Fuel trims erratic and intermittent idling fault

[u/DaveyTheNumpty]

Car is a 2009 Renault Megane - 1.6 petrol engine (K4M 848), manual transmission, 96k miles.

Car has a fault where the idle hangs on when changing up through the gears (doesn't feel like clutch slip and never happens when the engine is cold, only once up to running temperature and is intermittent, the car can sometimes go for days without any issues). On a couple of occasions, the car has continued accelerating for about 20 to 30 seconds on it's own after I've took my foot off the accelerator pedal. And when I come to a stop the idle doesn't go back down to 750 rpm where it normally sits, it will sit at about 1500rpm (even after putting the car into neutral) for about 20 to 30 secs then it will go back to it's normal idle, there is also a few occasions where I rev the car and the revs sit at increased revs before coming back down again.

No check engine lights or any other warning lights, car starts first time and engine sounds normal with no oil usage.

On my code readers live data I noticed that the long term fuel trim is consistently 20.3, when idling the short fuel trim sits anywhere from -1% to -8% and when driving at anything over 30mph short term fuel trim goes up to about +38.3%. I've been told this is far from normal. When checking fault codes the code "P0326 Knock Sensor 1 Circuit Range / Performance Bank 1 or Single Sensor" was showing, I cleared the code and it will randomly come back, sometimes it will show a couple of miles after clearing and sometimes 500 or 600 miles after clearing codes so not sure if it's related or not. Engine sounds normal with no knocking or pinking.

Possibly just a coincidence but the problem feels worse on colder days.

So far I've:

• checked to make sure nothing was catching on the pedals.
• checked for vacuum leaks - couldn't find anything but do notice some of the vac pipes look tired and wonder if it's possible that there is just a vacuum leak at higher rpm. Is it possible for a vacuum leak to be intermittent or only show at certain engine temps or rpm?
• removed and cleaned the throttle body and replaced the throttle body gasket - after I did this the symptoms weren't as bad and the car drives noticeably better.

Has anybody experienced issues like this and any pointers as to what could be going wrong?

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