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DIY: Do It Yourself
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Old 07-31-2019, 10:26 AM   #1
docmatt
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Join Date: Feb 2006
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My Ride: 2004 M3
Model: M3
Help with P0206 E46 M3

Hey everyone. I was wondering if I could get some troubleshooting help. I have a mysterious 206 on my M3. It has 133k miles and I'm the original owner.

I originally had this problem a year and a half ago and found it to be a faulty injector. I replaced the injector and o ring. I did a smoke test and no leaks were found.

The issue just started happening again. I did the coil test and it didn't change. So given the age of the car, I replaced all the coils and spark plugs. Car was running amazing and then it misfired again.

And it is a very subtle misfire. No engine shaking or anything. Just a slight hesitation and then it is gone. Always on the 6 cylinder.

I don't think it's a fuel filter or pump. Mainly because it is cylinder specific.

Any thoughts on where to go from here?

Thanks!

Matt


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Old 08-01-2019, 08:28 AM   #2
markusmarkus
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Help with P0206 E46 M3

Itís a misfire on cylinder 6. Misfires can be caused by a myriad of things:
- bad injector
- bad coil
- bad spark plug
- mechanical issues such as a chipped valve
- wiring harness issues
- DME issues.

Youíve changed the injector, the spark plug and the coil. So, unless by some odd chance that all the new parts are bad (verrrrrrrrry unlikely), they are not the problem.

The first step is to perform a compression check on all cylinders. This includes a cold check, a hot check and a leak down test. And deviation greater than 10% means you likely have found your issue. But, as itís an intermittent misfire Iím betting youíll find nothing out of the ordinary.

The next things to check are the engine harness connectors for the cylinder 6 coil and injector. Now, BMW has pretty high standards for its connectors. In fact, BMW bought a German connector manufacturer in the 1980s because it was having a problem getting consistently high quality connectors. So, check the connectors. Compare each connector with the corresponding connector of cylinder 5. Use a good LED light to look for any issues with the connectorsí metal parts such as burns, corrosion and misalignment. If all looks ok, the next thing to check is the DME.

Both the coils and the injectors have 12VDC as long as the key is in either the run or start position. The DME provides the ground to these parts for them to ďoperate.Ē That is, the DME grounds the return line in a coil when it wants the coil to fire. The DME uses metal oxide substrate field effect transistors (MOSFET) as the component that provides the ground. Each injector and each coil has its own MOSFET in the DME.

In case you want to know more about MOSFETs: https://en.m.wikipedia.org/wiki/MOSFET

So, what you could have is a MOSFET with a bad solder joint or a failing MOSFET. A friend of mine, a great tech and BMWCCA driving instructor, was chasing misfires in his E39 M5. Heíd rebuilt the engine with the best parts as he tracks the car (itís blazingly fast). But he was getting occasional misfires on 2 cylinders. He was pulling his hair out (figuratively). A friend suggested he pull the DME. Turns out the MOSFET for 2 coils we going bad. He had them replaced and the misfires disappeared. Now, how do you check the MOSFETS? You can pull the DME and check for obvious issues. I bet you wonít find any.

So the next step is to check cylinder 6 with a borescope. Compare it to cylinder 5. Btw, cylinder 6 runs hotter than any other cylinder.

Letís say the borescope shows all is well. The next step is to use test equipment to look at the engine harness wires that provides the ground to cylinder 6ís coil and injector. Thatís how my friend found his bad MOSFETs.

Last edited by markusmarkus; 08-01-2019 at 08:53 AM.
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Old 08-01-2019, 10:32 AM   #3
docmatt
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This is a great write up and thank you very much! So as a new DIY'er with my E46. This looks like it could be a fairly tough one. Do you recommend having the DME pulled by a pro and do the work (maybe need an ocilliscope, hot air system etc) or would you say that this is something I might be able to go after?
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Old 08-02-2019, 07:55 AM   #4
markusmarkus
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Do the easy things first:
1. Check the coil and injector connectorsí metal parts (an electrical connector has female sockets and make pins) for corrosion and damage.
2. Borrow a compression gauge and read up on how to use it. Be advised that you need to pull the DME relay or the DME fuses before performing a compression test.
3. Remove the DME cover to inspect the circuit board for obvious issues.
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