Occasional rev drop

[Voltaire53]

I just wondered if anyone has any ideas on this one:

I pull up to a junction and dip the clutch to put it in a new gear ready to pull away and the revs drop off to about 200-300rpm then after about 3-4 seconds come back up to the regulation 1000rpm. Occasionally they drop so low that the car stalls (a bit tricky when it happened halway around a roundabout as I coasted a little ... yes, bad habit I know).

The odd thing is this seems to happen for a whole journey *but* only about 1 in 20 journeys. If I stop for about 30mins+ and then set off again it seems to count as a new journey so it usually solves the problem.

So anyone got any clues as to the cause?

[Big Al.]

Stepper Motor?

[K4TRV]

I would imagine you're marginal in one of the following criteria necessary for the idle circuit to maintain the correct tick-over:

Speed sensor input to ECU (what's the gap - is it nearly too large or small?)

Stepper Motor - they get dirty, they also get tired??

Throttle Potentiometer setting - if it starts to get close to the 1.50 to 1.95 volts limits, then that will trigger symptoms.

Base idle setting in conjuction with stepper motor, throttle pot and air bleed screw - all may need tweeking.

Your problem now is whether you'll just live with it or eliminate each one-by-one?? But the suns shining here in the SW, so just go and drive .....!!!

Trev McM

[Voltaire53]

Thanks for the info - I think I'll probably leave it for now (at least while it's sunny ) and at least if it starts to happen more often it should be easier to track down ... maybe!

[chim 500]

mine done the same thing(loose wire in speed sensor unit under dash)

[tweety]

Mines just started doing this too. Thought I'd give the stepper motor a clean first to see if that cured it. Still pops and bangs tho so thats the main thing.

Al.

[alex.tvr]

Mine does this aswell, but am living with it at the moment cos as you say only happens about 1 in 20 journeys. Temp/fuel etc doesn't seem to have any effect.

Alex

[chim500nj]

Mine did this badly until I took it to Fernhurst where they cleaned the stepper motor and reset the base idle. It has only happened one time since then but I want to get to the bottom of it, I thought it may be the speed sensor but Fernies said if it was the speed sensor then the speedo would not work?...

So I have whittled it down to;

Throttle Potentiometer (bit of a tongue twister!)
or Speed sensor input to ECU..

It has only happened once in three weeks since the idel was reset but these niggles annoy me...

Is there anything else it could possibly be? considering car runs sweet as a nut all the time..



>> Edited by chim500nj on Tuesday 27th April 12:06

[Voltaire53]

Thanks again ... am living with it for now (worked fine the last few times) but, if it helps as a clue, it *occasionally* instead of revs dropping to 300 or so does the opposite and idles back to about 13-1400 before settling back to about 1000.

Well seems like a clean of the stepper is in order and a general "they all do that, sir" if the 100% cure cannot be found!