 90C when the car is overheating.

Vincent.

-----Message d'origine-----
De : Paul Gailus [mailto:gailus at mindspring.com] 
Envoyé : 16 décembre 2004 00:20
À : Mark Strangways; Vincent Frégeac; 'audi s-cars list'
Objet : Re: [s-cars] Local, intermittent overheating !???!?!

I agree with Mark that the thermostat sounds like it's defective and should
be replaced.

>Why the dash temp is right on 90C and very stable when the channel 51 
>says it's overheating and displays temperature from 92C to 118C.

A  possible explanation:
If the thermostat is closed then this directly blocks the return flow from
the radiator. This stops the flow of heated engine water into the top
radiator hose through the coolant manifold. Since the MFTS is on the coolant
manifold, under this situation the dash guage could read considerably lower
than the coolant temperature within the engine.

However, as long as the heater valve is open (the heat is on) then the
temperature sensor in the heater hose will get hot water flow directly from
the engine. Therefore channel 51 should track engine temperature reasonably
well, including any overheating that may be occuring.

It sounds like the thermostat may be sticking. Or perhaps some of the wax
inside the thermostat has leaked out so the coolant has to get very hot
before the remaining wax expands enough to open the valve. Once the
thermostat valve  does open, then there would be enough flow to get the
MFTS/dash guage and channel 51 to correlate reasonably well with eachother.
But driving on the highway may cool the flow through the radiator enough
that the thermostat closes and it starts overheating again.

Paul

----- Original Message -----
From: Mark Strangways <Strangconst at rogers.com>
To: Vincent Frégeac <vfregeac at sympatico.ca>; 'audi s-cars list'
<s-car-list at audifans.com>
Sent: Wednesday, December 15, 2004 7:45 PM
Subject: Re: [s-cars] Local, intermittent overheating !???!?!

> There just might be some air in the head, not likely though. With a 
> thermostatically controlled oil cooler it will get to 100 C or so
and
> hold there if it can.
>
> My best guess would be to get that thermostat out ASAP. Don't wait as 
> you risk trashing your head, the aluminum one, not the French Canadian 
> one.
>
> Mark
> ----- Original Message -----
> From: "Vincent Frégeac" <vfregeac at sympatico.ca>
> To: "'audi s-cars list'" <s-car-list at audifans.com>
> Sent: Wednesday, December 15, 2004 7:57 PM
> Subject: [s-cars] Local, intermittent overheating !???!?!
>
>
> Hi all,
>
> Once again, my S6 is puzzling me. On my way to work, this morning, I
noticed
> the check light was on. I started to look at gauges to see if 
> something
was
> really going wrong and, no, every needles are right where they should 
> be. Huh? Every needles? My car is one of those who shows 70 something 
> as
coolant
> temp on the highway when the channel 51 displays a very stable 89-90C.
This
> time, the coolant temp on the dash is right on 90C. So I started to 
> play with the HVAC controller and channel 51 shows 112C! How! The next 
> exit is within reach. In less than 500m, the channel 51 reach 118C. 
> Something
really
> goes wrong. So I took the first exit - to notice that channel 51 
> dropped
to
> 111C during the exit ramp, probably because I'm coasting - park the 
> car in the gaz station parking lot and pop up the hood. Coolant 
> everywhere, one boiling inch remaining in the bottom of the coolant 
> tank and electric fan working. That's the first time I see it working 
> and I notice it's out of balance or rubbing somewhere as it's 
> massively vibrating.
>
> So, first step, let everything cool down, the engine and my head. 
> Towing
my
> car is the last solution I want to think of as I'm in the middle of
nowhere
> and it will probably take two hours before I can have the required 
> flat
bed.
> I'm 30 min. from home. If I drive 10 min, stop 10 min, I can probably
reach
> my trusty 90Q20V and be at work before the second meeting. So, I look 
> for water - no way to find the pink stuff in the middle of nowhere - 
> feel the coolant tank, wait until the channel 51 display a confortable 
> 92 and start the trip back to home to see how long it will take before 
> I reach 105C,
the
> point where I've decided to stop and let the car cool down again. 
> After
1min
> of driving, the car have cooled down to 89C and stay there. 5km later,
it's
> still at 89C. So, as I'm not so far from work, I decided to take a 
> chance. And the channel 51 temp stayed at 89C, perfectly stable, when 
> the dash displayed the usual 70C. Some shit happened, I don't know 
> what, buit now it's gone. Good news.
>
> So this evening, back to home. I took no chance and tuned the HVAC on 
> channel 51 from the beginning. Temp is climbing at it's usual pace, 
> everything seems OK and when I reach the highway, 5min later, the 
> channel
51
> just reached 87C. 10 seconds later, it reach the usual 89C while the 
> dash
is
> still at 70C, everything is as usual. But it doesn't stop there, 90, 
> 91, 92... Up to 99C where it seems to stabilize, probably because I'm 
> going at
a
> slow 55mph to try to slow down the temperature raise. Looking at the 
> dash, the water temp is right in 90C, oil temp is just below the first 
> thin mark as usual. And I start the drive-cool down cycles until I 
> arrive home. At
one
> point, while driving on the highway, the channel 51 suddenly fall from
104C
> to 92C while the dash temp was falling from 90C to the usual 70C. 
> Then, it started to climb back to the 105-110C which was the maximum I 
> allowed
before
> I stop the engine for cool down.
>
> Now, in my mind, all that point to an intermitently stuck close Tstat, 
> especially the temp falling down from 104 to 92 while cruising on the 
> highway, but I've still some points I would need your usual wisdom:
> - Why the dash temp is right on 90C and very stable when the channel 
> 51
says
> it's overheating and displays temperature from 92C to 118C.
> - Why the oil temp stays perfectly stable, just below the first thin 
> mark, where it has always been - except during very spirited driving - 
> when the water temperature is everywhere between 89 and 118C.
> - Why the dash temperature is stable (independant of speed) when the
channel
> 51 fluctuate (overheating) and why the dash temperature fluctuate
(depending
> on speed) when the channel 51 is stable at 89C.
>
> Besides, I've never heard of a Tstat stuck close, intermitently or 
> not.
Any
> BTDT?
>
>
>
>
> Vincent.
>
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