Thanks for replies , yes it could be blocked exhaust or bad fuel but read on....


went back for more on this . re-checked fuel pump relay OK. Still cranking no start after several attempts.

Decided to put pico on coil packs to look at ignition in detail. Pulled back rubber boots and looked at first coil pack , the pulses were random and of different voltage peaks showing no ringing . But after a few more cranks the engine started . It was also noted that we had a smell of petrol just before it started. There was no smell of petrol during the previous extended cranking tests.

Engine revved freely, no sign of any misfire , fast idle soon settled to slow idle as it started to warm up as normal. Also stopped and started engine soon after it first started with no problem.

Looking at the scope , all 6 coil packs were showing clamped output at about 110volt peaks with seven 'pre-spark' ignitions before switching off but with no coil ring on any coil pack. This suggested all 6 coil packs were faulty. Not very likely!!!

But gradually I guess as engine warmed up the coil pack ringing appeared on all 6 coil packs and the output peaks reached 400 volt peaks , again clamped all to to the same peak, very strange that all seemed to be exactly the same.

The seven pre-spark peaks reverted to one single coil pulse as one would normally expect to see but only when the rev were raised slightly. Dropping back to idle always produced the seven spark pulse. I have seen multiple spark pulses before on BMW .I guess this is how BMW fire their coil packs to ensure complete combustion at idle i.e. multiple sparking events, reverting to one conventional spark as revs pick up.

I cannot decide what caused the car to start . However it might have been the change of fuel filter with its combined regulator but I am thinking its something to do with a duff ECU/Coil drivers.

I removed and inspected the ECU and it was in perfect condition inside , no signs of power transistors damaged in any way. Put back ECU and again car started and ran perfectly.

Ran a vacuum check. Vacuum was perfect and steady and doing a WOT check showed vacuum response perfect, so no valve problems.

Customer reported the car never drove better after a short test drive with much better response to acceleration. I put this down to removing air leaks after the MAF spotted on smoke test or possibly the fuel filter swop or cleaning of sparks.

Recommended fitting of a new set of sparks. Re-checked fault codes and cleared DME. Advised customer to run car and call me if problem re-appears . It starts and run extremely well so nothing more I can do for now.

George