Engine fan staying on

Hedgecutter

I have a 2019 VW passat. i have noticed lately that the engine fan is staying on after the engine is turmed off even on very short journeys. Is this normal. Not due a service till November.

Gregor Samsa

I assume it's a diesel.

If a DPF regen is interrupted, the fan will stay on after the car is turned off. If you're doing a lot of short journeys, it's more likely to happen. Take it for a good spin and let it complete the DPF regeneration.

The DPF filters out particulates (soot) in the exhaust fumes. When it fills up to a certain level, the car attempts to burn off the soot, raising the temperature of the DPF up to over 600 degrees C. The process is called regeneration and takes about 15-20 minutes, an happens automatically while you're driving. But if you're on a short journey, and the car is stopped before the regeneration process is completed, the car leaves the fans on after you turn off the engine to try to bring the temperature down. However, this means that the DPF hasn't cleared out it's soot. So it'll try again next time. If that's a short journey too, then the same thing happens. Eventually, if you keep interrupting the regenerating process, the DPF will get very full and throw a warning light on your dash. If you ignore that and don't do a decent journey to clear the DPF, it could get to a point that it won't clear from a regular journey and you'd have to get a forced regeneration done (by a mechanic or someone who has the diagnostic requirement to trigger one). If you keep ignoring it and your DPF fills right up, it won't be able to be regenerated at all, and will have to be replaced.

Short journeys are fine with a car with a DPF, but you need to do some longer journeys too. Just normal driving on an open road for 20 or 30 minutes after the engine has fully warmed up is usually enough, but if you're in doubt, get onto a motorway, get the revs up (drop a gear or two), drive at a constant and high speed, and do so for at least 20 minutes. Older DPF systems were a little more particular about how and when they regenerated, and some even needed a special fluid to help get up to the massive temperatures needed, but the more modern ones work easier. The car won't give any direct indication that it's doing a regeneration, but sometimes you can smell it, or you'll notice your idle revs are a little higher than normal. Note that the car won't attempt an regeneration unless there's a certain amount of fuel in it. Think you need a minimum of a third of a tank or about 20 litres, something like that.

If you have an OBDII device and an Android phone, there's a great app for VAG cars called VAG DPF that lets you monitor ever aspect of your DPF. Some other diagnostic apps have similar functionality.