idling high

Sounds as if you may have knocked off a vacuum hose while changing the plugs, there is no reason why just changing plugs will alter idle settings. so go back and have a good look around all the vacuum hoses and pipes that you may have disturbed.

It sounds like you installed the wrong spark plugs which change the timing of the computer. Here is a guide hat can help as well

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

Please let us know what happens.

Cheers, Ken

Engine Performance problem
1999 Renault Scenic Front Wheel Drive Automatic 67,000 miles

i have a Automatic Renault Scenic,when i start her up,She revs up at 2000 revs,after 5 mins or so she ticks over below 1000 revs,i was told this is common for automatics,but when im driving and sitting in traffic,her revs should tick over at below 1000,but her revs go back to 2000,she only goes to ticking over nicely after a good warm up, have i got a problem or is it cause shes cold ?

outside temperature is a factor in all idle speeds when the engine warms up so does the air around the engine thus sensor pick up on this ... a 2000 rpm tick over is still quite high even on the coldest morning .. have the cars computer scanned for fault codes ..check the IAC (idle air control valve)

i replaced dephaser pulley cause it was making that rettle nois on cold start so thats gone,but i still have low revs intermittendly. and it only rev up if i floor the excellorator

check the idle air control valve. Check throttle positions sensor.

I have a Renault Scenic atomatic 2005. It starts ok but when I take my foot off the throttle it cuts out, dosnt idle at all. I have managed to drive it with keeping my left foot on the brake and keeping the revs up with my right foot, but when its idling like this it is very slugish and lumpy. I have had it in the dealers garage and they hooked it to their computer and say its all fine.What shall I be checking, the sensor was replaced by renault in May

What sensor was replaced?
Check for vacuum leaks.
Remove and clean the Idle Air Control Valve.
Remove and clean the EGR, if so equipped.