Thermostat advice needed

[Monzoa]

Recently picked up a ko4 converted mk5 gti -

Should’ve really spotted it on the test drive knowing it’s a weak point but on my drive home I noticed the thermostat needle dropping from 90, eventually all the way down to just over 50... (3 hour drive). When I got home and let the car idle/cool down, temp gauge crept back up to 90 after a minute or two.

I seem to have in my head that if faulty thermostat it usually only drops to 75-80 ish, could something else be the cause? Or am I just being paranoid and it’s main thermostat? Coolant levels are spot on. Have also read there are two thermostats, is the second one easily accessible when doing first and should I change it as well?

And also is there anything else I should change/look at while I’m in there? Not planning to take front end off as I’ve read you don’t need to, it’s recently had valves decoked and chain/tensioners + belt&pump changed anyway but just if there’s anything else in there worth looking at.

Any help is much appreciated, cheers.

[Lewo]

If all that work why wasn't the thermostat done at the same time....?

[Jons1001]

Just found this thread where someone had the same issue. Looks like the OP didn't return but someone else at the bottom of the thread says they had the same issue and was the stat.

https://www.mk5golfgti.co.uk/forum/index.php?topic=114557.0

I do think some DSG may have 1 main stat and 2 in line stats. However mine had 1 main and 1 in line (mine was not quite getting to 90). I had the in line stat changed aswell, this is a cheapish part around £30 i think and can DIY quite easy.  Its the main stat which is awkward and pricier.

There are different scenarios i think where it could mean you might get away with just changing the inline stat. Hopefully someone will be along soon to clear this up.

[Monzoa]

If all that work why wasn't the thermostat done at the same time....?

I thought the same thing.. was done by previous owner. Maybe they ran out of funds.. 🤷‍♂️ There are receipts for it all.

[Monzoa]

Just found this thread where someone had the same issue. Looks like the OP didn't return but someone else at the bottom of the thread says they had the same issue and was the stat.

https://www.mk5golfgti.co.uk/forum/index.php?topic=114557.0

I do think some DSG may have 1 main stat and 2 in line stats. However mine had 1 main and 1 in line (mine was not quite getting to 90). I had the in line stat changed aswell, this is a cheapish part around £30 i think and can DIY quite easy.  Its the main stat which is awkward and pricier.

There are different scenarios i think where it could mean you might get away with just changing the inline stat. Hopefully someone will be along soon to clear this up.

Ok thanks mate, sounds like it is just thermostat then and will do inline one while I’m there. Doesn’t take long getting up to temp but I may as well while I’m there.  Going to attempt a diy.. (Y)

[Jons1001]

Just found this thread where someone had the same issue. Looks like the OP didn't return but someone else at the bottom of the thread says they had the same issue and was the stat.

https://www.mk5golfgti.co.uk/forum/index.php?topic=114557.0

I do think some DSG may have 1 main stat and 2 in line stats. However mine had 1 main and 1 in line (mine was not quite getting to 90). I had the in line stat changed aswell, this is a cheapish part around £30 i think and can DIY quite easy.  Its the main stat which is awkward and pricier.



There are different scenarios i think where it could mean you might get away with just changing the inline stat. Hopefully someone will be along soon to clear this up.

Ok thanks mate, sounds like it is just thermostat then and will do inline one while I’m there. Doesn’t take long getting up to temp but I may as well while I’m there.  Going to attempt a diy.. (Y)

Try the inline stat first you never know you might have a result.

Here's a guide for the main thermostat not sure how good it is though;

https://www.pelicanparts.com/techarticles/Volkswagen_Golf_GTI_Mk_V/31-WATER-Replacing_Your_Thermostat/31-WATER-Replacing_Your_Thermostat.htm

[LC5F]

It could be the fans running full pelt on the drive super cooling the engine, likely to be one of the 2 coolant sensors.

With the Golf being new to you it may be difficult to know if the fans are operating correctly - mine was running the fans on cold start up.

With the current fuel prices it is worth fixing quickly, the ECU will think the engine is cold and will be dumping extra fuel in to help it warm up.

If you end up replacing the main thermostat, its housing also contains one of the coolant sensors.
They are buried under the intake, alternator off - a right PITA to do with big hands -it is wise to replace the sensor at the same time - don't ask how I know...

[colesey]

Mine starting showing similar drops in temperature last winter so I had the main stat (bwa / manual) and temperature sensor changed. Since then, the car warms up quicker, the heater works sooner and it uses less fuel. Is a well known maintenance issue.

[Monzoa]

Ok thanks guys, appreciate the help.

I have ordered a main thermostat, in-line stat and coolant sensor + coolant (opted for quantum G13 which is hopefully fine). I’m sure that will sort it but will update with results.

Probably also worth noting I’ve scanned the car with vcds and it has come up with -

Coolant fan control circuit 1
001152 P0480 - 004 - electrical malfunction

Hoping that is just thermostat related, guess I’ll try change the parts and clear it and see what happens.

I do also have codes for a lambda sensor and camshaft positioner (bank 1 intake) but I can’t see how they’d be related. Will post a separate thread if I get stuck with them but I’m pretty sure that will just be new lambda sensor or it’s due to decat/map and other one looks to be the n205 which I think is oil pressure.

One last thing which maybe someone can advise on.. The car has recently had vis hpfp internals fitted (after being mapped)
 Can’t get back to the place for a map tweak until the 19th but this wouldn’t cause any issues with also having the faulty thermostat? Just thinking if that dodgy thermostat does cause the ecu to dump more fuel maybe it could somehow but I’m trying to go easy on the car and stay off boost.
Not always successful at that though..

[pudding]

It will be fine.  You could have a fire hydrant as a fuel pump but the ECU/Injectors will only allow so much into the engine. 

What were the codes for the cam phaser and which lambda, bank 1 S1, or S2?

[Monzoa]

Ok cheers Pudding, don’t feel so bad giving it the occasional boot now. 

The codes are -

Oxygen (Lambda) sensor b1 s2 heating circuit
P0036 - 004 malfunction/open circuit - MIL ON

Camshaft positioner (Bank 1 Intake)
P000A - 008 - slow response