Strange coolant temp guage movement - 98 A4 V6

This just started happening. On cold mornings the engine takes a while to warm up - say 4-5 minutes after driving. Used to be much quicker.
Now the really odd thing is the water temp gauge (no degrees, just
marks) used to be just to the left (cool side) of top dead center when warm. Now, it will go straight up, then in a few minutes, actually fall to almost where the "C" is and then move up again. Would a bad thermostat cause the needle to swing wildly like this? I originally thought it was the thermostat until the gauge did this strange maneuver and now I am wondering if is the sensor out of whack.
The oil temp seems to be fine.
98 Audi A4 V6, 2.8l Quattro, 5spd. US model.
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Could be the Tstat or sensor but it could also be low coolant and a bubble in the system.
Ed H. wrote:

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I considered low coolant, but the overflow is well above the "Min" line. I guess I'll take it in early next week to get checked out. Thanks.

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It's probably the sensor. I had to have mine replaced on the same car - around $200 at the dealer IIRC. Thermostats usually fail by sticking in some position so the behavior you describe is unlikely. .
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Ian writes:

I agree. But, on the chance it *is* a thermostat, while you down and in there, ya might as well replace the timing belt and the water pump.
Dave RS6
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there,
Yes, definitely, if you're replacing the thermostat, the TB and WP should also be considered unless they've been recently replaced. I still think the OP will discover it's the sensor.
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snipped-for-privacy@aol.comPirate (Dave LaCourse) wrote:

Well, I hope it is the sensor. I described the behavior to them Friday and they agreed with the general opinion. I had the timing belt and timing belt tensioner replaced in December. I'd hate to go through much of that expense again if it is the thermostat. :-|
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You /did/ have the water pump and thermostat changed at the same time, didn't you?

Indeed.
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No. :-( Lesson learned. Assuming my problem isn't the thermostat and it lasts another 60K miles, I'll get both changed at the next TB change at around 200,000 miles.
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wrote:

Funny you mention this because my own A4, identical model and vintage as yours, does/did the same thing after getting a whole new TB/TB tensioner, and water pump at 69k miles. Before this work was done, the coolant gauge would stay rock solid right on the middle hash mark, and afterward it would wander up almost to the next hash mark and back down and back again. This wandering behavior has stopped for the time being (our temps are freezing right now, with lots of snow and ice on the ground, reaffirming the perfection of the A4 for my driving needs). The gauge never passes that next hash mark except when the engine is turned off and the car is hot; once I get the engine started and the radiator cooling, the gauge moves back down again. I've lived with this now for almost $15k miles and just keep my eye on it for the reason you state: I really don't want to fork over the labor $$ to change out the sensor/thermostat.
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Odd. My gauge drops to "C" as soon as I turn the key off - always has. I assumed the gauge was electric and regardless of the engine temp, it would not register if the car was off.
The car warms up on the inside as fast as ever, so the thermostat can't be sticking open. The mechanic told me it would take 10-15 minutes before I got heat if that were the case, which makes sense. The erratic behavior has subsided a bit. Wondering if it is an anomaly similar to what you described after having your tb/tb tensioner replaced? I am going to keep an eye on it and the oil temp. The car is warming up - gets to *almost* 200F on the oil temp when driven for extended periods (longer than 30 minutes) and is well above 175F even for 5-10 minutes trips. Never gets above that in the winter. I've seen it approach 250F in the dead of summer - gets above 95F here in TN.
More and more this looks like a faulty gauge sensor.
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The easiest thing to replace is the temp sensor. I just had an issue with my 2001 A8 where my needle used to run about 1/4 on the gauge and then started going to half, and eventually 3/4. The car had plenty of heat and although it never overheated, it would run high enough so the electric fan would click in on high speed. It turned out to be a thermostat. If it were a Chevy, it would have been about 4 bucks and a 15 minute job but seeing how it's an Audi, it was $2026 and 2 days. It was about time to do a timing belt so that service, along with a water pump, pulleys, thermostat, tensioner, serpentine belt, and antifreeze were all replaced at once. Just the thermostat would have been about $1000 due to the labor involved in getting to the $14 part.
If your car has around 80k on it and the timing belt hasn't been serviced, you should get it done. I had a 98 A6 that I did the timing belt on and it wasn't that difficult. You have to buy a couple of special tools though. One tool locks the crankshaft at the top dead center position for the number one cylinder and the other holds the cams in place so you can keep them aligned with the crank shaft. I think they are about $140 for both. The parts you will need will cost you about $200-$300 Wich will include thermostat and gasket, water pump and gasket, timing belt, crankshaft seal, serpentine belt, tensioner pulley, and idler pully.
Good luck! Dennis
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