Unstable warm idle

[Tanguero]

My 4.2 Cerb has developed an instability whein idling with a warm engine. Revs 'pulse' sharply up by about 100 rpm then drop back about 1 per second. It also has a tendancy to stall if left to its own devices, I think the base idle speed creeps downward while pulsing, but I havn't confirmed that with the software yet. When cold it idles steadily at 900 rpm. I have hooked up a laptop with diagnostics and can't see anything obviously out of the ordinary. Fans are coming on normaly when the engine is fully up to temp, (indicated 100 on the dial and 85-90 on IR thermometer)

The car has TCS linked decats and a sports exhaust. The pulsing is not coincident with either of them being fitted so I dont think that is particularly relevant.

Any thoughts as to where to look next gratefully received!

[trackcar]

sounds like a throttle pot failure .. the spiking voltage from the failing pot is seen by the ecu as you opening the throttle so it adds accelration fuelling and ignition advance .. hence the revs fluctuate without you touching the pedal. you need the diagnostics software hooked up to confirm, or get it to your local specialist.

[Tanguero]

trackcar said:

sounds like a throttle pot failure .. the spiking voltage from the failing pot is seen by the ecu as you opening the throttle so it adds accelration fuelling and ignition advance .. hence the revs fluctuate without you touching the pedal. you need the diagnostics software hooked up to confirm, or get it to your local specialist.

Thanks Joolz, much appreciated - I will check that more carefully now I know what to look for!

[Tanguero]

Spot on diagnosis. Left bank throttle pot started going eratic and creeping downwards once it got hot. Just ordered a replacement from you - thanks Joolz