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So I got the car back and it had a timing belt change together with waterpump and t-stat.

At first everything is normal, water temp climbs pretty quickly to 90C. After a little while there it drops in the space of 30 seconds to just over 60C, like the thermostat opens and gets stuck. It stays there pretty much sometimes a little higher sometimes a little lower. Give it the beans and it will rise. At some point during the drive oil was 90C, water around 65 and I pulled away full throttle in 1st, 2nd, 3rd. That was enough to boost water to 75C and after a minute or 2 oil had also crept up to 100C (all from just 1 little sprint) but water temp started to drop again while parked.
The fan runs continuously while the engine is running. One side of the radiator is hot to touch, the other side is ouch nearly burnt myself hot to touch. But temp on the dash is around 65.

Any ideas? I find it strange how quickly oil temp will rise, though water stays much colder...though the radiator is pretty hot and the fan stays on.



Actually the very first time I drove the car home the w-temp also dropped steeply after reaching 90C....but after a few miles recovered within seconds back to 90. That did not happen today, stuck at 60-65 even standing still it slowly drops.

is it possible air got in the system...

Yea it has to be really, after thinking about it and posting on .com

Nothing is ever easy on these cars not even filling it with coolant

yes but you forget when its working properly.....there comes a time when my little laptop is constantly connected with Vagcom so troubles are soon to be discovered...i will have to heighten my seat to look over my laptop screen...although there are very small kind of displays/computers which can be connected constantly to the car diagnostic port to see live data, its mentioned on the forum a while ago...

It could be temp sender unit. I have had similar behavior

el_ringo wrote:It could be temp sender unit. I have had similar behavior

Yeah thats plan B I guess, for now I think its less likely because it started right after doing the thermostat/coolant change.

Symptoms of a duff thermostat, change the thermo

I wonder if the thermostat was put in in the wrong position. There's a way it goes in.

What happens when start the car and let it idle while the water temp comes up? Then if you drive it at constant 70mph? What does it do?

If it was a thermostat issue, I'd expect to see a correlation between the oil temp and the water temp at some point.

If you've got a Vagcom, log the water temp to see if it does anything weird. Could indicate a dodgy sensor. Or perhaps it's not been plugged in properly. You could try wiggling the wires at the connector while looking at the Vagcom readings to see if that affects the temp.

I wonder if the sendor not being immersed for the relatively long time that the engine was out has damaged it? Oxidation damage? It's a £30 part, and prone to failure. Worth changing if only to eliminate it from the diagnosis.

After you let it cool down, if your expansion tank particularly pressurised? Do you get a release of gas when you open it (after cooling down)?

Shoppinit wrote: What happens when start the car and let it idle while the water temp comes up? Then if you drive it at constant 70mph? What does it do?

I haven't yet tried to start it cold and let it idle to warm up, but I suspect it will not change a lot. When the temp is already "down" letting it idle only seemed to make it drop further, albeit slowly. After a restart it showed 90C and stayed there idling 5 mins, I did not wait longer...it collapsed within a few hundred yards of driving it.
There does appear to be a definite correlation between actual water temp and what the gauge says. It will go from 65 to 75C after flooring it briefly. And letting it cool down driving slow or idle makes it go down again.
It seems obvious that this problem starts around the time that you expect the thermostat to open. Initial warm-up is normal but once it hits 90C you can count down to the sudden 30 degree drop. It could be a t-stat issue I suppose, or perhaps the opening of the stat creates water flow that moves an air pocket slightly to cover the sensor...not sure if thats technically possible or likely.
The very first time it happened, water temp did suddenly recover at one point. I would say that points more to a sensor or air issue, I do not think if it was a sticking stat that suddenly closed the actual water temp could rise that much in a few seconds.

I did not try opening the expanstion tank when cooled down completely. I did open it yesterday a few minutes after shutting the car down and water at that point was pretty hot, there was significant pressure and there some boiling sounds for a few seconds. Also touching the radiator I think actual water temp must be higher than reported.

Might be worth testing for exhaust gasses in the coolant. If only to put your mind at ease.

Definitely change the sender. Consider it preventive maintenance; It's a 5 minutes job, no exaggeration.

The water temp on the dash is filtered and doctored. If the sendor suddenly reads -45.15847 the dash isn't going to display it, but might average it out into a figure it can display. If you don't have a Vagcom, you can display the realtime water temp on the aircon display.

To get the temp up, I believe the procedure is this:

Press recycle and up arrow at the same time for a few seconds.

Then use temp + or - to scan through the channels and press recylcle to display.

IIRC it's channel 49.

I'll go and try on mine in a bit to see

I can't find any way to get it into diagnostics mode. Maybe it's not possible on this version of climate control. That's what it looks like.

I have VCDS but unfortunately I busted the screen of my laptop the other day so I can't use it.

Well I went to warm it up with nothing but idle...I had a laser temp thingy with me to check in & out temps on the radiator hoses. Unfortunately it's very far from accurate the cheap little thing.

Temp on the dash reached 90C pretty quickly in this weather. Temp reading on the radiator hose was 90/25C at that point. The intake hose temp kept climbing slowly to a 115C+ reading (I believe the measurement is wrong though it was hot to touch but i could hold it for a second or 2) the out temp climbed much more slowly and seemed influenced more by the warm air being blown around it than water passing through. It took another 15-20 mins for oil to reach 90C, water still at 90C displayed as well. In/out on the rad 115/80C.

Went for a drive...both water and oil keeping steady at 90. Did a 100-200kmh run and a 1/4 mile with my GPS logger...water still showing 90, oil around 110C. 25C ambient. No anomalies to be reported.

Will let it cool down and drive again later...confusing stuff.

Maybe it is airclocked after all. Let's see what happens later.

Oh, it does work.

Sweet

I'll try to find out what the difference channels do and get back to you asap.