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Lean code on Bank 1 - MAF under-reporting?

523 Views 40 Replies 6 Participants Last post by  NZ00Z3, 
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2
  • OEM Siemens/VDO MAF (2nd hand - not sure of usage)
  • both brand new Bosch pre-cat lambdas
  • two ICVs tried
  • no vacuum leaks - 1000% positive,
  • injectors pulled, cleaned, and checked for spray
  • 02Pilot mod (last 1700 miles)
Had a lean code come up for Bank 1 today so I did a test-drive with INPA and got the following at idle, fully warmed up, no aircon:



Two things I noted:
  1. I've read in more than one place that the idle air consumption for this engine should be around 12.5 kg/h or 3.5 g/s; mine is 8.75 kg/h (2.5 g/s). Does this seem a bit low? I can't find any data on what it should be. Mine scales linearly with rpm, I should add.
  2. the idle speed of 700 is, I've read (again), a bit high. Can anybody corroborate?
First thing I will do is clamp off the 02Pilot mod and reset the trims for another run.
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A
02Pilot has been clamped for tomorrow's run... something about the fact that the vacuum is positioned right at the front of the engine makes me think it's going to affect Bank 1 more, but I suppose we'll see once the adaptations dial back in.
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4aceman
Arcsaber said:
OEM Siemens/VDO MAF (2nd hand - not sure of usage)
both brand new Bosch pre-cat lambdas
two ICVs tried,
no vacuum leaks - 1000% positive,
injectors pulled, cleaned, and checked for spray
02Pilot mod (last 1700 miles)

Had a lean code come up for Bank 1 today so I did a test-drive with INPA and got the following at idle, fully warmed up, no aircon:
View attachment 979774
View attachment 979775

Two things I noted:
  1. I've read in more than one place that the idle air consumption for this engine should be around 12.5 kg/h or 3.5 g/s; mine is 8.75 kg/h (2.5 g/s). Does this seem a bit low? I can't find any data on what it should be. Mine scales linearly with rpm, I should add.
  2. the idle speed of 700 is, I've read (again), a bit high. Can anybody corroborate?
First thing I will do is clamp off the 02Pilot mod and reset the trims for another run.
Click to expand...
I never understood the additive/multiplicative stuff. What are the fuel trims as % when you use a standard obd2 scanner?

A lean condition on one bank is very unlikely to be the MAF. On E10 petrol, the car will run about 5% lean, then add to that a 5% tolerance, you could have up to 10% trim without having any real issues.
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4aceman said:
I never understood the additive/multiplicative stuff. What are the fuel trims as % when you use a standard obd2 scanner?

A lean condition on one bank is very unlikely to be the MAF. On E10 petrol, the car will run about 5% lean, then add to that a 5% tolerance, you could have up to 10% trim without having any real issues.
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It's the code that concerns me - it's been on E10 for a couple of years and never chucked a code before.
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4aceman
Arcsaber said:
It's the code that concerns me - it's been on E10 for a couple of years and never chucked a code before.
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It shouldn’t, but using E10 will move the “normal range” up 5%. So even if you’re in this range you can throw a code.

I’m not sure what the exact total % is required for it to set a code. What are the obd fuel trim %s?
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4aceman said:
It shouldn’t, but using E10 will move the “normal range” up 5%. So even if you’re in this range you can throw a code.

I’m not sure what the exact total % is required for it to set a code. What are the obd fuel trim %s?
Click to expand...
I'll check with OBD tomorrow when I get home - it will need a few miles on the reset trims before it settles down.
I would be happy with +5/+5, to be hoenst... don't know why Bank 1 is the higher.

If the 02Pilot clamping doesn't fix it, my next step will be to pull the fuel rail out and test the injectors one by one, it situ.
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S
MAF should be around 3.5 g/s at hot idle of 700 RPM.
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S
Arcsaber said:
Thanks - I can rule the MAF out(y)
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MAF is under reporting which caused lean code.
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4aceman
Sapote said:
MAF should be around 3.5 g/s at hot idle of 700 RPM.
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Not conclusive proof of a bad MAF.

Say there’s a vacuum leak of 1g/s, engine gets the required 3.5 for idle, but only 2.5 passes and is measured by the MAF.
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4aceman said:
Not conclusive proof of a bad MAF.

Say there’s a vacuum leak of 1g/s, engine gets the required 3.5 for idle, but only 2.5 passes and is measured by the MAF.
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I tested the airflow at various rpms, and it scaled perfectly linearly - doubled at ~1500, trebled at ~2000.

If it were a vacuum leak, wouldn't the values at these rpms just be the correct values, minus 1?
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Does anybody know of a good UK-based OEM supplier for these MAFs? All the ".co.uk" sites I can find them on are based in Germany.
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4aceman
Arcsaber said:
Does anybody know of a good UK-based OEM supplier for these MAFs? All the ".co.uk" sites I can find them on are based in Germany.
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Cheapest would probably be from Autodoc. What's wrong with ordering from Germany? In a rush?
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4aceman said:
Cheapest would probably be from Autodoc. What's wrong with ordering from Germany? In a rush?
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Just ordered from Autodoc - would have prefered UK, but of F**KING COURSE there's a bank holiday on Monday, which would scupper any UK delivery anyway!
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Did you check fuel pressure?
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burntoutrubber said:
Did you check fuel pressure?
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Yep - and just installed a brand new (non-OEM, but doesn't matter) fuel pump, and nothing changed.

Old pump only had 87,000 on it, so it was probably still good.
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Non-OEM could matter. Not sure why people state that it doesn't.

did you test fuel pressure afterwards?
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burntoutrubber said:
Non-OEM could matter. Not sure why people state that it doesn't.

did you test fuel pressure afterwards?
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It had identical trims on the old pump; the new one hasn't changed anything.
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Arcsaber said:
It had identical trims on the old pump; the new one hasn't changed anything.
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Ok. Looks like you have it under control
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Update - potentially interesting result, if it sticks. I will test again tomorrow. These are from today, with the 02Pilot mod clamped off (back to stock, effectively):



Two things I noted:
  1. the air consumption is still reading low (2.6x g/s), assuming that 3.1x is the correct value, and
  2. the banks are now within a gnat's chuff of being symmetrical
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Yes, MAF=2.6g/s for 325i is too low at 700 rpm. If no intake vac leaking, then the MAF is bad.
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