Running temperatures that fluctuate?

[wolfsburggolf]

Hi all,

In the search for finding out why my car is weak over 3,500 rpm, I've recently had a brand new water temperature sensor fitted, and my gauge shows me that every so often the car falls below 90 degrees (round about 80-85 I'd say).

Is this normal?

Every other car I've owned has stayed at 90 degrees once it's there. Never owned a car that falls below and then goes back up to temperature randomly?

The new sensor isn't faulty because there are no error codes during diagnostic checks.

The car is an Audi A3 1.8T sport

[S0 What]

Ae you sure the gauge sender and the one the ECU reads for cold fueling are one and the same ? some are but most are seperate senders.
If its an RPM related problem i'd be looking at the TPS and or map/maf sensor although as you say they should throw up a fault code, TBH any emmisions related problem should bring up the managment/engine check light ?

[wolfsburggolf]

S0 What said:

Ae you sure the gauge sender and the one the ECU reads for cold fueling are one and the same ? some are but most are seperate senders.
If its an RPM related problem i'd be looking at the TPS and or map/maf sensor although as you say they should throw up a fault code, TBH any emmisions related problem should bring up the managment/engine check light ?

I thought the same. There are absolutely no fault codes. The previous faulty water temp sensor showed up in a fault code, but now....it's "green lights and go", APARANTLY? :S

[Mister3man]

Sounds like you may have a dodgy thermostat.

edit: and this would have very little to do with your lack of power, that sounds like a seperate issue all together.

HTH

[djdestiny]

Most probably the thermostat, or could be an air lock in the coolant system.
If it has climate control, you are able to bring up diagnotics and get a reading of the coolant temp from the sensor the climate uses and verify it that way

[fozzymandeus]

Audis don't usually fluctuate.

BMWs never do. It's all down to the latency in the cooling system.

French stuff usually does like fun. Don't worry about it, unless it looks like it's going to boil then shut it off sharpish.

[Mr MXT]

I had an MR2T that would get up to temp then drop back down again if I opened the throttle fully.

It was HGF.

[wolfsburggolf]

fozzymandeus said:

Audis don't usually fluctuate.

BMWs never do. It's all down to the latency in the cooling system.

French stuff usually does like fun. Don't worry about it, unless it looks like it's going to boil then shut it off sharpish.

Yea, it's never gone over 90 (apart from when the water pump failed.....hehe)

I'll keep a close eye but will see about a new thermostat anyway

[Purple Tom]

Classic 20VT thermostat problem and very common. Had the same with an A3 I owned a few years ago, changed the sensor and it made no difference.

The ECU is what controls the temperature gauge, the ECU being fed by the coolant temperature sensor. The gauge will sit at 90º as long as the coolant temperature of the engine is within 'normal operating parameters' - determined by the programming of the ECU. Once the coolant temperature rises or falls beyond normal operating parameters then the gauge will move to follow suit and alert the driver.

Don't worry about it - get your thermostat changed (or do it yourself, it's not too difficult apart from the inlet manifold obstructing access a bit) and the gauge will behave itself again.

You might find that performance improves again and you should also see an improvement in MPG as well.

Hope that helps :-)

[fozzymandeus]

Now I think about it we've had a thermostat failure on the TT!

Another typical characteristic is the "always open" behaviour: running at about 70deg, then rising to 90deg (exactly) when the car was left standing for a few minutes with the engine off "decay heat" styley.

New thermostat solved the problem despite EVERYONE trying to tell me it was the dashpod. The fools.

[Purple Tom]

fozzymandeus said:

Another typical characteristic is the "always open" behaviour: running at about 70deg, then rising to 90deg (exactly) when the car was left standing for a few minutes with the engine off "decay heat" styley.

Yeah, that's another classic characteristic of it too. Once the car is left standing and the coolant temperature rises into normal operating parameters then the gauge suddenly pings up to 90 degrees to tell you that all is ok.

It's in direct comparison to the fully linear gauges fitted to older VW's which would directly follow the coolant temperature. Quite alarming at times when you have a quick look at the gauge in traffic and it'd be way up towards 110 degrees. Obviously not an issue in a pressurized system where water boils at a higher temperature but I imagine that the alarming feeling might be why VAG moved towards ECU-controlled gauges - I wonder if they got lots of calls from people who's cars had 'overheated' but then actually hadn't at all?

Either way - get the thermostat done and it'll all be ok!