I have a 52 plate Skoda Superb 1.9tdi 130.

It's done about 120,000 miles.

Recently I've been driving and noticed the coolant gauge going down while I'm driving. It will later return to 90.

However yesterday I did a 420 mile journey from north Wales to the Cairngorms in Scotland. Driving at 70 down the A9 my coolant temp went from 90 to 0????

After about five minutes it went back up to 90. This is all while cruised at 70.

So I pulled over at the next service station, went to the toilet and when I came back I opened the bonnet to check the coolant. But it wasn't hot? It was Luke warm? How is that possible after 6 hours driving?!

Help and advice please!

How much lower/colder did the ambient temperature get as you climbed higher on the A9?

You maybe cleared an air lock/block.

Has your Anti freeze ratio been checked or your fluid/coolant changed recently.?

Maybe worth having done.

george

Could be your thermostat on its way out - staying open more than it should.

Gaz

Temp went from about 7 all the way down to -0.5.

Coolant hasn't been checked. But it was changed when I had the cambelt done and water pump done.

Is it okay to drive? As I won't be home till next weekend.

It'll affect your mpg, but will be okay to drive, short term. You don't want to run it cool for too long.

I don't suppose it's your electric fan at the radiator is running all the time is it?

My money's still on your thermostat being stuck open. Write up here:

http://www.myturbodiesel.com/1000q/b5/thermostat-replacement-vw-passat.htm

You can mitigate it a bit by lowering the cabin temperature. Not so you're freezing your nuts off, but if you keep it down at say 20c, the heater matrix won't be robbing you of engine heat.

Gaz

Well I averaged 54 but it is usually slightly higher. Ill get it looked at when I get home.

Any ideas how much I'm looking at to fix the thermostat?

Thanks for the replies!

re Thermostat stuck open.

I fail to see connection.

Car starts with cold coolant.

Engine is cold and thermostat is closed, (Coolant circulates around the engine untill the thermostat opens)

as the engine heats & the coolant gets to about 88 degree celsius, the thermostat opens and allows the coolant to flow around the system.

radiator (top hose first) , heater system, engine, radiator.

It flows while the car/engine is running,

it does not close during your journey, it does not close if the coolant was going to drop to even 50 degrees celcius.

'It is supposed to stay open'. So would not be 'Stuck open'.

Now if the thermostat did not open it would start to roast your engine.

(coolant cools engine running through the water jackets and cools the engine oil ,

engine oil is also a coolant.)

If the thermostat does not open the coolant in the block & the engine oil gets hotter and hotter even when just sitting not driving,

the coolant does not get to the radiator & the cooling fan can not cool the radiator.

Draw air through the radiator if the car is not driving into air. type thing.

If there was no thermostat in or it was stuck open.

It takes a while but the coolant does heat come time, its just its circulating before the engine is up to temp.

the running temperature is the same once up to heat and your moving.

(Same as a thermostat that opened correctly, actually.)

Thats a simplification.

But why i wonder why there is some suggestion the thermostat is stuck open.

The Antifreeze raises the boiling point & it is worth checking you have the correct ratio in.

(Antifreeze, summer coolant & rust inhibitor)

george

re Thermostat stuck open.

I fail to see connection.

George

A thermostat opens and closes during the normal running of an engine. Once an engine is up to temperature it doesn't just sit fully open, it continues to fluctuate between open and closed. Open mostly, but closing as the coolant reduces the engine temperature. Much as the secondary cooling fan cuts in and out to keep the airflow through the rad.

It's a very common cause of cool running on the 1.9 PD's.

Gaz

Hmm I had to top the coolant up myself, as it was slowly going down. I also added Rad weld. The coolant seems to have stopped leaking (going down). But could this be the problem?

I text my cousin, he's a VAG specialist, he said something about a g26 coolant temp sensor?

G62 - I thought about sensors and switches, and I could see these affecting fans and gauges, but you have lukewarm water, which suggests to me a more physical issue. What do your top and bottom hoses feel like?

Sounds like it'd be worth having your cousin look at it if he's able to.

Gaz

Yeah he'll have a look when I get home but I'm 420 miles away at the moment lol.

Typical it happens now.