MG Running rich(?)

[rpcee]

When I first start the MG, the revs are about 1200 then drop to about 900, but after a short run (3 miles) they're up around 1500, though this may vary the car always feels like it's pulling, and the fuel economy is pretty poor on any length run, so I'm guessing it's running rich. The temp. gauge behaves itself going up to almost the halfway mark, and the heater seems OK (it's a new thermostat).
What plan should I follow? Is it likely to be a faulty ECU or some other sensor?
Thanks, Richard

[G Force]

Hi, It is less likely to be a faulty sensor or ECU if the temp gauge is ok and the engine temp is ok. More likely to be a mixture of servicing and basic settings.

I would make sure there are no kinks, blockages, or leaks in the breather hoses and no vacuum leaks. Check that the air filter is clean, and the ignition system components are in good condition and spark plugs are gapped correctly. Don’t forget valve clearances affect engine performance, idle quality and emissions. Cleaning injectors makes a big difference to performance and economy.

If you are having problems with erratic idle speed you could reset the base idle. I would normally recommend cleaning the throttle body out before you do this. The best way is to remove the throttle body and unscrew the stepper motor, then give it a good clean with carb cleaner to remove all the gunk that accumulates. You will probably find the small breather port needs unblocking as it is nearly always blocked.

To set the base idle the engine needs to be hot. Then you need to ensure the stepper motor is fully closed (there is a special tool for this) but you can do it by using the following method.

Turn on the ignition and wait 10 seconds then remove the stepper motor plug, then turn off the ignition and wait 30 seconds or until you hear the main relay click out. Then replace the stepper motor plug and turn on the ignition and wait 10 seconds, and then remove the stepper motor plug again. Turn off the ignition and wait 30 seconds etc. Then repeat once more this will ensure the stepper motor is fully closed. With the stepper motor plug disconnected start the engine and set the idle speed to 650rpm with the throttle stop screw. Then replace the stepper motor plug.

After setting the base idle you will need to reset the throttle potentiometer with a voltmeter to read 325mv output with the throttle closed with ignition on.

Once you have done this with engine at normal temperature set the CO to 2% with the screw on the throttle body nearest the stepper motor, that’s if you have access to a gas analyser of course.

If the engine still stalls when coming to a halt it could be a faulty road speed transducer. To test the transducer disconnect the harness plug and connect an ohm meter across the transducer wires. Then jack up one wheel and rotate the wheel observing the reading on the meter, it should fluctuate between 0 ohms & open circuit.

Regards Gary :)

[rpcee]

Wow what a great reply, thanks! I'll start on that lot tomorrow.
Richard