Renault Megane - hunting idle problem

Hi all, this is one I've been fighting for ages...

2007 Renault Megane 2 litre petrol manual trans. For our American friends which never saw this model or engine, its a basic twin cam 4 pot with a MAP based injection, coil on plug and an on/off VVT on the inlet cam. Runs a Seimens ECU.

It behaves like a vacuum leak. Buts its not.... Or low fuel pressure. But its not... I have been learning a lot trying to fix this thing and before i learnt enough to start properly diagnosing I did the swapping parts thing from a second Renault I have with the same engine that runs fine. And ive swapped everything.... All inlet manifold gaskets done, manifold itself swapped even and then put back on the other car where the parts run fine.

Ive done it all. All sensors, even put in a new engine bay wiring loom from a known good running car!

I have the Renault specific diagnostic software, but it doesn't give useful fuel trim data for some reason. (good for codes and data feeds and configurations though). Have a basic EASYOBD software and an ELM327 thats giving me fuel trim and it seems to be running lean.

This was a car a recently assembled. The engine was from another car and it ran perfectly in that car. The car itself was also a good runner on its old engine. The dual mass flywheel was replaced with a Valeo brand in place of the OEM LUK item and the CAS runs off the flywheel that has the signal notches on it.

Any advice would be greatly appreciated on how I should attack this!

A couple of videos of the issue






Cheers from Australia

With a MAP engine any unmetered air in the intake will make the sensor report improperly causing the IAC to try to compensate. Hence the hunting effect. If absolutely sure no vacuum leak, look at the IAC.

Tried that with two other known good throttle bodies.

It doesn’t have a separate IAC, just the Bosch drive by wire throttle body.

In addition to the idle issue, the revs hang badly changing gear and it hesitates/jerks on a part throttle acceleration.

When it's reving up/hunting HOW the ECM actually commands the rpm to change: is it shifting ignition timing or is it changing the IAC?

Haakon wrote:

It doesn’t have a separate IAC, just the Bosch drive by wire throttle body.

Suggested the IAC as well but he says no IAC.

Neither - it doesn’t have an IAC, it operates the drive by wire throttle plate.

Ignition timing does change with revs, but I take that to be a normal to revs and load changes as it cycles up and down.

This seems to feature two coils in serious, right for #1&4, resp. 2&3, right?

If so, did you check for same coil resistances for each pair of them? Can you do an cilindro contribution test with the engine RUNNING, preferably with a good scope/software like USB autoscope or pico?

Yep, coils are paired. Control ground from the ECU.

I don’t have a scope, I’m still learning and not in possession of many tools. But I have swapped out the coils and plugs for known good items.

I have also swapped out the entire engine bay wiring loom from a known good running car… Like I said, I have done the parts swapping caper big time on this! It’s what’s made me start to learn how to properly diagnose it.

I will check resistance for the switched ground at the coil - maybe that would suggest an ECU fault if one of the supplies is different?

Anyone come across this setup? I reckon this might be something.

Page 17-7 of this pdf www.club-renault.ru/uploads/files/scenic/3307A.pdf

diyobd2blog.wordpress.com/tag/renault-can-clip/

Mine is a later model and the default base parameter is 50% rather than 128 here, but otherwise its the same. I've never heard of this, but my Richness Adaptive Gain and Offset are both stuck on 50% and not moving. Must have reset when the ECU was off the car for a few months when I was changing the engine.. Might have to give this road test caper a whirl...