Idle speed & emissions

There are two problems with my 1996 Rover 100 1113cc SPI that need attention: The idle speed is frequently very fast and the only way it will come back down is to switch off and restart. The fault has not gone on for long enough to let me pin down its precise pattern. 2. CO emissions are high and erratic. On fast idle (after running at high revs for a while) and where a maximum of 0.2 is permitted, it might typically read 0.9, dropping down after a few seconds to say 0.3, before suddenly jumping up again. I’m slowly working through the system and here’s what I’ve done and noticed so far: first, the very fast idle is caused by the stepper motor opening the throttle to around the same position that it parks in when the engine is off; second, I renewed the temperature sensor after discovering that the old one was under-reading by 20°C at 80°C.

I’ve checked the Lambda sensor, which is about three years and 15,000 miles old. The heater resistance is 4 ohms and there’s a 14V supply with the engine running. It’s also switching once a second or so with a max of 0.8V and a min of 0.2V. Enriching and weakening the mixture gives an instant response on the ‘scope. Initial leak checking with propane gas hasn’t revealed any obvious air leaks, although I need to double-check this, as well as doing some vacuum tests for leaks.

Fuel pump pressure is a healthy 15psi with the engine running.

I renewed the spark plugs and air filter despite being confident there was no need.

Much as I don’t like changing parts in the hope it fixes things, on this occasion, after fitting a new coolant temperature sensor, I swapped the body containing the injector and stepper motor before returning to have the CO emissions checked, just in case the injector was playing up despite its spray pattern being conical. The droplet sizes were bigger than I expected, but the pump is low pressure, so I am fairly confident the injector and the stepper motor are not implicated.

I have a good deal more troubleshooting to get round to, and a complete check of sensors and actuators is no bad thing, but I would be very grateful for any thoughts that come to mind as to where the problems may lie and where to concentrate my efforts.

There’s one important thing that I feel is certainly linked to poor emissions: with the car stationary and the engine running

at idle or even really quite fast, there is an irregular sound once or twice a second, like a missed beat from the exhaust. It’s not exactly a popping noise and you wouldn’t hear it unless you were listening for it. On first investigation, I couldn’t see any obvious link between this and the Lambda output waveform, but I intend to try putting the injector pulse on the second channel to see if I can detect this irregularity. That was why I changed the plugs, incidentally – the plug colours were entirely consistent with what I normally see on this engine.

It’s been a while since I ran a compression check and that’s on the agenda, but there is no audible variation when listening to the engine cranking before starting. Martin Rubenstein Given the extensive testing you have already carried out, you have covered many of the points I would have advised. Provided the compression test confirms that the compression is good, indicating there are no worn valve seats, the next area of examination would be the ECU.

You have already confirmed a good switching of the 02 sensor, and by changing the injector body and stepper motor you have eliminated these two components. This was a sensible measure as I have known these injectors to cause such problems.

The next step would be to get the ECU checked by either ecutesting.com or

bba-reman.com/uk, to ensure that this is not at fault. ECUS are easily corrupted by interference from radio waves emitted by the HT coil.