Hesitation

When the engine in my 2003 MINI Cooper R50 1.6 is cold it really stutters at low revs, such as when pulling away. I usually have to rev to more than 2500rpm and slip the clutch to pull away. Once warmed up, it’s a bit better, but there’s still some hesitation at low revs when accelerating. When cruising, I can feel it hesitate or ‘miss’, especially when slowing down.

I have fitted new spark plugs and air filter. I also tried changing the throttle body for one from a good running engine, but it made no difference. I tried a spare coil pack from another known good engine and installed a set of injectors, to no avail. The MAP sensor was in really bad shape, so I replaced it with a working spare, which improved things but didn’t solve the problem entirely. I have heard that there is a pedal sensor showing the pedal position, but I didn’t really think this could be an issue.

I have now run out of ideas. I have run injector cleaner and general engine cleaner through the car, as it had stood for a while when I bought it. I have been through about three or four tanks of petrol now, so the fuel should be OK. Do you have any other ideas to try?

Also, I heard that if I change the throttle body, I need to reset the ECU. Is this true? Tomas Hills

Autodata informs me that the common problems with the MINI R50 that could produce the symptoms you have include contaminated contacts on the throttle motor harness multi-plug, a poor connection in the throttle motor harness multi-plug due to poor manufacture or Internal failure of the throttle motor. These would be good points to check first. From the information I have, the throttle body on your R50 should not need programming, but I would disconnect the battery while fitting it; once fitted, the ECU should learn the new parameters of the throttle body.

As the problem you have is worse when the vehicle is cold, I would also check the parameters of the engine coolant temperature sensor to ensure this is operating correctly. If you have the facilities to read off live data from the vehicle, you should be able to check the readings to ensure that the CTS is operating correctly.

You mention that you swapped the coil pack and I would ask if this included the HT leads? The misfire could be the classic symptom of failing leads, which are often overlooked on modern engines. Other points to check would be the induction system for any air leaks and the 02 sensors to ensure that they are switching correctly. On a more basic level, I would also carry out a compression check to ensure that the base mechanics of the engine are operating correctly and that you do not have a valve problem.