Rover 200 & 400 Owners Club

That manifold pipe is a sniff of the exhaust gas pressure - feeds into the pressure sensor in the ECU. It's reading will be used to directly index the tune map, i.e. select injector firing length/timing - this could indeed be the source of the problem. I'd certainly check the other half of the pipe...

The oxygen sensor I'm suggesting could be the cause is a weird one - I'm only making it because I had direct experience of the same problem many, many years ago - but on the Honda engine. If you think about it logically, when the car is cold it's running open-loop - so presumably the O2 readings are being ignored??? Unless the "fault" is that the sensor is actually outputting sensible "I'm up to temperature" type readings when it shouldn't be?

Let us know how you get on...

Johnny 216GSi wrote:That manifold pipe is a sniff of the exhaust gas pressure - feeds into the pressure sensor in the ECU. It's reading will be used to directly index the tune map, i.e. select injector firing length/timing - this could indeed be the source of the problem. I'd certainly check the other half of the pipe...

I think you had a brain glitch there...
The ECU needs to sniff the inlet manifold pressure, or more intuitively put, depression, caused by, and thus indicating, the engine's instantaneous inlet air demand.

The same parameter on my carburettor 8-valve causes the carb piston to position itself such that its attached needle meters just the right amount of fuel. No electrons required. Magic!

crepello wrote:

Johnny 216GSi wrote:That manifold pipe is a sniff of the exhaust gas pressure - feeds into the pressure sensor in the ECU. It's reading will be used to directly index the tune map, i.e. select injector firing length/timing - this could indeed be the source of the problem. I'd certainly check the other half of the pipe...

I think you had a brain glitch there...
The ECU needs to sniff the inlet manifold pressure, or more intuitively put, depression, caused by, and thus indicating, the engine's instantaneous inlet air demand.

The same parameter on my carburettor 8-valve causes the carb piston to position itself such that its attached needle meters just the right amount of fuel. No electrons required. Magic!

No brain glitch! I understood it to be "manifold pressure" but never read/had confirmed it was inlet. I've obviously plumped for the wrong one in my explanation...

Of all the people who could have spotted my incompetence, I'm glad it was you

First thing to try is a stepper motor reset

Turn the ignition on (but don't start the engine)
Fully press and release the throttle 5 times in quick succession
You should hear a rapid ticking noise from under the bonnet for a second or two
Start the car and allow the idle to stabilise

Job done

If that doesn't help then certainly check and blow out the pipe to the ECU from the inlet manifold
If that doesn't help then removal and cleaning of the throttle body paying particular attention to the air bypass hole. If this gets blocked (and they commonly do), then the idle control gets a bit lumpy.

Also check and if necessary clean/repair the earth connection from the battery to the bonnet closing panel (slam panel) as this is also a common failure point causing all sorts of issues.

I ran the car with the lambda unplugged today...only on the drive and it seemed to improve things although i plugged it back in and was the same so im not certain.

The reset i have tried a couple times first as i knew about this from when my metro was playing up after a headgasket change (forgot a breather pipe)

I had the throttle body off and cleaned it all out. Is the air bypass the breather pipe from the rocker cover? I couldnt see any other 'holes' or pipe connections to clean out.

Earth is a good shout, it is all abit rusty on the slam panel due to its time off the road and general neglect. I will try all of the above again and take it for a good run.

Thanks all, Ill let you know how i get on.

Its been a long time since I looked at an old K series throttle body but usually there is a small air passage drilled into the throttle body itself that bypasses the throttle butterfly. Its job is to allow a small amount of air through even when the throttle is fully closed to help control the idle speed. This small passage way blocks up with gunk and then the stepper motor is trying to control the idle speed on its own which is trickier and sometimes it doesn't get it right. Having the air bypass clean makes idle control and the on/off throttle transition smoother.

That sounds a tad more mysterious than it actually is. There is a similar, but less refined provision in the poverty carb-fitted cars, where it is solenoid controlled within the carburettor body, and is stated to weaken the mixture at idle for emissions compliance.

In the MPI setup, the bypass pipe similarly bypasses the throttle butterfly, and then plugs onto the idle air control valve, which incorporates the stepper motor. The stepper motor drives a leadscrew which drives a tapered piston in or out of its seat in the valve, to vary the valve orifice cross-section area and thus control the bypass air flow.

In other words, the pipe and idle air control valve are in series, and both need to 'work' correctly; the pipe (with its unions) is merely passive and needs to be clear of restrictions. The IACV is the active element, and similarly needs to be clean internally so that the stepper motor can move the piston freely, and that the various motor steps (hence piston positions) produce repeatable orifice free-areas.

The pipe to keep clear is item 6, and the IACV is item 4 on this diagram:

That is the later car setup above with the idle control valve. I'm not 100% sure but I think a 1993 car will have the earlier setup which has the stepper motor instead. This has the air passage built into the throttle body and it is prone to clogging.

E_T_V wrote:This has the air passage built into the throttle body and it is prone to clogging.

Ah! I wonder if that's why they made the later one easier to strip down?

Mine is the later version...ill strip it down again and clean and look more closely.

Ill post some pics tomorrow...havent been out today cause of the rain.