When sat still the car revs itself like someone keeps tapping the accelerator,, imtrying to sort this out as it has recently failed its mot, with high emissions, and wondered if this had anything to do with that , any help gratefully recieved, car is a 2002 renault megane scenic 1.4
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if it is a carby engine then check the settings of the automatcic choke and the throttle cam screw settings
it works in a manner so that when you press on the accelerator the automatic choke flips a cam around and the idle speed screw rests on that cam giving a high idle speed until the engine is hot enough not to need a choke at which point the cam is held clear of the idle screw
if efi engine run the fault codes to check for problem accelerator position sensor or throttle position sensor.
You probably have a bad Idle Air Control Valve (IACV). In fuel injected carburetors when the throttle valve is closed the IACV opens and closes to control the idle speed. If it malfunctions the engine may rev up allowing the car if in gear to accelerate on its own . When I had a bad IACV in my Mitsubishi Lancer it would drive down the road at 50 mph with out touching the gas pedal. Fortunately, it could be slowed down by using the brakes (unlike the Toyota stories).
Check the accelerator linkage under the hood.
It should be just in the rear of the ngine compartment, right on top. You will see a lever and linkage, pull the lever with your finger and the engine should rev up......if it does, then trace the cable back to the firewall and to the pedal itself.....
start witha new fuel filter and perhaps a intank additive such as heat. also look inside the air box for nests. also common. if all is clear, let it run and idle for about 10-15 minutes look underneath the vehicle and look to see if anything under there is cherry red. if not try holding the accelerator down for a few momoents and check again. if it glows, replace the catalytic converter.
I've just had the same thing on my Elantra 2001. I took it to the dealership and they fixed it easily; it was a malfunctioning 'Throttle Position Sensor'. They fixed the faulty connection but they said a new one would have cost about 175GBP (275USD) fitted. Hope this helps.
I would sooner think it was caarbon on the throttle plates and a very dirty idle air control
Take off the air intake tubing so you can see the throttle plate on the rear side of the throttlebody there is a small round electrical item the connector will likely come in from the side on the end of this item it will have two (2) TORX head screws holding it on you can see the tim if you get a bright flashlight and look straight down into the hole in the throttlebody near the idle air controller you hould see a tapered plunger its likely all coated with carbon. its best to remove the IAC and thuroughly clean it but some good carb spray may do it on the plunger. Now open the throttle and put a stick or a brick on the accelerator pedal of course with the engine off!!!!! the throttle plate will wide open dont put your fingers in theere in case the accelerator snaps shut it will cut your finger nearly off, clean BOTH sides of the place till its spotless and the walls as far down as you can reach all around rinse it good with carb cleaner and take the block off the accelerator , put everything back togother and start it it will start hard but it will start and rev it up gently a few times and see if it idles better if not you need to remove the IAC and clean correctly.If this information helps please give me a fixya Rating thanks,Randy
contact me if you have any questions
you should use about half the can of carb cleaner to do this.