Our reader’s VW Touran is the 1.6TDI model and it appears to overheat on motorways when he exceeds 60mph for any length of time. Garages have given up trying to help him, other than telling him that when they plug their diagnostics in, it says the auxiliary heater has got too hot.
He’s had a new radiator, thermostat and circulation pump, all with no effect. Garages can agree that it’s not a head gasket problem that’s making his Touran overheat and there’s no fluid leaking from his cooling system. We asked our mechanical expert and he came up with some handy hints.
What is an auxiliary heater?
The auxiliary heater is a bit like a mini engine that sits in the engine bay alongside the main motor and runs off diesel. It’s designed to help diesel engines get up to temperature and run efficiently as quickly as possible in very cold weather. As diesel engines aren’t particularly fond of very cold weather, the heater helps to warm up the engine’s cooling water. This encourages the engine to reach its optimum temperature quickly.
On many cars, including the Touran, the auxiliary heater also pumps warm air into the cabin when the weather’s cold. The benefit of this is that it helps to demist the windscreen quickly. The auxiliary heater should switch off when the car gets up to temperature as it’s not needed.
Why might this be the problem?
When our reader’s Volkswagen Touran overheats, the heated air coming into the cabin appears to switch off and cold air blows in instead.
Of course, it’s very difficult to know without seeing the car. But our expert believes the auxiliary heater might be still running when it’s not needed, causing the engine to overheat. The car’s brain thinks it’s getting too hot in the cabin so it turns the heating off to cool things down.
What can you do?
We’d take the car to the garage and ask them to have a look at the auxiliary heater. If necessary, take them out on a road test with you. Intermittent problems are always really annoying because they’re very difficult to replicate on command.
If it was our car, we’d take it on a drive and attempt to get it to overheat. Then you could take the car to the garage and go out with a technician on the exact same route. The idea is that hopefully the Volkswagen Touran overheats again, they can interrogate the diagnostics and actually act on them – something other garages seem to be struggling to do.
Take it to a specialist
Just as there are specialists in Japanese cars, there are independent specialists in German motors. We’d take your Touran to one of these. Rather than a general garage – Jack of all trades etc… – there’s a much greater chance that the independent specialist will have experienced a similar problem to yours and know the best way of diagnosing and fixing it.
I’ve been writing about cars and motoring for more than 25 years. My career started on a long-departed classic car weekly magazine called AutoClassic. I’ve since pitched up at Autosport, Auto Express, the News of the World, Sunday Times and most recently the Daily Telegraph. When I’m not writing about cars and motoring, I’m probably doing some kind of sport or working in my garden.