Sporadic idle issue

GORDON LUNDIE

MEMBER
2015 PEUGEOT 308
1.2L
2WD
MANUAL
41,100 MILES

So my car listed above SW model developed a misfire due to a faulty spark plug two months ago. All the spark plugs were replaced but now sporadically when stationary at traffic lights, the engine will sit at idle and then for no reason rev up and dip down if revs and the whole car lightly shakes as it feels the engine is struggle to run properly. The car still pulls away fine. This issue doesn't happen each and every time. Some journeys it might not happen at all and then others, it happens enough that I am convinced that the car won't make it home. One other observation is that in second gear when I begin coasting there is a light kangaroo hop/jitter that I am feel as the engine doesn't want to smoothly work down the rev range. I have taken it to the garage who plugged in a diagnostics machine but nothing was flagged up.

It might be that this has nothing to do with the spark plugs getting changed but I didn't experience this issue before hand.

And help, suggestions or similar stories would be appreciated. The car is still under extended warranty so I'm fairly keen to know what it is so that I can try and get Peugeot to fix it.

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,645 POSTS

Hi and thanks for using 2CarPros.

The issue may have been caused by the plug replacement. If an engine vacuum hose was damaged, disconnected, or missed, it can cause a similar problem. Additionally, there is a component called the idle air control valve (IAC). The IAC is responsible for maintaining engine idle speed. Basically, with fuel injected cars, the throttle plate fully closes when you are off the gas. However, the engine still needs air to run. The IAC allows a metered amount of air to enter the engine. When they start going bad, the engine idle can go up and down.

I apologize, but this vehicle isn't offered in the US so I don't have technical information on it. However, what I described is relatively standard on all vehicles produced today.

What I suggest is this. First, confirm that no vacuum likes were damaged and +make sure the air intake tube that runs from the air filter box to the throttle body is properly attached and not allowing unmetered air to enter the engine.

Here is a link that shows how to check for engine vacuum leaks:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Since the IAC is operated electrically, check and confirm the plug is properly attached. Here is a link that shows how to replace one. It is general. However, it will help you identify the one on your vehicle. By the way, the IAC is mounted on the throttle body.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

Here is a link that discusses high idle issues:

https://www.2carpros.com/articles/engine-idles-too-high

I hope something here helps. Let me know if you have other questions.

Take care,
Joe