[scream5]

So I hit a new-to-me problem with my M5 today.

2-3 months ago, the car started misfiring on the highway. Pulled the codes and got the standard misfire codes for a specific cylinder. Replaced the plugs and the coil on that cylinder (82K miles). The car has been fine since -- I did the thermostat about a month and a half ago since my temps were running low.

Fast forward to today: I'm on the highway and the car starts misfiring and turns on the SES light. I was only about 5 minutes from home, so I decided to just limp it there. After I had slowed down and was on the side roads (through 2-3 stoplights), the car randomly stopped misfiring and the SES light went off. To see if the car was fine, I tried to accelerate at a decent clip and the car felt at about 90% up to 3500 rpm. By 4k, it literally had no power, even at full throttle. About two turns later, I was at my house, so I just parked the car and left it. Started the car to move it about an hour later and it still seemed a little off, but no misfires.

Then, 3-4 hours later, after the car had fully cooled, I decided to go for a drive and see what problems were still there. Oddly, the car was completely fine. It pulled really hard (maybe harder than it used to? probably a crappy butt dyno...) and didn't hesitate/misfire/cut power.

So my question is: What on earth could cause this? I really don't want this randomly happening....

 

[Sailor24]

It could be a recuring problem that fouled a plug before, and after replacement took some time to foul the new one. You are stuck in the same place you were, pull the plug on the cyl with the code and post a pic or two. Move coils. Unfortunately it has been documented many times that the misfire ident does not always get the correct cyl. Proper diag would be the key, has a cyl been identified? If so post the cyl. and I will give you the correct place to switch the coil to narrow it down.

 

[scream5]

It could be a recuring problem that fouled a plug before, and after replacement took some time to foul the new one. You are stuck in the same place you were, pull the plug on the cyl with the code and post a pic or two. Move coils. Unfortunately it has been documented many times that the misfire ident does not always get the correct cyl. Proper diag would be the key, has a cyl been identified? If so post the cyl. and I will give you the correct place to switch the coil to narrow it down.

Appreciate the help -- I will get the codes read tomorrow and pull the appropriate plug/post pictures. Hopefully that will help us sort it out!

 

[saedrin]

When have you replaced the CPS sensors? If never, that's about the mileage that mine started to fail..

 

[Sailor24]

Do you remember the original cyl? It helps when looking at the firing order and guessing where the real problem is.

 

[ReMi21]

I would look at CPS replacement history and then if possible, run a VANOS Test in DIS. I had similar issues years ago where I had a misfire, it would return to normal, and eventually, the car would only misfire. It ended up being a bad VANOS solenoid on my end.

 

[ard]

Id first buy a Peake code reader....

 

[scream5]

Scan/reset tool is on order -- went to get the codes read this morning and: no codes on the scanner. I'm not sure of the original cylinder, but I may be able to find it in my records. Also, pretty sure the CPS have never been replaced. Clutch is grabbing really high up on the pedal, so I may be pulling the trans and doing those at the same time anyways shortly. Car was fine again on decent distance drive again today.

 

[Sailor24]

With no codes I am going to guess a bad CPS exhaust to be specific. The exhaust note changes and the car will vibe a bit so it seems like it could be a misfire. If it was a real misfire it would have a code. For some unknown reason sometimes dead CPS still have no codes. Failed CPS can have long periods of running fine then one trip a failure the next fine.

 

[scream5]

I appreciate the knowledge here -- knowing that the CPS haven't been replaced, I think it's time. Now to start reading DIYs! Thanks for the help!