Air Fuel Ratio disparity between banks

[ajm55]

This is my first post on the forum. Lots of useful info here. I'll post pics of my ride soon. Its a 2013 F10 M5 Comp Pack. In the meantime, I'd appreciate some feedback from those in the know about an AFR issue that I've recently detected.

The issue is that when going WOT (and to a lesser extent, when idling and at part-throttle) there is disparity in the bank 1 and bank 2 AFRs.

I can't upload the data log, but the disparate data on a WOT 3rd gear run is illustrated below:


5,000 rpm: 14.4 on Bank 1 14.7 on Bank 2
5,500 rpm: 13.2 on Bank 1 13.5 on Bank 2
6,000 rpm: 12.3 on Bank 1 12.6 on Bank 2
6.500 rpm: 11.9 on Bank 1 12.2 on Bank 2

This is measured on a DashDAQ unit via the OBD. I understand that the refresh rate may be a little slow but there is clearly a pattern - Bank 2 is leaner than Bank 1 and I am guessing that the extent of the disparity is indicative of an underlying issue.

I plan to switch the O2 sensors and test.

Apart from catless down pipes, the car is stock.

The bank 1 injectors were replaced about 2,000 kms back. I had not logged AFRs before then and therefore do not know whether the disparity was an issue prior to their replacement.

I'd appreciate input from anyone who thinks he may know the possible cause/s/

[allmotor_2000]

This level of difference is minor. Keep in mind that this is a cross-bank system so two cylinders from the driver-side mixes with two cylinders from the pass-side (and vice versa).

The system is closed loop and will always try to hit the target lambda. Check where STFT's are. If they are pegged you have different issues.

[ajm55]

Originally Posted by allmotor_2000

This level of difference is minor. Keep in mind that this is a cross-bank system so two cylinders from the driver-side mixes with two cylinders from the pass-side (and vice versa).

The system is closed loop and will always try to hit the target lambda. Check where STFT's are. If they are pegged you have different issues.

Thanks for the feedback. I'll log STFTs as suggested

[technician117]

How many miles on the "primary" or otherwise known as "precat" O2 sensors? It could be as simple as one of them not working as well as the other. Best to replace in pairs and to not exceed 50k on them. I replaced my originals at 60k and there was a noticeable improvement in driveability which means they actually needed to be replaced sooner than 60k. My car is completely stock, no tune.

[ajm55]

Originally Posted by technician117

How many miles on the "primary" or otherwise known as "precat" O2 sensors? It could be as simple as one of them not working as well as the other. Best to replace in pairs and to not exceed 50k on them. I replaced my originals at 60k and there was a noticeable improvement in driveability which means they actually needed to be replaced sooner than 60k. My car is completely stock, no tune.

Thanks for this. The car has done 55,000kms. Today we switched the primary O2 sensors and tested - this made no difference. I'll log the short term fuel trims over the weekend as suggested by Allmotor to see if that reveals an underlying issue. I don't know whether the disparity is unusual on these cars. I would guess that not many log AFRs on both banks

[ajm55]

I've logged fuel trims (long term and short term) while cruising (part-throttle). I don't know how to upload the data logs but they show this:

The extremes reached on STFTs on both banks range from about -7 to +7. They are not pegged.

The LTFTs are usually about 8 apart. The bank 2 LTFTs range from about -6 to +2 while the bank 1 LTFTs range from about 0 to +8

I have an extremely basic understanding of fuel trims but what confuses me is that I would expect the bank 1 LTFTs to be in the negative and the bank 2 LTFTs in the positive because the bank 1 AFR is richer than the bank 2 AFR. However, the opposite is true. The bank 1 LTFTs are consistently in the positive and the bank 2 trims are, for the most part, in the negative.

Can anyone make any sense of this?

Apologies if this makes no sense and if someone can explain how one uploads a zdl data log, I'll try and do that

[ajm55]

Sorted by removing the cat foolers. Seems the down stream O2 sensors will tell the ECU to add fuel if it thinks that the cat is blocked. Also established that running catless downpipes without cat foolers does not trigger a CEL - so no cat foolers required on this car - something I did not know

[Craze]

It will

Just need catless tune

[ajm55]

Originally Posted by Craze

It will

Just need catless tune

If you mean running catless dps without a tune and without cat foolers will trigger a CEL, that would seem to be incorrect. Been driving it like that for a month without issue. I removed the cat foolers on the advice of a friend who has been running catless dps without a tune on his M5 for about 6 months - without any CEL being triggered

[SapphireM5]

Originally Posted by ajm55

If you mean running catless dps without a tune and without cat foolers will trigger a CEL, that would seem to be incorrect. Been driving it like that for a month without issue. I removed the cat foolers on the advice of a friend who has been running catless dps without a tune on his M5 for about 6 months - without any CEL being triggered

Sounds strange, are you sure your car is not tuned? Running these cars catless on stock tune will most certainly throw a check engine light.

[ajm55]

Originally Posted by SapphireM5

Sounds strange, are you sure your car is not tuned? Running these cars catless on stock tune will most certainly throw a check engine light.

Bought it secondhand. When buying, BMW checked for a tune - a precondition to transferring the motorplan - and concluded that it was stock. I’m boosting about 19/20 psi - which I think is stock boost. My mate’s car is also stock. Is it possibly a widespread misconception, I wonder. Most people who do downpipes, do a tune simultaneously I assume. So they would not know from personal experience

[SapphireM5]

Originally Posted by ajm55

Bought it secondhand. When buying, BMW checked for a tune - a precondition to transferring the motorplan - and concluded that it was stock. I'm boosting about 19/20 psi - which I think is stock boost. My mate's car is also stock. Is it possibly a widespread misconception, I wonder. Most people who do downpipes, do a tune simultaneously I assume. So they would not know from personal experience

There are a lot of guys who do catless down pipes and don't tune right away, they get check engine lights. I have never seen a car that you can remove the cats on stock tune and not throw a CEL.

[ajm55]

I hear you. Perhaps someone with personal experience will chime in. I’d be keen to know for certain

[SapphireM5]

Originally Posted by ajm55

I hear you. Perhaps someone with personal experience will chime in. I’d be keen to know for certain

It's a very basic design though, a reading is taken before catalytic converter then a reading is taken after the cat and it compares the two readings to show the cat is being efficient or not. If the car is inefficient it throws a code, if the second reading shows there's no difference before and after the cat, which it will if there is no cat, then it throws a CEL. Either way it's going to throw a code for catless downpipes on stock tune.


Unless I'm wrong, which I could very well be, I'm pretty sure that's how it works.

[ajm55]

Originally Posted by SapphireM5

It's a very basic design though, a reading is taken before catalytic converter then a reading is taken after the cat and it compares the two readings to show the cat is being efficient or not. If the car is inefficient it throws a code, if the second reading shows there's no difference before and after the cat, which it will if there is no cat, then it throws a CEL. Either way it's going to throw a code for catless downpipes on stock tune.


Unless I'm wrong, which I could very well be, I'm pretty sure that's how it works.

Technically that makes sense. However, factually my car and another I know of indicate that this does not apply to the F10 M5. My mate tells me he removed his cat foolers on the advice of a BMW mechanic. Anyone personally experienced a CEL prompted by catless dps on the stock tune?

[Craze]

I know several people that had check engine lights

Its possible due to ambient temps and altitudes that its not 100% the case in all circumstances

As a general rule it would appear

And personally i wouldnt run catless downpipes unless car had a tune to suit

[ajm55]

Originally Posted by Craze

I know several people that had check engine lights

Its possible due to ambient temps and altitudes that its not 100% the case in all circumstances

As a general rule it would appear

And personally i wouldnt run catless downpipes unless car had a tune to suit

Your reference to altitude may apply. Can you elaborate? My mate and I are both at 5,500 feet above sea level.

Would you opt for a tune with catless dps simply to extract the full performance benefits or for some other reason? If the latter, for what other reason? I will get a tune done - just weighing the options but I ask as I will expedite my decision if driving with catless dps without a complimentary tune is potentially harmful to the motor. This is not what you’re suggesting, is it?

[technician117]

Strange that you're not getting a CEL, but could be as simple as South African emissions standards compared to where most of us are from. As a separate market, it's possible the DME's are programmed differently.

As for tune/no tune with DP's: the car should adjust to the extra flow/less restriction without having a tune. There shouldn't be any concern over harm. Extracting maximum power from the modification is the main reason to tune with DP's.

[ajm55]

Originally Posted by technician117

Strange that you're not getting a CEL, but could be as simple as South African emissions standards compared to where most of us are from. As a separate market, it's possible the DME's are programmed differently.

As for tune/no tune with DP's: the car should adjust to the extra flow/less restriction without having a tune. There shouldn't be any concern over harm. Extracting maximum power from the modification is the main reason to tune with DP's.

Many thanks for this. Understood. Yes, we literally have no emissions testing here so it is possible, I suppose, that the DMEs are programmed differently.