Corsa 1.0 12V 1999 Eratic Engine Speed

[tonger]

This has me stumped

IAC valve failed a month ago and replaced it with a new item. Started car the other day and it coughed and put on the ECU light on dash, read out the codes using Vauxcheck and I have a P0505 idle system malfunction. First thought was that the valve had gone again but the symptoms are weird. With the throttle set so that the IAC is not in contact with the throttle (so not doing anything) the engine RPM varies up and down making the car hard to drive....really lumpy.

If my understanding is correct, the IAC is just a stepper motor that adjusts the rest position of the butterfly to maintain correct idle. So maybe the fault code is a red herring as the ECU is just having trouble controlling the revs. If I rev the car to 3k and hold for a while the light goes out for until it idles again.

Any help or suggestions appreciated

[haydn1987]

Yes its most likely that the ECU thinks there is a fault with the idle control motor because it can't control the revs.

irratic idle can be caused by a vacuum leak check the hoses that go into the throttle body and inlet maifold for cracks or loose fittings.

have you done anything to the fuel pressure regulator, Lambda sensor or injector? they could also cause problems.

[tonger]

Thanks for the tips.

Had a look tonight and couldn't find any air leaks, listened with a hose and also squashed pipes one at a time to see if the fault changed.

Interestingly, discovered that the erratic revving only occurs when the engine gets warm. When cold it is a bit rough (possibly because the ECU has gone into a safe mode) and engine speed control seems ok at all throttle positions. As it warms up, opening the throttle sounds like a lot of induction noise for a moment then the engine speeds up, then slows and induction noise increases etc etc. Almost like the ECU is overcompensating for something. As the engine speed increases, this cycle changes speed too, taking longer to go from slow to fast and back, then at higher throttle settings the engine is smooth again.

Beginning to suspect the EGR and associated airways, is the EGR only open when warm.......what is the sequence of operation for the EGR? Anyone know?

Might have to resort to professional help but don't like giving up.

Does anyone know of any freeware diagnostic programs I can plug into my ELM327 interface to give me real time engine data, apparantly Engcheck from Gendan won't work on this model.

Cheers

[tonger]

Problem fixed

It was the idle control motor failed again, exchanged it and all ok.
It would appear that the throttle closed contacts in the idle unit were high resistance causing the ECU to sentence it as defective. The ECU then tried to control the idle by adjusting the mixture or ignition advance, this caused hunting at medium revs but only when the ECU is running in 'closed loop' mode, i.e. the engine is warm. That's why the hunting stopped when I disconnected the temp sensor or the TPS as both of these force it into open loop (warm up) mode.

Thought I'd update just in case anyone else had the same fault......looks like the ECU error message was right all along.

Cheers