BMW lean code, diagnosis and fix

[Flexray]

I'm a bit slow at work today, I figured it would be fun to write a small case study about a vehicle I fixed this week.

Warning, long and technical post.

2013 BMW 550i with the N63 engine. The complaint is a check engine light and a reduced engine power message. I check the engine module for trouble codes: 118001 fuel-air mixture lean bank 1.

With the engine idling in my bay I start looking at some data on the scan tool. Lambda (air-fuel ratio calculated from the 02 sensors) on both banks is 1.00. This is good. 0.70 is very rich, 1.30 is very lean. I now look at my fuel adaptations. Bank 1 is 26%, bank 2 is -22%. This is bad. These numbers are how much fuel the engine module is adding or subtracting to keep lambda at 1.00. The rule of thumb is no more than +- 10%. Adaptations run out at +- 25% and rich/lean codes will set.

I pull a spark plug out of each bank. Rich fuel mixture will cover the plug in black soot while a lean mixture will make the porcelain bone white. Both banks have a normal tan color to them and are a little worn, but that won't cause this problem. I clear the adaptations so now both banks are at 0%. I drive the vehicle for about 5km while watching the adaptations. Bank 2 moves to -1% and stays there. Bank 1 slowly starts creeping up. Under acceleration it is 10% but at idle it is back to 25% and 118001 sets again.

With the engine idling in my bay. I use my oscilloscope and take a look at the mass air flow sensor signals. Each bank has its own MAF, turbo, throttle, cooler, and intake manifold. Each side of the engine basically has its own induction system. The MAF sensors are a hot film type that produce a frequency for a signal. I back probe the signal wire of bank 1. It has a nice 0 to 5v square wave, looks good. I measure the frequency, it is 1950hz. I move to bank 2, same square wave but has 2000hz. Bank 1 is flowing or measuring less air than bank 2.

Using my scope and a pressure pulse transducer (also called a delta pressure transducer) this measures small vacuum changes and plots it onto a graph. I connect it to bank 1 intake manifold, give the engine a crank, save the waveform and do the same on bank 2. As the intake valves open and the piston travels down its bore, this induces a vacuum in the intake manifold. This vacuum pull is how much air the cylinder is ingesting. On the scope it shows up as a series of humps. Each hump is a cylinder pulling air out of the intake manifold. A larger hump means that cylinder pulled less air out of the intake. All cylinders on both banks are pulling fairly even amount of air. This means the engine is mechanically ok and bank 1 mass air flow sensor is under reporting the amount of air that is flowing past it.

I replace bank 1 mass air flow sensor. Both sensors are now producing 2000hz. Fuel adaptations for both banks are now under 5% under all driving conditions and no trouble codes have set. The vehicle is fixed.

[mr2mike]

Good explanation.

[03ozwhip]

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I'm a bit slow at work today, I figured it would be fun to write a small case study about a vehicle I fixed this week.

Warning, long and technical post.

2013 BMW 550i with the N63 engine. The complaint is a check engine light and a reduced engine power message. I check the engine module for trouble codes: 118001 fuel-air mixture lean bank 1.

With the engine idling in my bay I start looking at some data on the scan tool. Lambda (air-fuel ratio calculated from the 02 sensors) on both banks is 1.00. This is good. 0.70 is very rich, 1.30 is very lean. I now look at my fuel adaptations. Bank 1 is 26%, bank 2 is -22%. This is bad. These numbers are how much fuel the engine module is adding or subtracting to keep lambda at 1.00. The rule of thumb is no more than +- 10%. Adaptations run out at +- 25% and rich/lean codes will set.

I pull a spark plug out of each bank. Rich fuel mixture will cover the plug in black soot while a lean mixture will make the porcelain bone white. Both banks have a normal tan color to them and are a little worn, but that won't cause this problem. I clear the adaptations so now both banks are at 0%. I drive the vehicle for about 5km while watching the adaptations. Bank 2 moves to -1% and stays there. Bank 1 slowly starts creeping up. Under acceleration it is 10% but at idle it is back to 25% and 118001 sets again.

With the engine idling in my bay. I use my oscilloscope and take a look at the mass air flow sensor signals. Each bank has its own MAF, turbo, throttle, cooler, and intake manifold. Each side of the engine basically has its own induction system. The MAF sensors are a hot film type that produce a frequency for a signal. I back probe the signal wire of bank 1. It has a nice 0 to 5v square wave, looks good. I measure the frequency, it is 1950hz. I move to bank 2, same square wave but has 2000hz. Bank 1 is flowing or measuring less air than bank 2.

Using my scope and a pressure pulse transducer (also called a delta pressure transducer) this measures small vacuum changes and plots it onto a graph. I connect it to bank 1 intake manifold, give the engine a crank, save the waveform and do the same on bank 2. As the intake valves open and the piston travels down its bore, this induces a vacuum in the intake manifold. This vacuum pull is how much air the cylinder is ingesting. On the scope it shows up as a series of humps. Each hump is a cylinder pulling air out of the intake manifold. A larger hump means that cylinder pulled less air out of the intake. All cylinders on both banks are pulling fairly even amount of air. This means the engine is mechanically ok and bank 1 mass air flow sensor is under reporting the amount of air that is flowing past it.

I replace bank 1 mass air flow sensor. Both sensors are now producing 2000hz. Fuel adaptations for both banks are now under 5% under all driving conditions and no trouble codes have set. The vehicle is fixed.

Always hate to hear CEL and loss of power on an N63. Glad it was fairly simple. Are you a tech at BMW?

[Flexray]

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Always hate to hear CEL and loss of power on an N63. Glad it was fairly simple. Are you a tech at BMW?

I work for an aftermarket shop, the flat rate dealer world is not for me.