Radiator / cooling thermostat

JulesW

A2OC Donor
I seem to have fluctuating engine temperature. I'm sure when I bought my 1.4 TDI in May that it was dead central and showing 90 C. But since then it runs lower (and very occasionally at the vertical 90 C position). Sometimes it barely reaches the 80 C mark. This is all very frustrating as I had requested it be swapped as part of a full service I just had. The guy said he had measured it and it was fine for the TDI.

So my first question is at what temperature should the thermostat open? I presumed 90 C or just below.

My second question is does a cool running engine impact on mpg or general running of the car. Certainly I don't want a cool engine in winter if I want a warm cabin I guess.

Is it a big job and does everyone recommend genuine VAG thermostat or are there good equivalents?

Thanks in anticipation.
 

Special edition

Admin Team
Fuel economy can be affected by a faulty thermostat in a negative way. I personally would stick with genuine Audi thermostats and they are quite easy to fit if you know how.
 

audifan

A2OC Donor
Genuine thermostat starts to open at about 81 C. Personally I would replace the thermostat AND the temperature sensor for genuine parts. I would then flush and refill the system with the correct G12-G13 coolant. Get premixed coolant far easier and you can top up straight from the container if needed. Bottom fixing on the thermostat bit awkward but several threads on here, temp sensor get a second clip and tie string to it so you can retreive it when you drop it into the engine bay...
 

Proghound

Admin Team
The standard thermostat should be fully open at 87°C. What you might have here is a faulty temperature sensor i.e. the thermostat could be just fine but what the gauge is showing you is wrong.

Cool running engines do indeed use more fuel, the ECU uses different fuel mappings depending on engine temperature. The leanest mapping is employed from 87° i.e. when the engine is up to temperature.

But, and here's the fun part, the temperature sensor has two circuits, one that feeds the gauge on the dash and one for the ECU. So, independently of what the gauge is showing you, it might or might not be telling the ECU the wrong temperature and leading it to use the wrong fuel mapping. Only viewing the live ECU data using VCDS will confirm what it's seeing (unless you're lucky enough to have a Colour DIS from @timmus).

Normally you would replace both thermostat and temperature sender together to be sure all is well. As already advised, go OEM on both. It's not a big job in garage labour terms.
 
Last edited:

Pelican

Member
Cool running engines do indeed use more fuel, the ECU uses different fuel mappings depending on engine temperature. The leanest mapping is employed from 87° i.e. when the engine is up to temperature.
On my 1.4 petrol, while driving at longer distances, but with "relaxed" speeds of around 30-40 mph, and low demand on the throttle, I find that the coolant temp needle drops slight below 90ª, sometimes it´s around 80º. Then, if I drive "harder" it goes back to 90º.

I´m assuming this has nothing to do with a faulty thermostat, right?
 

Proghound

Admin Team
On my 1.4 petrol, while driving at longer distances, but with "relaxed" speeds of around 30-40 mph, and low demand on the throttle, I find that the coolant temp needle drops slight below 90ª, sometimes it´s around 80º. Then, if I drive "harder" it goes back to 90º.

I´m assuming this has nothing to do with a faulty thermostat, right?
Assuming that your temp sender is telling you the truth, that sounds like a thermostat sticking open to me. It should continually adjust itself more open or more closed in order to maintain the engine temp at 90°. I had similar on a Vectra in a previous life, it would cool down from 90 as I went down a long incline on a motorway for example.
 
Last edited:

Bargepower

A2OC Donor
On my 1.4 petrol, while driving at longer distances, but with "relaxed" speeds of around 30-40 mph, and low demand on the throttle, I find that the coolant temp needle drops slight below 90ª, sometimes it´s around 80º. Then, if I drive "harder" it goes back to 90º.

I´m assuming this has nothing to do with a faulty thermostat, right?
I do many long distance journeys, and in the three different 1.4 A2's I've had (well, four if you count the brand new one I had in 2001), the needle is pegged at 90 degrees.
I'd replace the thermostat if I were you. It's a 20 minute job.
 

JulesW

A2OC Donor
Thanks for the very prompt and numerous responses.

I have one of those Infra-Red temperature sensors. So will aim that around the engine bay and see what measurements I get from that. If that is all around 87-90 then I guess I can assume that the physical thermostat is correct. But compare those readings with the temperature display inside and, if they are far apart, then it is the sensor which is probably faulty.

Thanks for the info on there being two temperature sensors.
 

dj_efk

A2OC Donor
If it helps, the part number for the sensor is 059919501A; the part number for the thermostat is 044121113.

I decided to replace both on my car as a precaution with genuine parts as I've read of others on here faffing about with cheaper / aftermarket items (even from relative good OE supplier makes) that have turned out to be defective.
 

audifan

A2OC Donor
Do not misunderstand. There is ONE temp sensor that internally has two circuits, one for the gauge the other for the ECU. Either faulty not good. Ask is @CreweAudi can supply both thermostat and temp sensor then change both now rather than wait for the inevitable winter rush. If the thermostat is slow to react then the temp can drop. Situation is worse on the diesels as they produce less heat.
 

Silverpig

Member
Morning all!

based on the above recommendation I’ve bought a thermostat in the hope that it will sort things out once and for all.......this issue is truly my nemesis!

Having removed the 6 month old Circoli unit, I’m now slightly puzzled by the o-ring which is supplied with the new one. Which side of the thermostat does it go?

It’s such a hateful job to undo the housing and faff around with the coolant replacement so I don’t want to have to mess around again, so can anyone advise on the correct siting of the rubber seals?

enclosed photo shows the new one on the left and the one I’ve just removed on the right.

many thanks!!!
 

Attachments

audifan

A2OC Donor
Forgot to mention, only the correct thermostat really fits into the plastic pipe. It is twisted in the pipe and the little plastic fingers in the end of the pipe lock the thermostat in. If the wrong thermostat has been used the fingers will be broken off as the upper part of the thermostat had the wrong shape.
 

Silverpig

Member
Thanks, it’s a very slighty different looking set up on my TDi, I’ve put it back in like that, just curious that the Circoli thermostat has an o-ring pre-fitted which fits around both sides of the thermostat and the ‘genuine’ one just sits on top between the thermostat and the plastic housing.

Going to refill the coolant now, you’ll probably hear my screams of despair if I’ve got it wrong and it starts leaking.....
 

Sootywg

A2OC Donor
Thanks, it’s a very slighty different looking set up on my TDi, I’ve put it back in like that, just curious that the Circoli thermostat has an o-ring pre-fitted which fits around both sides of the thermostat and the ‘genuine’ one just sits on top between the thermostat and the plastic housing.

Going to refill the coolant now, you’ll probably hear my screams of despair if I’ve got it wrong and it starts leaking.....
Did all go well? I am about to replace mine, so encouragement would be welcomed! Had a trial at slackening off the lower bolt to no avail. Considering removing the alternator!
 

audifan

A2OC Donor
@Sootywg
Here is my thread that links in another, both helpful.


WP_20200401_23_33_32_Pro.jpg


My version on the right.
 
Top