Poor tickover when hot

[bluechim]

Hi There. My first guess is stepper motor problems, but I took my steeper motor out and cleaned it up a few weeks ago.

Yesterday (it was very hot) towards the end of a 40 mile run around with a few stops and a few good blasts, but mostly just taking it steady the car started to play up.

It’s a 2002 450

The symptoms were a no more than a few pops and bangs out of the exhaust to start with, but then the revs started hanging up around 1200 to 1500 when my foot was off the gas (its been doing this for a while, which was why I cleaned the stepper motor in the first place)

When sitting idle the car would rev up to 2000 revs and back to tick over on its own in a cycle taking 15 seconds or so and sometimes dip right down so low (300 / 400 revs) it looked like it would stop.

I closed off the air bypass pipe and the car barely ticked over at a few hundred revs, but did not fluctuate (again pointing to the stepper motor?)

Any ideas please?

[chassis 33]

Could you have an airleak somewhere either from a failed gasket or damaged hose? If you squirt WD40 along suspect areas and the idling stops playing about then you've found the problem. If anywhere hisses then that would be my prime suspect.

Regards
Iain

[tvr keith]

My 2000 450 did the same, the dealer found a small vacum pipe was off

[GreenV8S]

If you disconnect the stepper motor with the engine idling at the right speed, does the hunting stop? If so this does seem to point to a problem with the stepper motor, wiring, ECU or road speed sensor. If it still does it you're looking for a more mundane problem such as an air leak or excessive fuel pressure (eg from a vac line failure on the fuel pressure regulator, which would *also* cause an air leak).

[bluechim]

Hi There

The small pipe that was off - where was it please?

Thanks

[GreenV8S]

It was the vac line from the back of the plenum to the fuel pressure regulator.