Z4 roadster misfiring - help pls!

petecullum · Post by **petecullum** » Mon 02 Mar, 2009 23:37

Hi - just joined, but have had the Z4 for about 5k miles and about 6 months - love it apart from one or two problems and the odd rattle.

Latest issue seems to be a very bad misfire which can be temporarily cured by switching off and re-starting. Basically, from cold the car is fine. As soon as it warms up it misfires at low revs very badly. This is getting worse so I've stopped using the car. This has to be ignition I'd say, but I'm wondering if it is a common fault that someone else has come across.

Any thoughts and is it easy to trace?

Thanks in advance - Pete

pop_sausage · Post by **pop_sausage** » Tue 03 Mar, 2009 06:38

hi, welcome to the forum.

Could be anything but I've had problem like this when coils have gone bad (common problem) and the MAF sensor go wrong. The OBC should have recorded any misfires.

petecullum · Post by **petecullum** » Tue 03 Mar, 2009 09:54

Hi - thanks for the reply.

Ok, so I have no idea what a MAF sensor is - air or fuel sensor of some kind? And the OBC? SOme kind of black box thingy for a BMW?

Looks like coils are pretty cheap to replace - £17. Will my scientific method of pulling the spark plug caps off while the engine is on tick-over tell me anything? Probably not come to think of it. So, I am looking at going to a stealer - ouch!

Thanks again!

Pete

Gazza · Post by **Gazza** » Tue 03 Mar, 2009 10:27

The coilpack is like a plug cap but not as simple. If you are unsure just buy one coilpack and try one cylinder at a time.

OBC is On Board Computer.

MAF is Mass Air Flow.

Welcome to the Forum

PhoenixCoupe · Post by **PhoenixCoupe** » Tue 03 Mar, 2009 10:56

I'd agree with the other's diagnosis.
An indy should be able to diagnose the problem - if it's a coil pack, it should be easy enough to replace. You could get one and try swapping it in to each position yourself - they are held on with 4 bolts and a single connector. (once you get to them by removing the engine cover)

Post by **Robert T** » Tue 03 Mar, 2009 11:08

Does the Z4 have a camshaft position sensor? Assuming it does, then I would also suspect that as the symptoms are very similar to those on my Z3 when mine decided to give a duff signal.

I would advise against the unplugging one lead at a time route. I have been doing some serious reading on the subject and there are two reasons why I wouldn't attempt it: 1) the coils in modern cars operate at higher voltages and can deliver more current than older cars - there have supposedly been a couple of people killed when doing the same to a 7 series beemer; and 2) it can upset the ECU and damage the cat etc as you have unburnt fuel going down the exhaust.

The plug-in diagnostics are sophisticated enough to tell you which cylinder has a problem, so I would always get that checked first.

Cheers R.

petecullum · Post by **petecullum** » Tue 03 Mar, 2009 23:11

garythefish wrote:The coilpack is like a plug cap but not as simple. If you are unsure just buy one coilpack and try one cylinder at a time.

I've bought a Bosch coil pack (just shy of £20 from GSF) - when I get five I'll have a go at fitting it. I've read elsewhere that it's a common item to go (cylinder No 6 rumoured to be favourite) and my symptoms seem consistent. Gary's suggestion sounds the cheapskate's best plan so I'll be digging my tools out of storage in the next week or two. Will let you know how I get on

Thanks all for your advice so far

Pete

petecullum · Post by **petecullum** » Mon 23 Mar, 2009 18:26

Well... I bought 1 coil pack and fitted it to the back cylinder which immediately seemed to make the car run smoother. BUT! Having recently taken it for a proper run the car is now rougher than ever. My choices are - another coilpack and start swapping them over (the down side being the car has to be up to temp before it makes any difference), replace all of them (just another 5 to go) or spend £50 plus labour and a day at a specialist who has the kit to plug the car in.

I'm torn. Part of me thinks the car is 6 years old and I've been told the coil packs go - just do all of them. The other part of me thinks it's worth getting the thing checked out. Any thoughts?

*AL* · Post by ***AL*** » Fri 17 Apr, 2009 18:55

I had this, from what i remember it was either the cam or crank sensor.

petecullum · Post by **petecullum** » Fri 17 Apr, 2009 21:23

Sorry to hear about your cam sensor

I bought a coil pack - £20 from GSF - a Bosch part. I did what a previous post suggested and kept swapping all of them out until the issue was cured. Didn't take long - cylinder No 5 was the culprit and the car runs fine now.

I did a google search on coil packs related to BMW and it sounds like this is a very common fault - something to do with them overheating/getting old.

Tools needed: 10mm socket to remove engine cover. Time taken 2 minutes.

If it comes up again and a new coil pack doesn't fix it then I'd go for diagnostics next - cheapest I got quoted was about £50+VAT. The problem is that garages need to make money. Swapping coil packs doesn't make enough to pay the bills I suspect. I was advised to change all, not just 1, to avoid upsetting the ECU. I was also told it was likely to be the cam sensor. I asked a friend who used to design ECU's for a living who didn't really buy either of those. I've gone with him on the strength that I believe he knows more about vehicle electrics than a garage ever would - but if my ECU does go pop he owes me a serious favour!

Thanks very much for your input - sorry to hear yours was the cam sensor.

*AL* · Post by ***AL*** » Sat 18 Apr, 2009 08:38

Glad to hear you have it sorted

, misfires can be caused by many things.Start at the easiest/cheapest first!

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