Idle revs varying wildly
Discussion
As above.
A sticky stepper motor can momentarily get out of sinc with its pulse/ movement settings which throws the ecu reading out of step with the steppers actual position making the Lambds’s react trying to correct the now incorrect air/ fuel balance.
Firstly if it happens try turning the car off.
This re sets the stepper position back to zero.
You may find the car drives ok for awhile before stepper sticks again and the problem returns.
Often in the good old days when we often met up in our cars you could swap your stepper out of a mates car known to run well to see if this cured it.
Stepper motors or reliable ones can be hard to find.
If you do find a good one or yours works after a good clean and after fitting I’d be tempted to wrap it in heat proof foil as used on the inner wings etc to protect it a little from the heat when in traffic.
I think I’d try doing it with the coil too as heat is often the reason these components fail.
A sticky stepper motor can momentarily get out of sinc with its pulse/ movement settings which throws the ecu reading out of step with the steppers actual position making the Lambds’s react trying to correct the now incorrect air/ fuel balance.
Firstly if it happens try turning the car off.
This re sets the stepper position back to zero.
You may find the car drives ok for awhile before stepper sticks again and the problem returns.
Often in the good old days when we often met up in our cars you could swap your stepper out of a mates car known to run well to see if this cured it.
Stepper motors or reliable ones can be hard to find.
If you do find a good one or yours works after a good clean and after fitting I’d be tempted to wrap it in heat proof foil as used on the inner wings etc to protect it a little from the heat when in traffic.
I think I’d try doing it with the coil too as heat is often the reason these components fail.
Typical symptoms of an air/vacuum leak. Worse at idle as vacuum is highest at idle.
Hi vac sucks air in thru leak revs go high,
Stepper kicks in to slow the idle
Vac leak stops as vacuum strength drops
Stepper kicks in to raise revs
And repeat....
Go leak hunting .....get a "smoke generator" to do it properly.
Hi vac sucks air in thru leak revs go high,
Stepper kicks in to slow the idle
Vac leak stops as vacuum strength drops
Stepper kicks in to raise revs
And repeat....
Go leak hunting .....get a "smoke generator" to do it properly.
Hmmm, interesting.
I did clean the stepper motor but it didn't seem dirty at all, however it was quite loose, don't know if that may affect matters.
It seemed to settle down the idling, however I'm aware that my lambda emissions is always close to the MOT limit (1.028 against permitted maximum of 1.03) and with an MOT looming next week I'm keen to identify any possible causes - which I guess would be air leaks? any clues?
P.S. Where would I find the sensors, etc referred to so I can check/clean them?
Cheers
I did clean the stepper motor but it didn't seem dirty at all, however it was quite loose, don't know if that may affect matters.
It seemed to settle down the idling, however I'm aware that my lambda emissions is always close to the MOT limit (1.028 against permitted maximum of 1.03) and with an MOT looming next week I'm keen to identify any possible causes - which I guess would be air leaks? any clues?
P.S. Where would I find the sensors, etc referred to so I can check/clean them?
Cheers
I can’t answer your question, sorry, but I bet a loose stepper was causing some nonsense.
I took mine out to clean it (didn’t need it at all), but I did put a single layer of ptfe tape on the thread before refitting.
I also had noticed that the vacuum advance module on the distributor was loose - held on with one screw, so did that up too. Not any noticeable change but made me check other things in the engine bay and a found there was a quarter turn on the rearmost exhaust manifold bolts.
Might be near to needing new gaskets, but that job looks like a real pita.
Worth going around checking stuff periodically…
I took mine out to clean it (didn’t need it at all), but I did put a single layer of ptfe tape on the thread before refitting.
I also had noticed that the vacuum advance module on the distributor was loose - held on with one screw, so did that up too. Not any noticeable change but made me check other things in the engine bay and a found there was a quarter turn on the rearmost exhaust manifold bolts.
Might be near to needing new gaskets, but that job looks like a real pita.
Worth going around checking stuff periodically…
Any air leak upstream of the lambdas won't give you a weak lambda reading at the tailpipes, this is because the lambdas/ecu acts on that weak mixture and the ecu richens the fuelling to suit. You could drill a 1/4inch hole right into the plenum and (assuming there's enough headroom in the lambda trimming) the mixture would still present as lambda=1 at the tailpipe, just that the ecu will be doing massively richer trimming to achieve lambda=1 because of the extra (unmetered in this case) air.
You may well have slow reacting lambdas, or exhaust air leaks after the sensors, or the mot equipment is always 1-2percent out, or they haven't stuck the probe far enough into the tailpipe .. but if the lambdas are working *as they should* they don't care where the air comes from, if it presents weak or rich at the sensor the ecu will trim accordingly, up to its available working limit.
You may well have slow reacting lambdas, or exhaust air leaks after the sensors, or the mot equipment is always 1-2percent out, or they haven't stuck the probe far enough into the tailpipe .. but if the lambdas are working *as they should* they don't care where the air comes from, if it presents weak or rich at the sensor the ecu will trim accordingly, up to its available working limit.
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