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Discussion Starter #1
Hello

My E39 M5 has started misfiring and is lacking some power.
It pulls OK, but it feels like it has lost about 100hp

Vanos solenoid maintenance has been done 5K ago, (this problem started occuring before that was done)
No vacum leaks.
Peake reader give no codes and no check engine light
The car runs worse with maf's unplugged.

What I can think of is CPS or O2 sensors? Anything else to check before starting to swap those?

Car is built in 1999

Any help is greatly appreciated :)
 

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How old is the fuel pump?
 

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I'm leaning toward fuel issue as well. With bad CPS or O2 you'd have codes.
 

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Maybe check fuel filter. Could be clogged cat. Could be fuel pump. Could be injectors. Could be coils/plugs. Could be ignition wiring. Could be MOSFET/drivers on MSS52 DME. Could be (really, really, really) dirty intake air filter. Could be MAF(s) or the intake air temp sensor in whichever side your's uses it on (look for 5 pins in MAF connector).

Also, if it's truly misfiring - you'd have basic OBD2 codes for it, and more information available with an ISTA+/INPA setup. In my experience, MSS52 DME does not seem to do fuel/ignition shutdown upon detection of sustained misfire (almost all other BMWs I've done diagnostic work on seem to)

Good luck
 

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Discussion Starter #5
Thanks for the replies! :)

Now that you mention fuel pump its been 140K since it was changed, I'll start with checking the pump and fuel filter.
 

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It could also be faulty MAF sensors...sometimes they don't throw codes and can make your car feel like it's being robbed of power.
 

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Discussion Starter #7
@le nitro
I tried driving with the MAF sensors unplugged and the car ran worse without them.

Is it still possible that its the MAF's?
 

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The computer recognizes that the MAFs are faulty and enables a predetermined a/f mapping so you can continue to drive the car. (kind of like an emergency protocol) If it ran worse without it then maybe it's not the MAFs but I wouldn't know for sure.

Maybe a bad coil pack or spark plug? Have you ever replaced them or performed a full tune-up recently?
 

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How many miles did you put on the car with the MAFs unplugged? If they are the issue then the adaptions in the car will have changed to accomodate. When you unplug the MAFs, if at fault the issue disappears, but the adaptions remain and take time to adjust to the proper air reading going in. After 50 you likely get to 50% change in adaptions but to get to 100% it takes a lot more miles. If you drive for 50 and notice some changes go to 100 and if there is improvement it is likely worth the gamble of throwing MAFs at the car without further diag. Generally if the car gets better or worse when unplugging the MAFs it is a fairly strong indication that something is wrong, but it might not be the only thing that is wrong.
 

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Discussion Starter #10
Hey guys thanks for all replies :), I'll update with the solution that fixed the issue. (sort of)

First I changed the fuel pump & fuel filter, didn't help but it was due for service anyway.

After that I did some live data reading while driving and found out it was misfiring quite badly on all cylinders, I cleared all driving adaptions from the car ECU and for some reason after that it started running as normal again.

then I did more live reading and found out I still had some misfires on cylinder 1,2 & 8

Changed sparkplugs and cylinder 8 stopped misfiring :)

right now the car runs even better than when i bought it, but I still have some misfires on cylinders 1 & 2.

Could the timing be off, and would that be able to make only cylinder 1 & 2 misfire?
 
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