Revs all over the place!!!!!!

[Meatball]

I could do with some help my cars running like a bag of s4it. It will hold revs on idle at 1500-2000rpm then suddenly drop to 1000rpm then run normal then spike back at 1500-2000rpm.
Ive tried the following
Re set the ecu
Re set the throttle pot at .33 - .35 (would a duff throttle pot cause this?)
Cleaned the butterfly and linkage.
The allen key bolt is still there (not worked loose)
The butterfly is opening and closing correctly and not catching.

And the suns out great

[pwd95]

Sounds like an air leak somewhere along the inlet after the AFM.

[Ballistic Banana]

All together now stepper motor!!

[Chris Smith]

Ballistic Banana said:

All together now stepper motor!!

Certainly a good starting point

[Meatball]

Ballistic Banana said:

All together now stepper motor!!

Steppers clean and working fine

[Chris Smith]

Is it just the tickover that is bad, or is it then subsequently bad throughout the rest of the rev range too?

[Meatball]

[quote=Chris Smith]Is it just the tickover that is bad, or is it then subsequently bad throughout the rest of the rev range too?[/quote

Its happens on both im wondering if its the throttle cable sticking??? because i have to blip the throttle to release the sticking point. If i press the throttle down and release the revs stay on for a bit longer than they should.
Weird huh?

[blitzracing]

The reason the car cycles the RPM, is the ecu applies a series of pulses to the stepper to slow the engine down, and then waits for the idle to stablise for 3 or so seconds. So example the ecu would expect the the engine to drop 400 rpm in 3 seconds, say from 1200 rpm to 800 rpm. The problem occurs when the engine RPM drops faster than this value, so after 3 seconds the RPM might be 600 rpm, so too slow, the the ECU winds the stepper out again, and waits, so the RPM shoots up again, and the cycle repeats. There are multiple reasons for the RPM to drop to fast, most of them are mixture or air leak related. As the engine gets near idle, any slight air leak in the plenum area has a much greater effect on the idle mixture, so the engine can get near stall very quickly as the mixture leans out. Its also possible that an ignition problem could cause it, if say a plug was mis firing at low RPM, so the engine power output is unstable. To eliminate the actual stepper, wait until the car is warm, and ticking over correctly, then disconnect the stepper, and see if the idles then OK after you drive it. Not a 100% test, but a pointer. You say the stepper is OK, I hope its not a nice shiny new one of ebay, made in China ??

Mark

Edited by blitzracing on Monday 29th September 13:11

[Meatball]

blitzracing said:

The reseon the car cycles the RPM, is the ecu applies a series of pulses to the stepper to slow the engine down, and then waits for the idle to stablise for 3 or so seconds. So example the ecu would expect the the engine to drop 400 rpm in 3 seconds, say from 1200 rpm to 800 rpm. The problem occurs when the engine RPM drops faster than this value, so after 3 seconds the RPM might be 600 rpm, so too slow, the the ECU winds the stepper out again, and waits, so the RPM shoots up again, and the cylce repeats. There are multiple reasons for the RPM to drop to fast, most of them are mixture or air leak related. As the engine gets near idle, any slight air leak in the plenum area has a much greater effect on the idle mixture, so the engine can get near stall very quickly as the mixture leans out. Its also possible that an ignition problem could cause it, if say a plug was mis firing at low RPM, so the engine power output is unstable. To eliminate the actual stepper, wait until the car is warm, and ticking over correctly, then disconnect the stepper, and see if the idles then OK after you drive it. Not a 100% test, but a pointer. You say the stepper is OK, I hope its not a nice shiny new one of ebay, made in China ??

Mark

Hi Mark thanks for the info, Ive played with the adjustment on the throttle linkage and it has brought the revs back down to the 1000rpm range on idle so thats a start. My stepper looks to be the original one, i pulled the pin/plug out today and cleaned it just to be sure but it wasent dirty.
I`ll re seal the plenum again and see if that helps.
Would the ecu throw a code on a low rev miss fire? as i think i can get my hands on a fault code reader.

Edited by Meatball on Sunday 28th September 20:37

[big geoff]

i can pop around and take a look after work just call me geoff

[Meatball]

big geoff said:

i can pop around and take a look after work just call me geoff

YHM i mean text

[blitzracing]

Meatball said:

blitzracing said:

The reseon the car cycles the RPM, is the ecu applies a series of pulses to the stepper to slow the engine down, and then waits for the idle to stablise for 3 or so seconds. So example the ecu would expect the the engine to drop 400 rpm in 3 seconds, say from 1200 rpm to 800 rpm. The problem occurs when the engine RPM drops faster than this value, so after 3 seconds the RPM might be 600 rpm, so too slow, the the ECU winds the stepper out again, and waits, so the RPM shoots up again, and the cylce repeats. There are multiple reasons for the RPM to drop to fast, most of them are mixture or air leak related. As the engine gets near idle, any slight air leak in the plenum area has a much greater effect on the idle mixture, so the engine can get near stall very quickly as the mixture leans out. Its also possible that an ignition problem could cause it, if say a plug was mis firing at low RPM, so the engine power output is unstable. To eliminate the actual stepper, wait until the car is warm, and ticking over correctly, then disconnect the stepper, and see if the idles then OK after you drive it. Not a 100% test, but a pointer. You say the stepper is OK, I hope its not a nice shiny new one of ebay, made in China ??

Mark

Hi Mark thanks for the info, Ive played with the adjustment on the throttle linkage and it has brought the revs back down to the 1000rpm range on idle so thats a start. My stepper looks to be the original one, i pulled the pin/plug out today and cleaned it just to be sure but it wasent dirty.
I`ll re seal the plenum again and see if that helps.
Would the ecu throw a code on a low rev miss fire? as i think i can get my hands on a fault code reader.

Edited by Meatball on Sunday 28th September 20:37

Its very unlikely you will get a fault code, as even though the idle is unstable, the lambda probes are still likely to be switching at least some of the time. It takes some time period of no switching activity, to flag up a lean or rich error.

mark

[Earl Grey]

i recently found that the same problem .. it was the rotor arm . and dizzy cap.