Stop/start struggles to restart engine when COLD

[meerkat]

Hi all,

My Polo 6C (1.2 TSI) has gradually developed a problem with the stop/start. Whenever the outside temperature is cold (around 10 deg C or less), the engine will start fine but struggle to restart if stopped by the stop/start system - sometimes the revs will be unstable and hover around ~300rpm and then either recover or stall, other times it'll take a lot of cranking before it eventually restarts. Not convenient when I'm stopped at a junction!

The strange thing is that once the engine has warmed up, the stop/start works fine. The first engine start (with the key) also works fine, and the car drives fine too. Also if the engine restarts with unstable rpm, the car ignores all throttle input until the engine idles normally.

It's definitely not the battery since that's been changed fairly recently and a code scan doesn't show any error codes.

Does anyone have any ideas? I'm guessing it's a faulty sensor somewhere but don't know what to look for. I tried using my OBDEleven to look at some values but couldn't see anything obvious. The only thing I found so far was the radiator outlet temperature was stuck at -40 deg C but I'm guessing that's just a glitch with the obd eleven?

[doc_777]

If the radiator outlet temp is reading as minus 40, the fuelling will be all wrong and the car will struggle to get off idle as you’ve described. Sounds to me like a bad sensor more than an obd11 issue. Obd11 is only displaying what the car is telling it

if the ECU “thinks” the temp is way cold, it’ll be injecting much more fuel than needed (cold air is denser than hot air and requires more fuel so as not to run lean) and the car will be effectively flooding. If you stood behind the car in this situation I bet you’d smell unburnt petrol at the exhaust.

[meerkat]

Hmm I'll get the sensors checked but I'm not 100% sure that it's the cause of the problem - if the car was running rich all the time then surely the issue would be worse as it warms up? Also, I thought the fuel mixture was determined by the coolant sensor in the engine, and not the radiator?

The reason I'm not sure about the readings from the obdeleven is that if the radiator sensor was really reading -40 then I'd expect a fault code to be generated for it. Another reason I'm not confident in the obdeleven values is I get other nonsense data like "Coolant temperature max -48 deg" along with "Coolant temperature min 143.3 deg"!

[doc_777]

Have you checked the OBD11 forums to see if anyone else is reporting this kind of behaviour in the app?

[h100vw]

outlet temperature was stuck at -40 deg C

That's an indication of a failed sensor.

Gavin