High Revs

[goonerjon]

Hi
I have a 54 Megane 1.4 16v Dynamique with high mileage (115,000) and the car is revving very high when sat in traffic or when idling i.e. randomly spiking to up to 4,000 or even 5,000 rpm. Anyone got any idea what it might be? I've had the throttle body cleaned and this has made no difference. Someone has said that it could be a new throttle computer or something else? The garage I've taken it to is running out of ideas but is looking to charge approx £500 to source & fit a new cpu (only trouble is I can't remember which cpu!) Has this happened to anyone else's car?
Any help would be appreciated.
Thanks
JB

[davelowe]

This is odd. Do you have any OBD codes - this will help with the diagnosis. It could be as simple as a vacuum leak, or be as complicated as a new engine.

Other contenders are the throttle pot on the 'go' pedal being duff.

[goonerjon]

Hi
No, I have no OBD codes. Apparently it's a new injection computer that I need. My current part has been sent away to be tested but no fault was found which makes me a little suspicious as to why it would get fixed with a new one??
Again, if anyone has any ideas it would be appreciated.
Thanks
JB

[Keyholder]

High revs in traffic usally means, either 1 of 2 things.

TPS - throttle position sensor ( attachs to the end of the accelerator cable - to know how far your foots down , i,e how fast you want to go)

or

there could be a problem with youre O2 / lamba sensors -

Either of which would of thrown a error code up on the ol computer system.

So im betting due to idle controll being on the ECU - Not cpu , id say you could need a new ECU.

What you should do is get the garage who you took the car to, to do a live test. basically take the car out for a drive, whilst the techie is sitting in the passenger seat with the computer plugged into the cars obd port and watch the live readings.

Do the high revs happen when the engine is hot or cold ?? or does it happen regardless of how long the engines been running ??

[goonerjon]

Thanks for the advice
It seems to be when the engine is hot.
Why would the garage think it would be the injection computer?
It's starting to get very annoying now so need to get it sorted out, but trying to avoid spending £600+ on it if I don't have too.

[Keyholder]

somethings telling me exhaust lamba sensor, of course which works when the engine is upto normal operating temp, then again new ones nowadays warm the sensor up tellilng the ecu that the cars upto temp thus making the idle drop down and use less fuel etc etc.. exhaust lamba sensor dont work when the engines cold, untill upto the right temp.

Do you have a vacum leak anywhere. when the engine gets warm / hot it could make a pipe all soft and create an air gap, ?? rare but can happen,... Can you get another lambda sensor from a local scrappy and see if it cures youre problem,, then again if it were a dodgy sensor it would of shown up on the computer.

if it were an injection problem, im sure it would do it 100% of the time .. air flow meter maybe ??

Be handy if you can swap some sensors over with another friends car. - Or failing that try another garage see if thier computers pick up a fault. You might find that the garage you took it to dont have the correct equipment to read the codes, but thats just a wild guess. ???? im sure someone will come along with some more input.

[davelowe]

You should definitely check whether the car is going from closed loop to open loop fuel trim. If it isn't, then either there is a sensor to blame or the ECU is kaput (which I sincerely doubt).

I'd wager you have either a vacuum leak, an intermittent connection or bad sensor - any good auto-electrician should be able to diagnose the problem within an hour especially if the are familiar with the CLIP computer. Phone around and ask them if they have CLIP - any that don't, well, possibly avoid.

The ECU is coded to other computers in the car and also the injectors. I would imagine that a brand new one would cost £600, never mind the coding time and expertise. Usually these are sourced second hand with the UPC and card reader bundled.

[Skidms9]

I've had the same problem with 2 meganes and the alternator was at fault in both cases. Alternator output is monitored and regulated based on electrical load, A/C, heated screen, lights etc. In both instances the alternator falsely reported low output to the ECU so in response the ECU increased engine RPM to increase alternator output. This could be the cause of your problem. Alternator output can be checked using the Clip diag tool. Might be worth checking out.

[davelowe]

The above is an excellent idea.