My car accelerates without pressure on the pedal - what's wrong?

I drive a 2017 Skoda Superb 2.0-litre diesel. In December, I noticed the car had started to intermittently idle at 1000rpm or slightly higher and the fan would kick in at the same time. I contacted my local Skoda dealership, who said as it was an intermittent problem I should monitor it and, if it became worse, to re-contact them. The matter worsened and a second fault developed, whereby when I took my foot off the accelerator in a bid to slow down, the car actually increased speed. I booked it into Skoda, who said the over-revving was normal. Regarding the accelerating, they couldn't find anything wrong. If the over-revving was normal, why hasn't it been doing it since I got the car new in March 2017? Just wondered if you had any idea what the problem could be.

Asked on 22 March 2018 by Paul Mathers

Answered by Honest John
If you regularly drive a diesel car short distances from a cold start, the DPF does not get hot enough to passively regenerate the soot created by the cold starts. Active regeneration may then kick in, whereby diesel fuel is post-injected to the DPF to fire it off and burn off the soot. If this process is interrupted by switching the engine off mid-active-regen, the post-injected diesel sinks to the sump, contaminating the sump oil and gradually raising its level. Since a compression ignition engine can actually start to run on its own sump oil when the level is too high, your sump oil level needs very carefully monitoring. The fan coming on confirms that this was happening because active regeneration creates a lot of heat.
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