Having been around cars for a long time, this has me stumped.

2005 Renault Megane Estate with 1.4 K4J engine. Approx 125 000 km. Engine is restless at idle. Revs are unstable and feels like misfiring.

Been checking diagnostic data from time to time, but nothing that would point to an issue. Sometimes I get P0638 and P2120, but very rarely. Has to be a cold and wet period with about a week without driving to get those codes to appear.

I took my small scanner and some photos of the live data when running at idle and fully warmed up: LINK

I believe the scanner window is 15 seconds long.

To me it looks like an ignition problem, but the coils seem fine (common issue on this engine and according to previous service info they have been all replaced 2x now) and spark plugs are fine. When cold it runs fine and when you raise the RPMs a little. Also, when under load, like cruising on the motorway with a trailer, small surges can be felt as if the engine is starving for fuel.

LTFT is also a little too high for my liking but the STFT looks fine. Not sure about the ignition timing, it goes all over the place. Both O2 sensors seem to be cycling normally, the second one is a little slow and "hangs" but the ECU does not flag it as a fault. MAP also reads normal and stable.

What else to check? I can post more data if needed.



STFT1 (2)



Spark Advance

Spark Advance Graph




O2S12 (2)


  • Welcome to the site. I'm curious as to why you believe the MAP reading to be fine (44 kPa) seems fairly high if this is a naturally-aspirated vehicle.
    – Zaid
    Jan 3, 2021 at 21:43
  • It is an NA engine. It is a bit on the high side, but not terrible. It is stable and behaves predictably to throttle. I should point out that this engine has a fully electronic motorized throttle body, without IAC and other accessories.
    – eerik
    Jan 3, 2021 at 21:51
  • 1
    I just wanted to add this here: +50% ltft is massively out of acceptable. I would personally consider anything over 10% to be unacceptable and I would investigate anything over 5%. Now to be clear I'm not trying to bash you here at all but any specification I've worked with on cars will call issue to ltft long before where you're at. A vacuum leak is definitely where I'd look and although it could be fuel starvation I'd be looking more at vacuum first.
    – Techlord
    Feb 4, 2021 at 4:39

1 Answer 1


This thing has vacuum leak written all over it:

  • 44 kPa manifold pressure at idle seems high for a naturally-aspirated engine
  • At nearly +50% the long-term fuel-trim (LTFT) is working overtime
  • The lambda response appears to be sluggish
  • Lumpy idle observed

You might want to review this case with a similar set of symptoms.

  • For the lambda response, do you mean the O2S11 or O2S12 response graphs? The O2S12 is after the catalytic converter and O2S11 is before. On the pre-cat sensor, I do not like the random hanging in high state. Also, I did test for vacuum leaks and found that when I artificially create one, the engine revs up (as it sees the intake pressure rise as if the throttle is being opened).
    – eerik
    Jan 3, 2021 at 21:54
  • Yes, I was referring to both O2 voltage readouts. O2S11 isn't too bad, but O2S12 should be pegged rich if the fuel management is doing its job properly. See the diagram in this answer to see how the waveforms should look like. I'm not implying that your cats are bad, if the system is struggling to keep the air-fuel ratio stable you can get similar waveforms
    – Zaid
    Jan 3, 2021 at 22:00
  • @user1849874 just saw your updated comment. Regarding your vacuum leak check, I would recommend you do it another way: block the air intake completely and see if the engine continues to run. If it does, you know you have a vacuum leak.
    – Zaid
    Jan 3, 2021 at 22:09
  • To be clear here if the engine does stall with such a test it does not rule out a vacuum leak. The leak would be extremely severe to allow the engine to continue running.
    – Techlord
    Feb 4, 2021 at 4:33
  • Had the intake tested by a professional mechanic with special tools. No leaks were found. MAP sensor was also cleaned and tested, no issues found. I was however recommended to replace the pre-cat O2 sensor.
    – eerik
    Mar 18, 2021 at 17:03

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .