setting the base idle

[groomvroom]

Trying to sort out my low idle speed, around 600 rpm, have checked and cleaned the stepper, stripped the stepper housing, stripped and cleaned the plenum and butterfly, re-sealed everything, set base idle at 2.5 turns from closed as per bible, have see 1.5 turns quoted on her for cat cars. Reset the ecu, still no joy, any ideas?

Edited by groomvroom on Saturday 24th February 22:28

[Ozstyle]

Hi,

there is a section on this in the bible. You need to clamp the air by-pass hose to stop air getting to the stepper motor housing, then adjust the idle screw to 400 - 500 rpm. Release clamp on by-pass hose and idle rpm should settle to 900 - 1000 rpm.

[groomvroom]

Ozstyle said:

Hi,

there is a section on this in the bible. You need to clamp the air by-pass hose to stop air getting to the stepper motor housing, then adjust the idle screw to 400 - 500 rpm. Release clamp on by-pass hose and idle rpm should settle to 900 - 1000 rpm.

yep done this, revs rise to about 2000 when unclamp then settle to 600, regardless of the idle screws position.

[Cider Andy]

The stepper motor may be faulty: have you tried swapping it with one off one of your other cars?

It might also be caused by an air leak somewhere between the airflow mass meter and the inlet manifold, although I'm not sure if that would cause the revs to rise instead of fall.

[groomvroom]

Cider Andy said:

The stepper motor may be faulty: have you tried swapping it with one off one of your other cars?

It might also be caused by an air leak somewhere between the airflow mass meter and the inlet manifold, although I'm not sure if that would cause the revs to rise instead of fall.

Good point, will try swapping the stepper motor over, will have a spray around with easy start and check for leaks, the revs rise only after releaseing the bypass tube, then the stepper reduces them to 600 again, can the idle be adjusted in the ECU?

[Cider Andy]

Ultimately, yes, but not unless you know how to create your own map and write it on to a chip! Effectively, no.

[groomvroom]

Cider Andy said:

The stepper motor may be faulty: have you tried swapping it with one off one of your other cars?

It might also be caused by an air leak somewhere between the airflow mass meter and the inlet manifold, although I'm not sure if that would cause the revs to rise instead of fall.

Ok swapped over the stepper motor, reset the base idle as per bible, reset the ecu, still the same. Will grab some easy start and check for leaks, could the lambdas upset the idle? other than the tickover, the engine goes very well. When I had the plenum off (which had a 4.5 badge on it) it was marked under the throttle housing in black pen as a 4.5, is the 4.5 plenham any diferent to the 4.3? as in throttle diameter ect?

[spend]

Try blocking off the purge valve pipe ~ this could well be leaking (its dark & dingy down in the undertray ~ so easier just to block it off at the plenum base to test). The early cars have a pretty HeathRobinson separate valve from Cannister arrangement (easy to convert to Rover 2/4/600 / LR / TVR later integral valve&cannister setup)

Dave

[groomvroom]

spend said:

Try blocking off the purge valve pipe ~ this could well be leaking (its dark & dingy down in the undertray ~ so easier just to block it off at the plenum base to test). The early cars have a pretty HeathRobinson separate valve from Cannister arrangement (easy to convert to Rover 2/4/600 / LR / TVR later integral valve&cannister setup)

Dave

Ok, will do, off to the local meeting now, will have ago after.

[GreenV8S]

The way to get the base idle is by blocking off the bypass hose and then adjusting the bypass screw on the throttle body to give you a 500 rpm tickover. This is more reliable than trying to count turns etc. While you have the bypass blocked off the ECU will try to compensate by opening the stepper motor. When you release the hose the fully open stepper motor will cause the revs to flare up and the ECU will close the stepper motor to achieve its target idle speed. I can think of three possible reasons why the revs would go back down to 600 rpm afterwards.

The ECU has overshot and is trying to open the stepper but the stepper is sticking and won't open.

The ECU thinks the revs are right despite what your tacho tells you.

The ECU thinks that 600 rpm is the right idle speed.

It seems strange for the idle to be wrong so consistently, and this makes me doubt that the stepper is sticking. I wonder how accurate your tacho is. Do you have access to an electronic strobe with a tacho readout so you can verify it?

If the problem is caused by the ECU moving the stepper motor then you can prove this by disconnecting it when the revs are about right. If the revs consistently stay about right afterwards then it suggests that the ECU/stepper are causing the problem.

Does the engine run smoothly at 600 rpm? I would have thought it would be quite lumpy, so it may not be obvious, but it's possible you have a misfire that is causing the revs to drop off. One reason for this would be an unmetered air leak, so check the condition of all hoses to the plenum, confirm that noting changes when you squash the vacuum line to the brake servo and the carbon cannister purge line, check all leads are secure.