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Noticed today the engine temp dropping whilst out, always been bang on 90c once up to temp. Today dropped to 70c when driving through town, went back to 90c on the motorway then dropped again. It was cold out, about 5c , but not proper cold, cabin heater was off. Any ideas? Recently had cam belt and water pump replaced, 13 plate 1.6 diesel Volkswagen Touran.
Air in cooling system?
Or thermostat stuck open?
Could be allsorts, I'd probably start with fan stuck on from fault relay or thermostat/switch.
Sounds like air across a resistor that is supposed to be immersed, reads wrong then correctly
Or thermostat had a wibble amd opened up letting the entire rad of warm water round the system
Or your head gasket has gone and compression gas is leaking into the coolant. See 1.
Thermostat would be the best place to start
Classic thermostat problem. Cheap part.
The thermostat diverts coolant away from the radiator when it's cool to keep the temps up. It shuts fully when you start off then opens up but if the weather is cold and you're driving the radiator would be too effective and it'd cool the engine too much from it's ideal temperature so it closes partially. But not apparently in your case.
Thermostat - This was the issue with the Passat.
There is another Thermostat on the gearbox if you have dsg
Temp sensor on block
May be others but I believe these should be your first ports of call with a VAG diesel
Ok thermostat seems to be consensus, think I read online somewhere they can be integrated into the water pump? Just asking as that was replaced with the routine belt changes.
Take it to a garage. If you don't know about thermostats or how to find out how your engine is set up then you're unlikely to want to deal with the faff of draining the coolant and getting all the bubbles out when you refill it.
Unless you are genuinely interested in fixing it yourself in which case first port of call is finding out your engine code.
Right amount of liquid in the radiator?
Molgeips, fully intend to, just wanted to know of it was likely to be one of things or whether it could ne related back to recent work done on it. Ill get it booked in.
May be Stat but I'd expect to see long warm up times if it was wide open.
Or sender unit fault maybe.
Does it have an auxiliary waterpump?
AFAIK , No the thermostat wonnt be integrated into the water pump. MOst VAG stuff the water pump is tooth driven off the cambelt and hidden inside the plastic cam cover. Ballache to change if not doing the cambelt , but if doing cambelt adds an extra 10 mins to the job.
First thing to do , see if there is a bleed port up on the scuttle somewhere .
Second thing is actually drivee it for 30 - 40 mins , seeif it does it again , chances are the main stealer has left a load of air in the system and its working its way round the rad .- have heater on full chat
Occams Razor.- we know the coolant system has been drained, we know it didnt do it before. so the least guesswork is its related to the draining of the coolant system
cabin heater was off.
Who drives around in this weather with the heating off?! 🥶😳
Check coolant levels in the reservoir and for signs of leaks, has the coolant level dropped? If it has then there may have been some air in the system after the pump change. Top it up and keep an eye on it, if it keeps dropping then you have a leak somewhere.
It could be the thermostat but doesn't sound right, usually these mechanically collapse in the half open position, temperature will be about normal when trundling around town but will drop rapidly when on the motorway due to the increased air flow through the radiator, then recover when you slow down again. From what you've written yours does the opposite.
From what you’ve written yours does the opposite.
Correct, although that does correlate with how much effort the engine is putting in, if it's not closing properly I can see it cooling down quickly when the engine is under less load. It wasn't a main dealer who did the water pump / cam belt, it was a local trusted independent.
No the thermostat wonnt be integrated into the water pump
Further Googling confirms that, thank you. It needs a couple things doing in the New Year, will get him to look at it again. It's not a major problem at the moment, it stays on temperature at motorway speeds which is much of my commute, when i'm actually commuting. Important thing is it's not getting hotter than normal at any point.