Help! Temp sensor replaced now reads lower!! Genuine VAG

[mac456]

My car takes around 7 miles to get to normal, not sure if this is normal but it does take longer than my old mk4

[xjay1337]

Yes these engines can take a bit of time to warm up, it is normal. When it's colder outside it will be slower..

[OSB]

If the coolant  temp got up to 90 in a reasonable time with the new (tested) thermostat and the old temp sensor, and now the temp reads low/uneven with the new (untested) temp sensor - suggest you change the new sensor back to the old one and see what happens.

There must be a set of figures for temp/ohms resistance for a new sensor somewhere! Couldn't find one when I looked a few weeks back ...

[callis]

As above if your old sensor worked then pop it back in. I brought my first thermo from Euro car parts, and that didn't work, so i brought a gen one from VW and had no issues after fitting it.

Still sounds like a dodgy thermo tbh, as i had same issues before i changed mine. Maybe the TPS one works at a slightly different temp. My car takes around 10mins to heat up full, but then again i am in scotland at the mo and its bloody nippy.

[Itguy]

Ok - update.

I left the new temp sensor in today and after driving about 15 mins/15 miles earlier it did get to 90 on the gauge and stay there, so it is ok. Outside temp was 4.5c.

I'm coming to the conclusion  that this is just a slow engine to warm up, I can't see anything else wrong.

Does anyone know/understand how the plumbing works on the AXX engine? I'm wondering if it's a problem with the thermostat bypass function for example?

How do I test / see if the aux coolant pump is running too (without VCDS which I don't have)?

Thanks

[Itguy]

Ok, another update.

Seems as though something still isn't right as tonight on the way home the temp went up to 75 and hovered there. I then sat in stationary traffic and it went up to 90 and sat there.

Moved off again and it settled back at 75 on the gauge.

It did this several times up and down depending on the traffic I was in.

It didn't do this before I fitted the new sensor so I'm suspecting it is under reading the temp and what I can see on the gauge is just normal fluctuations of temp going between 85 and 100 which would normally be shown by the gauge as 90. I've read somewhere that the gauge shows a small range of temps as 90 to stop the driver from worrying that things are wrong when it's minor fluctuations.

Having just got home I need to leave the car to cool down a bit before I can swap the old sensor back in to check the theory.

Obviously if it does the same with the old sensor in then it's the thermostat.

[xjay1337]

That is PURE symptoms of a faulty thermostat. (heats up stationary and cools when driving).

[doylebros]

Or as you said the bypass thermostat or V50 pump.

I've emailed you some component drawings and operational temperatures which may help.

[Itguy]

Well it's -2c outside now so it will have to wait for another day when it's a bit warmer!

First step I think is to put the old temp sensor in and see what happens.

I did test the new thermostat as previously said, it didn't even start to open until 86c in my boiling pan of water. Unless it's failed straight after fitting?

Bypass thermostat? Is that somewhere else?

[vRSAlex]

Before changing my thermo it wouldn't get above 70 even in traffic. With the new one in it gets up to full temp within 5 to 10 mins. Always sits around 90 now.

Boost is back too ;)

[Itguy]

The more I read the more I'm convinced now it is one of the two(!) additional bypass thermostats that are in the cooling system.

My car does warm up but slowly.

My car has one in the coolant hose that runs the aux pump for turbo cooling after switch off and then another for the gearbox oil cooler by the looks of it.

Why they had to make a cooling system with three thermostats in is beyond me. I know the new main thermo is ok because ive tested it.

[doylebros]

Have a look here at the cooling section

http://www.mk5golfgti.co.uk/forum/index.php/topic,1797.0.html

[doylebros]

http://www.mk5golfgti.co.uk/forum/index.php/topic,75108.0.html

Online Parts catalogue

Go to illustration 121-030

That shows the full system for your vehicle

Then go to illustration 121-075 shows the regulator ( thermostat )

[Itguy]

Right.

Next step for the weekend is to change the two bypass thermostats. They are c.£20 each so another £40 but they are the only remaining 'active' parts of the system with the exception of the aux pump.

I suspect my local TPS won't have them in stock so it will need to be a weekend job but fingers crossed this will sort it once and for all. I'll test them both before fitting too. What what I've read they are supposed to start opening at 75 and be fully open at 87.

[Itguy]

7-10 days for getting a new bypass thermostat so instead of ordering blind I'm going to test the ones in the car at the weekend to see if I need one, two or none at all! Yes it will mean I need to drop the coolant twice but hey ho, at least I will know if it is actually the fault and what to order.