Replace stepper motor or plenum chamber?

[taylormj4]

Have been having trouble for ages with idle revs sticking at around 1800-2000rpm. Have cleaned the stepper before and this cured it.
Cleaned it and inside the plenum again on Monday as problem had returned. Have to say it wasn't really that dirty. Ok for 1 day and then problem reoccurs again today!
Have tested the resistance of the stepper as per SH's book...all Ok.
Does this mean that the stepper is catching in the plenum chamber and I need to replace one or both?
Any ideas?
Matt

[Boosted LS1]

I can't see the point in changing the plenum. It's a lump of aluminium and not likely to change much. I'd make sure airways and ports are cleaned out and go back to the stepper. Are all your pipes in the correct places? Nothing sticking on the throttle blade or linkage? Hidden bypass screw set ok for emmissions and is it the right plenum for your engine?

>> Edited by Boosted LS1 on Wednesday 12th November 21:16

[MOD500]

Sometimes I find the revs stick at 1200 - 1500 revs too, cleaned the stepper as you have, this cured it for a while. Get the same problem now and then, I find the best way of getting round it is to warm the car up, then pull over turn the car off then re-start. This cures it for me, but I only find this prob infrequently, how often do you find it happening?

[GreenV8S]

If the problem is still there, it might be worth disconnecting the stepper motor when it's idling about right, and see whether this immediately cures the problems. If it does, you might be looking at an ECU/wiring fault, duff stepper motor or simply that you aren't cleaning it properly. This would rule out things like a sticky throttle spindle, vacuum leak and so on.

[Boosted LS1]

I've just realises something which may be helpful or not. If I'm right in my recollection and the stepper is drawing air from the rocker cover into the plenum then air has to be allowed into the engine via the small breather at the back of the rocker cover. If that gets blocked there will be an increase in vacuum and air won't flow easily into the plenum. The beather has a very small hole so will be easy to block.

[taylormj4]

Boosted LS1 said:

I've just realises something which may be helpful or not. If I'm right in my recollection and the stepper is drawing air from the rocker cover into the plenum then air has to be allowed into the engine via the small breather at the back of the rocker cover. If that gets blocked there will be an increase in vacuum and air won't flow easily into the plenum. The beather has a very small hole so will be easy to block.

No, the stepper draws air via a tube connected to the air intake of the plenum, before the throttle disc. Have checked in there and there doesn't seem to be a blockage. Stepper motor is shiny clean. This is why I wonder whether there is a burr on the plenum chamber that is catching the stepper cone. Can't really see in there though to check / clean which is why I was considering replacing.
Matt

[taylormj4]

GreenV8S said:

If the problem is still there, it might be worth disconnecting the stepper motor when it's idling about right, and see whether this immediately cures the problems. If it does, you might be looking at an ECU/wiring fault, duff stepper motor or simply that you aren't cleaning it properly. This would rule out things like a sticky throttle spindle, vacuum leak and so on.

Once the engine is idling Ok, it remains there until the throttle is blipped or engine loaded and then you try to return to idle, e.g. stopping or changing gear. The throttle actually seems to stick open so the car tries to drive off with a fair acceleration. If you dip the clutch the revs rocket up to 2000rpm again. It's as if the ECU sees the low revs during setting off from standstill as the engine trying to stall so it whacks up the revs and leaves you fighting the car with the brakes.
Matt

[shpub]

Sounds like a sticky throttle to be or a duff speedo setting toi the EC or a duff ECU. Time to get the analyser plugged in.