Engine coil ( 5th cylinder ) keeps failing

[tk421]

Hello.

In the last 2 weeks, I've had the engine light come on in the morning. Having it towed to a shop, it turned out the that engine coil in the 5th cylinder had failed. I had it replaced. The mechanic put the new coil in the 4th cylinder, and the coil from the 4th into the 5th cylinder slot.

7 Days later, the same problem occured, again with the 5th cylinder. Now I am starting to panic.

At this time, I am waiting to pick up the car, but I am starting to pre empt possibly costly repairs.

A couple of things however need to be mentioned. I have recently run the car on normal unleaded ( 95 ). Additionally, the car had a small bingle ( front end ).

I have my suspicions as to what the cause may be. Any ideas before I have it fail again and send it away for more through diagnosis/analysis by a BMW dealer ?

I am in Melbourne, btw.

Cheers

 

[Kurts05Z4]

I assume that he (the mechanic) is getting a misfire code for #5 cylinder. By swapping the coils he eliminated the coil as the culprit. What's left is the spark plug, wiring and connections to the coil (if he's getting a circuit fault for #5 coil instead of, or as well as, a misfire code for that cylinder), the injector wiring, connector, and injector for #5, a vacuum leak at the intake runner seal for that cylinder can also cause a misfire and be intermittant. Plastic manifolds expand and contract quite a bit with themperature. Generally a seal failure at the intake port will be most evident on a cold engine and get gradually better as it warms up (heat expansion sealing the gap). This can be easily diagnosed with a scanner capable of diplaying the data stream. He needs to watch the short fuel trim (short term fuel corrections) and rpms while carefully spraying some carb clean or brake clean around the runner where it meets the head. Best done on a cold engine. The cleaner acts as fuel enrichening the mixture as it's sucked in to the vacuum leak. This will show up as a change in the trim (lean trim due to rich condition) and an increase in rpms due to that cylinder firing the extra fuel. If it's electrical, he's going to have to catch it in the act or begin a tedious (and possibly expensive) process of elimination.
I hope this helps.

Ps. if you can post back with the exact trouble codes he's getting that will help narrow it down.