Slight Knocking - 2003 540i

Have driven BMW 5 Series cars for 30 years. To save money this time I bought my first used car. It’s a 2003 540i but a true M-Sport. Came with all service records and is a well maintained vehicle. Trouble is that previous owner may have not used high octane premium. Car runs and idles great but on shut down it knocks just slightly. I have burned (quickly by the way) 5 tanks of Premium. Knocking on shutdown still persists. Plugs were last changed about 15,000 miles back. Should I add any gasoline additive or change the plugs?

Nice car.

I don’t have your answer, but I am fairly sure it it not fuel related. How many RPM is your idle? An air leak that makes the idle a bit high might cause a rattle on shutdown.

I would go to a BMW enthusiast forum like Pelican Parts, Roadfly, Bimmerforums etc. and post this question there in the forum that is specific to your car model.

You are describing a gasoline engine that is continuing to run after key is turned off, this is not “knocking”. I must says I never saw a BMW do this, something is wrong and it is not related to the previous owners use or non-use of preminum fuel.

Proper diagnosis should be used here. This is truly a case that doing nothing or the wrong thing could be very bad.

If you pay attention to the thread I will ask my BMW Master Tech friend about “dieseling” 5-series M-cars and I will add to the thread.

To respond to your question: RPMs…A nice steady 700 RPM idle.

Do you know what “dieseling” is? Is this what your car is doing? Why are you making a connection with engine “shut down” and this noise? Does it happen after you turn the key to off?

I thought dieseling was a thing of the past with EFI, since they (should) shut off fuel on shut-down. Maybe a leaking injector??

It’s not dieseling. I called it knocking but…I’m referring to the clack sound that I hear for 1/2 second while the engine shuts off (yes, after turning the key to off).

Perhaps it is one of your catalytic converters rattling, I have replaced many cats. for rattling on many different BMW’s.

Is it a manual transmission? If so, that car uses a dual-mass flywheel. The first indication of a failing dual mass flywheel is a rattle or metallic churning sound immediately after turning off the key. A new flywheel is $1000, so I would try to live with it until I needed a new clutch.

I’m unfamiliar with the dual mass flywheel system problem that Manolito referenced, but if it turns out to be dieseling I’d suspect a leaking injector.

Dieseling is caused by the cyliinder beiing hot enough to cause spontaneous ignition AND a source of fuel.

In days of carburators, and when engine temperatures were raised and engines run leaner to reduce emissions, a descending piston could pull fuel from the float bowl and the engine would keep dieseling until that source ran down. Manufacturers solved this by adding an “idle stop solenoid” that would allow the throttle plate to close completely when the engine was turned off and choke the engine to a stop.

With EFI, the only source of fuel would be a leaky injector. The system stays pressurized immediately after shutting the engine off and enough gas could spray into the engine to cause a bit of dieseling. Since it’s’ multiport, more than one injector could be leaking.

It’s a 6 speed manual alright. I wonder if a failing flywheel is acutally my problem? Is there any way to check this out easily or do I need to visit the dealer?

A mechanic who specializes in BMWs would be able to tell by the sound it makes.

I would not take an M5 to the dealer for clutch work if there were any good reliable independent shops around. We have several great BMW specialty shops around Sacramento. Actually, we have a great BMW dealer in Sacramento as well, but not cheap!