Tick over rises as car starts to roll-WHY !!!

[lakemaligne]

I have owned a 1996 2Ltr 16v LX petrol 406 saloon from new ,which has just rolled over 158,000 miles. I would like to know what is causing the following fault please chaps.

With the car stationary at traffic lights with the engine ticking over at the normal 950 rpm,if i let the car start to roll forwards or backwards,the tick over starts to climb all by itself to around 1200-1400rpm.If i stop the car rolling,the tickover drops back down to 950rpm.It only does this every few days or so,but when it does ,it is accompanied with a "jerk" as i lift on and off the throttle.
I have replaced the Oxygen sensor,throttle potentiometer,Stepper Motor,and throttle cable so far,and was wondering whether I should get another ECU from a scrap yard,but not sure whether i could just do a straight swop or would i have to do any setting up.

[DaiRees]

Hmm, I'm not so sure that that is a fault, seems strange it only does it intermittently though. I have occasionally knocked the car into neutral when rolling up to a junction or down a hill and noticed that the revs stay up a bit until you actually stop, at which point they drop to normal tickover revs. I always assumed this was the ECU keeping the revs up in order to provide power to the brake servo, PAS etc?

[rapierKen]

The tick over is related to the speed sensor which is connected to the engine ecu etc.
When my speed sensor failed I noticed when I declutched that the revs dropped lower than normal?
The ecu keeps the revs about 100 rpm higher when any speed is detected (wether in neutral or not).
It makes for a smoother gear change when moving.
I don't know why yours raises to 1400rpm.
Is yours a petrol?
If it is does it have an idle air control?
Maybe just needs a clean, it might be sticking trying to keep up!

[neildavies]

Mine has always done it, I put it down to providing power for the brakes and steering.

[robb]

My 2.0 petrol has done this all the time Ive had the car it does seem to be a normal thing the revs dont drop properly until Ive stopped

[lakemaligne]

Hi Rapierken, and others who have replied so far.Can this speed sensor you mentioned go intermittent every now and then,and if so where is it located and how can i test it. Yes mine is a 2Ltr petrol.

Regarding the high tickover,my 406 only started over revving about a year ago ,and up that point the revs alway's smoothly fell straight down to the normal 950rpm if i knocked it into neutral while coasting. When my 406 is miss behaving ,the rpm has some times bombed down to 300-400 rpm ,and has at times tried to stall.These tick over symptoms always coincide with a very noticeable jerk ,which makes my wife and daughters head jerk back and forth when it occurs,so this is diffinitely not normal. It is this "jerk/Jolt" which is the most irritating aspect of the fault. It's almost as if the power abruptly comes in a second after i depress the throttle ,and then suddenly shuts off a second after i lift of the throttle,almost as if the fuel is suddenly shut off. Then it will drive very smoothly for several days,then for no apparant reason,it gets upset again.Does this sound like an ECU fault of a sensor fault anybody ?

The stepper motor on my engine opens an air idle valve which bypasses the butterfly valve when it is is shut during low engine revs,and is opened by the stepper motor during idling,and this has been replaced.

I know i am clutching at straws here,but i was wondering if it could have anything to do with the fact that my 406 still has the original CAT fitted bearing in mind it's 14 years old and has 158,000 miles on the clock. As the Oxygen sensor is located very near to the CAT ,could it somehow how be getting contaminated from it,which the Oxygen sensor reads as an incorrect fuel mixture and sends the wrong data back to the ECU,which adjusts the fuel flow incorrectly. Just a theory,bit far fetched i know!