Not running smoothly

[esor]

HELP please!

My 1998 540 touring auto with the 4.4L engine (petrol) is not running very well.

 

It starts great and sounds fantastic.

 

However, when driving the car 'surges' and the rev counter 'twitches' - the surge is minor - a bit like a miss-fire, but not.

 

It's a bit better when driving in manual mode.

 

As the engine gets hotter, the problem becomes more noticeable. There is no lack of power and kick down works as it should.

The problem is only noticed when at a constant speed (e.g. 50 mph)

 

It's been in the workshop for a week now - the owner is an ex BMW technician and is amazing with the older BMW's. 

However, he is stumped and has never come across this problem before. No faults showing.

 

Has anyone ever come across this issue before or have any ideas how to fix?

 

Thanks for any help and advice.

[X5smw]

Check the ‘knock’ sensor.

it tells the engine when to fire the injectors etc. It’s located near one of the pulleys.

my pulley actually failed At the vibration damper on the m43 engine and caused the pulley to move back and forth away From the sensor, causing Sporadic spikes and dips on the revs.

nightmare to diagnose, I could be completely wrong but I would start there. 
 

[esor]

Thank you X5smw - I'll pass that along

[Greydog]

Morning Nigel

As a BMW tech I guess he has checked all the usual suspects such as Air/Vacuum leaks, Worn Throttle body, fuel delivery, partially blocked cat or cats (often overlooked on older models)

Dave

[esor]

3 hours ago, Greydog said:

Morning Nigel

As a BMW tech I guess he has checked all the usual suspects such as Air/Vacuum leaks, Worn Throttle body, fuel delivery, partially blocked cat or cats (often overlooked on older models)

Dave

Thanks Dave. I will pass this along - CATs have just been replaced. I'll ask him about the other suggestions.