MegaSack DRAW - This year's winner is user - rgwb
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I seem to remember reading somewhere about a typical fault on diesels that results in engine speed fluctuation.. Something like a boost leak maybe.
Engine idles fine but if I apply throttle stationary up to 2000rpm or so it goes up, then lowers to about 1800 and sort of drifts a bit.
faulty air flow sensor is another cause.
Engine drives fine but auto gearbox seems to have trouble matching revs when shifting. As if it requests certain revs but the engine doesn't quite hit the mark. MAF is a plausible explanation. Will clean it up.
Egr is anotehr culprit.... I seem to remember you still have that horrible bodge attached to your engine .
EGR is clean - doesn't make a difference, I've tried disconnecting it which closes it but doesn't change the shifting issue.
I won't remove the egr either cos I don't like NOx.
Sounds like you need to see a specialist.....;)
Seen similar issue with crank sensor and a cam sensor.
Cam sensor is recently replaced. So is the cambelt, but it did the same before the change.
Auto box will get it's load signal from the MAF, not usually able to clean them but try some brake/carb cleaner.
Do you have a scope? You could prob the maf and see if you have readings that jump about a lot. I'm going through this process myself at the most ment.
I've got VCDS which graphs these things.
MAF signal will vary, because engine speed is varying - have to work out if it's varying the right way or not.. still - I can pretty easily take it off and give it a squirt. It's the wire kind.
£100 for a new one though.
Sorry I was not clear. Varying with revs fine. Jumping inconsistently, not fine. Make sense?
Also can you log the fuel trim and see if that suddenly jumps when you experience the odd revving? Does it happen more when cold?
Yes, makes sense.
I dunno about fuel trim - have to see what measuring values are on offer, can't remember off the top of my head.
Haven't tried it when hot or cold. It was medium when I checked this afternoon.
The gearbox thing though - it used to match revs perfectly on manual downshift - so you'd be driving in say 4th at constant throttle, you could switch to third and keep constant speed, and there'd be no jerk. Now if you do it it starts to slow down in the lower gear, you can feel a bit of engine braking during the shift; and if you downshift it accelerates a bit.
Gearbox requests a certain torque, but doens't get it. Possibly boost leak I guess too. Otherwise drives perfectly normally so whatever it is, it's not severe.