Tdi amf engine pulses on start up ??

Hi now and again on starting up of the car it revs up pulsing a few seconds then dies down to normal tick over any one come across this bit disconcerting as it revs quite high ...sounds like rin nin nin ...I know that’s sounds daft but it really does sound like it ..
 
How high does it rev before settling down?
Did you notice if the glow-plug light came on at all?
Perhaps it may be a sign of the temperature sensor being faulty, so might be worth getting it scanned to see if there are any error codes and/or to check the temperature that the ECU is picking up.
 
Will do I will car down to garage sometime Monday and get it scanned ...as for glow plug light and revs to be honest it’s so disconcerting when he does it I didn’t concentrate and look at the red counter because it’s only for a few split seconds rin nin nin i think I blip the throttle and it’s back on tick over runs sweet otherwise only done 39 k ...
 
My project car does that maybe once in every 20 start ups from cold. Just after the starter motor disengages it does a 'boy racer' burst of revs with the three peaks like you've just described for a second or so, then back to normal, and runs completely normally thereafter. No obvious reason why, no errors logged etc barring a very occasional "coolant temperature sensor open to ground 'intermittent' fault". I wondered if I was being a bit hasty starting rather than giving it the second or two for the glowplugs to preheat on occasion without thinking but no other ideas. For the revs to happen there has to be fuel but where is it coming from?
 
Is this not a case of injectors dripping a little diesel whilst the engine is not running. Once it starts with the correct amount of fuel being injected and is firing on that cylinder then the excess fuel which as dropped onto the piston crown will star to be burned this will take a few seconds because it is not atomised but it will be burned hence causing the increase in revs. It is likely that not all three cylinders will burn this excess of fuel at the same time or even start to burn it at the same time hence the three peaks on the revs until all excess fuel is burned then revs return to normal to match the injector fuel demanded from the ecu
The ecu will be monitoring this increase in revs and trying to establish a correct tick over but the best it can do is cut all fuel to all injectors but with the excess of fuel in the cylinders the revs still rise until this is burned off
The pd engine compresses the fuel via the cam lobe so depending on the position it stops in one or more injectors can be ready to inject meaning there is several thousand bar of fuel pressure in that injector

That’s my logical theory anyway
If it bother you try some injector cleaner liqui moly diesel purge is really good run the engine on it near via a small slave fuel tank
A litre normally does the trick

Failing that an injector service
Cheers Paul


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All that makes sense much appreciated except yesterday I drove 6 miles stopped at friends for 20 mins came out and it did it again so not just when engine cold on ist start up ...I’m popping down to garage this morning to plug in diagnostics ..
 
If an injector is leaking it will leak when the pressure is greatest which is immediately after the engine stops
So I don’t think it will only do it when cold
Paul


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