Topic

How steady should the revs be on idle I find mine go up and down a little from 810 to 840 is this normal ?

Mine is pretty steady.
Variation suggests that the base idle may be slightly slow and the iacv is working to lift it. 850 seems to be a stable target.
Try a quarter turn in on the idle screw on the throttle body.

This is an extract from the MG Rover Engine Management System - MPi MEMS 1.9 document;

"Idle speed control
With the throttle pedal released and the engine at idle, the ECM uses the fast response of ignition timing to maintain idle stabilisation. When loads are placed on or removed from the engine, the ECM senses the change in engine speed, and in conjunction with adjusting the idle air control (IAC) valve, advances or retards the ignition timing to maintain a specified idle speed. When load is removed from the engine, the IAC valve returns to its original position and the ignition timing returns to the idle setting.
NOTE: Due to the sensitivity of this system the ignition timing will be constantly changing at idle speed."

So in short, yes.

David Aiketgate wrote: wrote:

....Try a quarter turn in on the idle screw on the throttle body.

Only just spotted this, in the same document referenced above (final note is relevant point) ;

"Idle air control valve
The idle air control (IAC) valve is mounted on the inlet manifold and controlled by the ECM. The IAC valve opens a pintle valve situated in an air passage in the throttle housing, allowing air to bypass the throttle disc and flow straight into the inlet manifold. By changing the amount the IAC valve is open the ECM can control engine idle speed and cold start air flow requirements by adjusting the flow of air in the passage. During cold starting the ECM indexes the IAC valve open slightly to provide a level of fast idle, dependent on engine coolant temperature. As the engine warms, fast idle is gradually decreased until normal operating temperature is reached. The position of the IAC valve can be checked using TestBook and should be within the range of 20 to 40 steps when the engine is running. If it is identified as being outside this range it can be adjusted to within range using TestBook. This ensures that the IAC valve is at the optimum position within its range for providing further movement to compensate for changes in engine load or temperature in accordance with signals from the ECM.
NOTE: The position of the throttle disc is preset during manufacture and the throttle position setting screw MUST NOT be adjusted."

So I would advise you not to touch the idle screw on the throttle body.

EDIT: Just checked the 'throttle position screw on the throttle body actually has an epoxy resin type coating to stop you making any adjustment to it.

I had an issue with high idle; found to be a warped face on the crappy plastic inlet manifold (drawing air) I fitted an alloy manifold and 52mm throttle body from a Freelander, this cured the high idle but now I had hunting at idle. A re-set would cure the hunting for a bout a day but then it would come back.

I suspected the IAC so I fitted a new unit, this made no difference.

The cure for me was to get the engine up to normal operating temp and do a TPS re-set, re start the engine, unplug the IAC, remove the plastic inhibitor on the head of the idle screw, wind in the screw until the RPM just started to rise and then wind the screw out to get your 850 RPM, switch off and a final re-set.

Since then the idle (about 3k miles) the idle has been rock solid steady.

Brian.

I agree with Paul's quotes. In theory. Don't mess with the set screw.

Unfortunately time, modifications, wear and tear etc.can mean that the factory set idle screw does need to be adjusted.

It's something that may need doing after you check all the usual culprits. From experience I know that too low a level of idle will result in hunting (the iacv trying to lift the base idle to 850). A base idle set too high will result in either a high idle speed or an effect where the idle hangs at eg 1100 before eventually coming down nearer 850.

A quick test is to have the engine at operating temperature and flick the throttle to about 2000 revs. Allow the revs to die naturally (foot off the pedal) and watch the rev counter.
Ideally the revs should come down quickly to about a 1000 then move smoothly to 850/
If the revs quickly drop to below 850 and then pick up the base idle is set too low. (assuming everything else is working)
If the revs come down to 1100-1200 and hang, maybe not dropping at all or only slowly. The base idle is too high.

A quarter turn is a v small adjustment, but should show an improvement. if not, it's easy enough to turn it back.

If you have the Torque App on your phone and a bluetooth adapter in the OBD port you can observe the idle speed with greater accuracy.

Badger wrote: wrote:

I had an issue with high idle; found to be a warped face on the crappy plastic inlet manifold (drawing air) I fitted an alloy manifold and 52mm throttle body from a Freelander, this cured the high idle but now I had hunting at idle. A re-set would cure the hunting for a bout a day but then it would come back.

I suspected the IAC so I fitted a new unit, this made no difference.

The cure for me was to get the engine up to normal operating temp and do a TPS re-set, re start the engine, unplug the IAC, remove the plastic inhibitor on the head of the idle screw, wind in the screw until the RPM just started to rise and then wind the screw out to get your 850 RPM, switch off and a final re-set.

Since then the idle (about 3k miles) the idle has been rock solid steady.

Brian.

Gonna try this method as Felix is intermittently hunting. I'll do a TPS reset and he's OK for short time then it's back!