Idling probs - revs up and down like a see-saw

[itm]

I've got a 1994 Griff 500. Having not been driven for a couple of months I took it out last night and hit a problem with idling. From time to time the idle revs will continually shoot up and down like a see saw (from just about zero to 1,500). After several bounces the engine will sometimes stall - but not always. This happens when fully warmed up as well as when cold.

Does this sound familiar to anyone??

TIA
Ian

[llanelli-rob]

Do a search on stepper motor cleaning on this forum!

[itm]

Thanks for the reply. The stepper motor was cleaned and checked last year for a similar problem, but wasn't found to be a problem.
Are there any other obvious things to check?

[Hoover.]

after a service and a clean of the stepper motor mine does that..... takes a few thou miles to get back to normal...... wierd I know....

[lig]

Reset the ECU,

Keys out - Unplug it, wait for 15secs, plug back in and away you go..................................

Lig.

[v8 racing]

there are loads of things that can cause this but one of the most common is a faulty coil or rotor arm

[itm]

Thanks - I'll try the ECU suggestion. Would a faulty coil/rotor arm explain the rev needle literally bouncing up and down between 0 and 1,500 rpm? It's not just "lumpy", it's actually a bizzarrely regular bouncing pattern on the rev counter!

[v8 racing]

yes it would the rev counter is a direct reading from the coil

[rfisher]

www.pistonheads.com/sales/64328.htm

[tvr4ever]

rfisher said:

www.pistonheads.com/sales/64328.htm

Excellent piece of equipment, just bought one myself, not even that expensive.

Fred

[rfisher]

tvr4ever said:

rfisher said: www.pistonheads.com/sales/64328.htm Excellent piece of equipment, just bought one myself, not even that expensive. Fred

Even less to hire I hear!

[tvr4ever]

rfisher said:

tvr4ever said: rfisher said: www.pistonheads.com/sales/64328.htm Excellent piece of equipment, just bought one myself, not even that expensive. Fred Even less to hire I hear!

Hiring one is cheaper, but only if you hire it for one week...

Fred

[itm]

Excuse the dumb question but....I've just been tangling with the ECU and can't see where to "unplug" it. Is it possible to remove the main connector on the ECU? (I can't see any obvious clips or screws). The cable disappears out of the battery area and I'm hesitant to pull it in case it disturbs anything else.

Could anyone offer assistance??

[mongoose]

itm said:

Excuse the dumb question but....I've just been tangling with the ECU and can't see where to "unplug" it. Is it possible to remove the main connector on the ECU? (I can't see any obvious clips or screws). The cable disappears out of the battery area and I'm hesitant to pull it in case it disturbs anything else. Could anyone offer assistance??

You can unplug the ecu at the multi pin connector.There is a little tab at the one end of the connector,you just squeeze this inwards as you lift the connector away from the ecu body-dead easy

[itm]

Thanks - thanks did the trick!

[itm]

Oh dear things seem to have taken a turn for the worse. After resetting the ECU I took it out for a run. Everything was fine for the first 10 minutes, then the see-saw idling started again, with the engine stalling regularly. Five minutes later and it was cutting out every time I let the revs drop.
The most worrying thing was a loud whirring noise from the engine bay, which persisted for a few seconds even after I'd taken the keys out of the ignition.
This sound familiar to anyone?

[mongoose]

Sounds to me like your stepper could be knackered?

[itm]

Presumably the whirring noise being the sound of the dying stepper motor?
How much are these to replace?

[tvr4ever]

itm said:

Oh dear things seem to have taken a turn for the worse. The most worrying thing was a loud whirring noise from the engine bay, which persisted for a few seconds even after I'd taken the keys out of the ignition.

With the above mentioned fault code reader you would probably see faultcode 48

48 Stepper motor fully open above 500 rpm or fully closed above 750 rpm Sticking stepper valve, incorrect base idle speed adjustment, air leak on non-cat cars, incorrect stepper motor adjustment, incorrect throttle butterfly adjustment, rough running because of fuel or ignition or mechanical faults.

Bought mine for 30 dollars, money well spend.

Fred

[GreenV8S]

You should be able to hear the stepper motor re-park itself if you switch the ignition on and off again. If not then perhaps there's an electrical fault, or it has mechanically jammed.

Rob from V8D recently pointed to another common cause for these symptoms, as far as I can remember he said that it can be caused by a faulty rotor arm. Worth a search to check, because the symptoms sound very similar.