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Which thermostat to look for or avoid for 2003 S16 138
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Bordonbert

Posted: Wed Mar 29, 2023 4:36 pm

Joined: Jul 05, 2022
Posts: 16
Trade Rating: 0

Hi guys. The thermostat on my 2003 GTi 138 seems to be stuck open as temperature is staying one notch low at around 80deg. If I am stuck in a crappy tailback on the M3 it takes a few minutes to climb to 90 and drifts on to just under 100 but then drops back to 90 as the fan kicks in. As soon as I drive off again it sinks back to 80. There seem to be no other issues with its running so I'm changing the thermostat as a first move.

I know the job is pretty simple, just drain, remove hose, remove 2x10mm bolts, swap, refit and bleed. I've got a lot of experience in general so don't see any real problems, but I'm a bit bewildered as to the number of makes of thermostat + housing kits there are around. I can go for something generic by guessing of course but are there any particular names to look for or avoid?


	They mock and say ask a stupid question? "A moment of embarrassment is better than a lifetime of ignorance".

Bordonbert

Posted: Thu Apr 13, 2023 3:23 pm

Joined: Jul 05, 2022
Posts: 16
Trade Rating: 0

No thoughts offered on this one so I just did what came naturally and replaced with a mid-price generic type. As parts were cheap I changed out the temperature sensor at the same time though that was just for convenience.

I opened the bottom hose connection and caught the coolant to have a look at it. It has been changed just a year or two ago and, while it is not overly expensive to replace, it was absolutely clean and fluorescent so I only drained a "headful". I checked this by opening up the highest point, the radiator top hose connection, to allow air in and followed with the actual thermostat housing connection afterwards. I did lose a little coolant in the process as you can imagine but it was less than a glassful and was easily topped up with a little fresh 50/50 mix. Checking the level in the header tank is a royal pain in the rear! A torch shone in the top or thought the side doesn't get through in daytime so I had to check it in the dark.

Now the point is not to chart the replacement of a thermostat, that's been done many times before, but there is one unusual aspect. I had received a couple of engine light warnings for "Anti-Pollution Fault". That was odd because the exhaust and cat are not too old and the sensors were replaced just before that too. The first had occurred when running at motorway speed. I checked this out with Torque and the fault code showed a Lambda Sensor Fault on Bank 1 Position 2. The rear sensor, only there for engine diagnostics, seemed shafted. However, as they do, it cleared itself a few starts later. Then a couple of weeks later it appeared again when I started up from cold. This time it wouldn't clear even through Torque.

So I replaced the thermostat which I had and and ordered a new sensor. Whaddya know, while waiting for the sensor to arrive the fault cleared again. I'm now wondering if the thermostat being soft and causing the engine to run colder than normal (steady 80degC instead of a steady 90degC) would have affected the mixture slightly and caused that fault to appear when the rear sensor was not reading what it expected permanently. I wonder was the diagnostic program assuming the sensor was faulty because the temperature was permanently low no doubt screwing what it expected to see for the mix.

Anyway, just an interesting observation for anyone who sees something similar. Anyone have any similar experience or knowledge of what was going on?


	They mock and say ask a stupid question? "A moment of embarrassment is better than a lifetime of ignorance".

Steve206

Posted: Sun Apr 16, 2023 4:25 pm

Joined: Feb 07, 2010
Posts: 2716
Trade Rating: +6
Location: UK

The engine was probably running a bit rich as the ECU thought the engine was cold. I know what you mean about reading the water level, that plastic is so thick light doesn't shine through!


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