A4 1,8 TS coolant temp reads low

Hi all,
My 2001 A4 Avant 1,8 TS (8D chassis, corresponding to B5 saloon, motor variant AJL) used to have coolant temp at a consistent 90C (is that
192F) during suburban driving, ambient temp between 0C-10C (32F-50F)
About one month ago it started showing coolant temp at a lower temp of 86C (183F).
Oil temp has been and still is consistent at ca. 80C = 175F.
I suspect that either 1) the temperature sensor (mounted on top edge of head, aft) may be giving a bad reading - or that 2) some thermostatic valve is stuck, allowing 100% of all coolant to flow throught the radiator, even if ambient temp is low.
But - on Audiworld I read articles that broken temp sensors tend to fluctuate wildly, which mine does not do. Mine just reads low. So this may indicate that I have something else wrong - but what?
Those of you - having a similar experience yourself - who managed to fix this, can you pls tell me what you did?
Thanks, Robert in Gothenburg Sweden, where we're still waiting for winter.
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Robert wrote:

It sounds more like a bad (open) thermostat. They respond to engine temperature not ambient.
Changing the temp sensor is probably worth a try and is a good idea anyway as they are prone to failure, easy to change and cheap but the thermostat is more likely to be the problem.
Changing the thermostat is usually done when the timing belt is changed. It is a large job and if you chose to do it be sure to have the timing belt and all associated parts done at the same time. The parts are the cheap part of the job and it needs to be done on or before scheduled time / distance anyway.
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TonyJ knastet inn: :

changed thermostat

agree
... when they also (should) change the water pump.
Changing the thermostat, is quite easy - done in 30-60 minutes.
But - changing just the thermostat alone, may be /the last nail in the coffin/ for the water pump [if it's old], due to a higher pressure caused by the increased temperature afterwards. And then, if you have to replace the water pump, replace timing belt as well.

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Frank wrote:

Thanks guys,
I will indeed change the temp sensor.
As far as water pump goes, I am told by my Audi indy that the water pump on my motor variant (AJL) is *not* driven off of the timing belt, though it seems to be a universal truth on this forum that all water pumps are.
My timing belt was changed one year ago at 110 000 km and I find myself sleeping better at night as a result.
. . . I will nonetheless be left with a more weird problem, that the car is very hard to start about 10-30 mins after shutoff, requiring the starter motor to spin for ca. 15 sec before the motor starts . . . more in another thread I will start shortly.
Cheers, Robert
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True - your water pump is driven by a small belt taken from the power steering pump - which is spun by the big ribbed belt...
/daytripper '00 s4 6spd
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Robert knastet inn: :

That was not the point ...
To replace the water pump _or_ the timing belt, 90% of the job is done - replacing the other one as well. The thumb rule - is to change both at the same time, to save work hours
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Change the timing belt, water pump and any idler bearings while replacing one or the other. Sometimes it is considered "Preventative Maintenance". Kinda like if you need to replace the clutch...........change the rear crankshaft seal and pilot bearing too!
AFAIK that has been the rule since the Audi 5000, although the rule is stronger when the timing belt and interference engine could be damaged by a damaged water pump. :-)
Some Audis, like my 4000S with the 4 cylinder, that rule is NA! :-)
JMHO later, dave (One out of many daves)

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I just ran into the same exact problem on the same application. It was the thermostat.I picked up the part at the dealer.If you are not mechanically inclined do not attempt to replace it.It has a plastic housing and is behind the alternator and steering pump.It also has a sensor on the housing.I went through the same thing replacing the green sensor,it was not the problem.when i removed the whole housing and took it apart i found that the plastic tabs that held the thermostat in place broke off which caused the thermostat to remain open.I purchased the whole housing,it came with everthing that i needed,it was a little over a $100 but worth it.replaced it and problem resolved.
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moparmadman wrote:

Thanks for this good info. I have just replaced the temp sensor but I notice that the temp is now only a couple of degrees higher, so it may indeed be a sticking thermostat.
Some questions:
1) was your only problem a low temperature or did you have other symptoms that caused you to want to replace the thermostat?
2) did you have to remove more than the undertray to get to the pump, or did you also have to remove the cowling around the timing belt?
3) were the plastic tabs broken through general wear and age, or did they get broken as a result of your dismantling job?
4) Can you say more about this sensor on the housing - I cannot see one in the pics on the Haynes manual, but their material is incomplete as a matter of course.
Grateful for any additional info you can provide.
Thanks, Robert
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1.yes low temp was the only problem. 2.the thermostat was not near the pump or the timibelt area,it was behind the power steering pump.if you follow the lower radiator hose from the bottom up it leads to the plastic housing that bolts to the block,inside it is the thermostat. 3.it was wear and age.dont forget the housing is plastic.and gets very hot and cools down constantly. 4.if you get the thermostat from the dealer the sensor is on the housing. hopefully this helps.
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moparmadman wrote:

Yes thanks, that's a useful set of answers. I think I'll go buy a thermostat in the next week or so, and try to make do with the existing housing.
Cheers, Robert
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