Uneven idling and flat spot.

Hi Guy's,

My '93 440Li 1600cc Multi-point injection B16F engine, has an uneven idle and a flat spot at the very first few millimetres of pedal travel (car sometimes cuts out if you push pedal down 2-3mm, otherwise runs great throughout the rest of the throttle range).

I have changed the plugs, leads, cap and rotor arm, air filter and Lambda sensor.

Has anyone got any clues as to what else I could try to clear this fault? Any help would be appreciated.

Bungee.

Check the throttle position sensor.

Is there a procedure for checking the throttle position sensor? Someone has told me that I need to check the resistance of the unit at various points of the throttle travel. If the sensor does not match the manufacturers specs, do I have to replace, or can it be cleaned etc? In fact, Is it servicable at all? Are these units available from motor factors, or do I need to go to Volvo with a large wad of cash?

Yep, check its resistance from closed throttle through to WOT. Haynes has a procedure for the turbo, not sure about yours as I don't have my manual here. Make sure you take the pipe off and look that the throttle is in the right place during checking though.

There may also be one or two microswitches In it. One for closed throttle, and one for full throttle. You have to check these do what they should if they are there.

Some TPS' are adjustable. If it is, you may need to reset it where it should be. They are really sealed units so if it doesn't do what it should you have no option but to replace. You can get them from places other than dealers (Usually it's just a bosch part).

Thanks Chris, I'll look into it and let you know how it goes.