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

[Itguy]

Ok. Can someone check my logic on this diagram below?

Bottom left you can see the secondary thermostat for the turbo. If this is stuck open I can clearly see that this would mean coolant flow around the system would be bad.

The other secondary thermostat top right near the gearbox cooler wouldn't be as bad but would just slow the warmup down.

Make sense?

[martziniuk]

Are you diluting your antifreeze at the correct rate?

[Itguy]

I haven't changed it. Just using what was already in there.

Does it make a difference to warm up?

[martziniuk]

I was just thinking if it was too strong it would take longer to warm up.

[xjay1337]

You are over-thinking the problem.
You had an immediate change in the behaviour after changing the main thermostat and Coolant temperature sensor. So it is OBVIOUS to me that the "problem" is with these parts.

Just my opinion

[Itguy]

Undertaken some more diagnosis this morning.

Dropped the coolant and removed both of the bypass thermostat valves to check them.

Here's the first (as my car is a DSG)



Here is the second (turbo coolant line)



These look like this



And then opened up look like this



You will notice that there is a brass active element in these. The pin position on the one in the photo here is fully closed but i had to use pliers to get it there. Both valves allowed me to blow freely through them in both directions when I took them out of the car, clearly both brass active elements are shot.

I put the brass thermostats in a cup of boiling water and they did expand, but then when i cooled them down and tried to press the pin back in it was very stiff, way harder than the small spring would overcome.

I've ordered them from the local TPS, c.£24 each inc vat.

I also tested the new temperature sensor against the old one (took ohm readings at various temperatures) and they are basically the same, so the old one was fine.

I've re-assembled the car with the old bits for now as we need to use it, but I've manually closed the thermostat pins so at least for the first warm up cycle i'll be able to see if it really has solved the slow warm up issue - I'm pretty certain it will have.

Through the research I've done it's very interesting to note that the cooling system design changed mid-05.

http://volkswagen.workshop-manuals.com/golf-mk5/index.php?id=5042

and then to

http://volkswagen.workshop-manuals.com/golf-mk5/index.php?id=5043

Spot the difference? Yep, no bypass thermostat for the turbo coolant pump line on the revised one. It's still there for DSG however.

Early 05 manual cars = Main thermostat + bypass stat for turbo cooling pump
Early 05 DSG cars = Main thermostat + bypass stat for turbo cooling pump + gearbox cooler bypass stat
Late 05+ manual cars = Main thermostat only
Late 05+ DSG cars = Main thermostat + gearbox cooler bypass stat

In short, just make sure you buy a 06 plate manual GTi :)

This thread here is also worth a read, many people seem to have the same problem and overlook the small thermostats. http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://golfgtiforum.nl/forum/viewtopic.php%3Ff%3D12%26t%3D1582%26start%3D100&prev=/search%3Fq%3D4e0%2B121%2B113%26start%3D10%26client%3Dsafari%26sa%3DN%26rls%3Den%26biw%3D1572%26bih%3D883

[Itguy]

I have now worked through the theory of what is going on - see diagram below.

If (3) is faulty and just is open all the time, as the engine warms up it will pump warming coolant through the turbo and down that pipe, round to the junction at the bottom and directly into the main pipe towards the main thermostat (4). As this gets hotter and hotter it will make (4) open quicker than it should, rather than staying shut until the water from the engine is warm enough to open it.

I can also see how went sat in traffic my temperature would reach a nice 90c on the gauge (i.e. not much heat from turbo being produced and low water flow rate) but then when i pulled away a lot or drove where the turbo would be partially used it would drop down again to 80ish, because the water flowing around the turbo coolant line into the engine will be HOT, making the main thermostat fly wide open letting loads of cold water from the rad in.

[Itguy]

Have just got home after a run out in the car and just as expected it warmed up straight away to about 80c within 5 minutes from stone cold.

Took a while after that to get to 90 but then it stayed there too. It was 4.5c outside when I left home, just for reference.

I fully expect it to be back to its old bad self tomorrow morning as I did have to manually close the brass active thermostat elements in both the bypass stats to get them to shut, but they key thing today was to find the root cause of the problem which I've done.

So, a message to anyone who has a gti that takes longer than 5-10 minutes to get to 90c, "it's not normal and should be fixed!".

[OSB]

So, a message to anyone who has a gti that takes longer than 5-10 minutes to get to 90c, "it's not normal and should be fixed!".

Yep - second that!

Great write up. Well done and thanks.

[Itguy]

Thanks, i'll make sure I post up once I've fitted the new two bypass stats just to close the story and do a final confirmation that they were the problem.

Annoying thing is that I didn't need to fit the new main thermostat or coolant sensor (£70 in total) but at least I've got peace of mind they are done anyway.

[OSB]

That would be really good if you could 'finish the story'. Good info for the Forum.

No bad thing to change the 'stat though. According to my local Indy -and  they have changed quite a few - he thinks there were a dodgy batch supplied to VW. Not made to the right tolerance and getting stuck with general muck and/or limescale. I can't back that with any hard evidence. I forgot to keep mine when I had it changed. If you still have yours and don't mind sacrificing it, you could cut it apart and have a look for evidence!

[doylebros]

Well done with your research and tenacity   

[Itguy]

I just have a pet hate of things that don't work as they should do. Just because the car is 8 years old doesn't mean it can't perform just like it has been delivered from the factory.

Just needs a bit of TLC !

[xjay1337]

Useful.

[ub7rm]

Its two years since I got rid of my mk5 GTI and it was an 05 and despite replacing the temp sender and what I now know was the 'main' thermostat it still wouldn't reach temp in the winter except on a very long drive and it drove me ABSOLUTELY NUTS as I knew it wasn't right despite being told by many that since it was colder it was to be expected.  40 mins and not reaching full temp? pah!

At the time I posted on here and did notice a few others saying the same thing, all 05 ish age hence AXX engines.

Well done on your perseverance and for laying that niggle to rest for me - I knew I was right