Practically Classics Forum

Hi,
Can anyone help me diagnose the source of a misfire on my 1991 E34 520i?
It started with a pop through the exhaust and then intermittently missing under load and got progressively worse till it was running very rough all the time.
A change of plugs cured the problem for a few weeks and it ran fine but then the problem returned with lots of popping on initial take off but clearing when flooring the throttle.
A friend suggested it seemed like the back (nearest the bulkhead) cylinder by disconnecting each coil pack in turn so I put another plug in this cylinder and it improved dramatically but still with the odd pop heard through the exhaust with accasional misfire. I could change the rest of the plugs again but not cheap on a 6 cylinder! The plugs all look fine with no signs of fouling or contamination even on the suspect one I replaced. There is an individual coil pack for each plug so no conventional HT leads, could one of these be failing?
It has been suggested that maybe a blocked fuel filter but I can't understand why changing the plugs improves the situation in the short term. I would be grateful for any suggestions.
John.

Does your BMW have OBDII? you can get a cheap reader from ebay and it will tell you which cylinder has the missfire.

You say you think the mis-fire is the rearmost one, Try swopping the coil pack with another on the engine , If the mis-fire moves as well you may have a coil pack breaking down and changing the plug may well be able to overcome that coils shortcomings for a while.
Early Renault Megans and scenics suffered from failing coil packs and this method often pinpointed the duff coil. On those coils because they were grey in colour you could often see slight burn marks down the sides. I would imagine your coils will be black so you probably won't see any marks.

As above, very likely a failing coil on that cylinder. Worth having a spare, so either buy one and swap it in, or swap them around to show you the fault moving from one cylinder to another.

Don't drive the car with a misfire, or the unburnt fuel will ignite in the Catalytic converter and melt it.

Cheers

Thanks guys,
I think I've got a code reader somewhere but might need the lead to fit the BMW socket. It makes sense that the new plug could be compensating for a duff coil pack so I've bought a couple on ebay at £3.50 each, got to be worth a shot for that price! Will let you know how i get on.
Regards,
John.

Hi,
Just to update you all now that the forums back up and running, my £3.50 ebay coil pack fitted to No.6 cylinder has completely cured the problem and she now runs like a dream again.
What ever did we do before ebay? !

cheers,
John.

Breakers' yards

OneCarefulOwner wrote:Breakers' yards

OOOHHHHHH...Cant use those nowadays....Health and safety etc
Gone are the days of clambering round 3 high piles of wrecks in Ozzies or Piggots in York for Mk 1 Fiesta bits
Nowadays,it's all U-pullit with hard standings and Hi Viz vests and hard hats