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When idling at 850 rpm, revs suddenly rise to 1300 and then go back and forth from 1300 to 1500 for possibly half a minute before returning to normal 850. Happens every time car is used but not every time engine is idling. Peugeot dealer AND very reputable specialised motor electrical engineers have both investigated and NOT found what is causing the fault. Any suggestions would be appreciated. Thanks in advance.

Not familiar with the TD engines yet, but if it were a petrol engine i'd be looking at the TPS, Idle control valve and coolant temp sensor.

Do you have a Haynes manual?

do you have air con ? if the air con is on the revs can rise and fall when the compressor turns on and off

sounds like the pump is breaking down

mine used to do that for a wee while, but only when cold, and worse when I was driving around town (sudden revving up and then down - a bit embarrassing, lurching about). Turned out to be a temp sensor, coolant I think. Cost about 30 squid to replace. I got the fault codes read and it turned up on them. Hope this might help. Anything is better than the fuel pump .... except the turbo, as far as I can tell :-)

turbo is mucho cheaper than the pump

Yeah I bet it is! It was just that I read here that in the 2.1td the first instruction when changing the turbo was "remove engine". That sounded quite severe (and expensive). :-)

Yes, Tiny Tim, I do have a Haynes manual. It only shows 2 or 3 of the 7 or 8 sensors (assuming it's a sensor problem)

Yes, darrenwall, I have air-con (climate) which I use (on auto) quite a lot. The problem persists regardless of air-con or not.

I would have thought, jameslxdt , that it would have expired by now as I have had the problem for the last 25k (2 years) but it has got progressively worse in last few months.

Thanx pavlf, temp sensor main dealer only, one of them at £12.99 (in stock) 2nd one at £22.00 (nil stock and no history of having sold one). Will buy and fit next week when re-open. Will let you know .

Thanx 2 u all 4 ur time

Just to back up pavlf, I had same problem and fixed it with the coolant temp sensor.. same price too iirc

that seems a common problem with alot of french carsnof that age... i used to have a renault 21 turbo and alot of people that had idle problems could trace it back to the coolant sensor.. or ISCV

I Have this problem with mine now and again, i spray the throttle sensor plug with wd40 dry it job done hope this helps.