Hunting when hot

[LenChim]

2001 4.5li. Drives, starts and runs ok until its hot, if your driving its ok as you stop the rev's stay at 1500 then drop to 500 back to 1500 down to 200 almost cut out then back up and so on. The car is all standard. I have read the earlier post but do not have knowledge or access to the equipment that angus337 has. I have cleaned the stepper motor and looked at no's 1 & 2 plugs they look the same and would indicate that it is running rich.

[rev-erend]

Sounds like a stepper motor issue..

I think Jules once posted that the ideal solution was to block the air pipe to stepper and drill an 8mm hole .. to reduce the effect the stepper has.

[angus337]

One option to confirm if it is the stepper would be to start the car normally until it warms up, then disconnect the stepper motor plug with the engine running. Go for a drive and If you still have the same problem then it's something else.

Assuming it's running the standard Lucas system, Other causes could be:

Plugs
HT leads
Dizzy
Coil
Ignition amplifier
Coolant sensor
AFM
Etc............

[blitzracing]

Its unlikely to be the stepper as long as its clean- whats happening is the idle is too high so the ECU sends some pulses to the stepper to close it- now the software in the ECU does this on a timed basis- a couple of seconds apart to the the RPM settle. On an an engine thats running correctly, the drop in idle speed is predicable, so lets say we start and 1200 rpm and want to reach 800 in 3 seconds that would be fine. Whats happening is its starting say at 1200, and dropping to say 600 that 3 seconds, so its now under speed, so the ECU does the reverse and opens the stepper and but the idle speed goes up too quickly and the cycle repeats.

To cause the idle to drop too fast it could be an air leak in the plenum area or around the stepper as this weakens the mixture as the RPM drops, or a problem with ignition- i.e. a defective plug or lead so its drops a cylinder at low RPM- anything that causes the engine to drop power rapidly towards idle. It is possible that the AFM output is out of calibration at low air flows but it would not be my first point of call.. Sure enough you can block the stepper air feed and run on the base idle as it will mask the under laying problem, but you will have to hold the throttle on cold starts, and it may possibly put the ECU into a fault condition as it cant control the idle as it should.

Edited by blitzracing on Thursday 10th August 09:07