No MAF - CAr runs perfect - With NEW MAF - several codes..

[95BMWIC]

2002 BMW 325i - 205,000 miles - Ive owned it since 2008 and 85,000 miles.

Been driving it 6 months now with MAF unplugged maybe 4000 miles- I get one code (basically MAF error) other than that - runs good. Mileage not too bad either.

Plug the MAF in and look out - runs rough, trouble with idle, misfires - throws several codes (fuel trim and misfire codes)

I have had the car a long time - so many things have been done - The CCV vent system is all been replaced, the Idle control valve checks out good, the vanos has been done , the DISA valve was replaced (and still has vacuum and flap has resistance), the throttle body has been replaced.. Vaccuum hoses have been replaced and no vacuum leaks oil filler tube seal replaced etc. etc..

I have a brand new MAF AND I have replaced the MAF from a known good one from another E46 and still get the same exact issues..

Unplugged MAF - runs great

Plugged in MAF - runs horrible

Thoughts on what to check next?

[whiskychaser]

If you disconnect the MAF, the DME doesn't know how much air is being drawn into the engine. So it reverts to a stored map. As I understand it, that map is for a warm engine so it probably doesn't like cold starting a lot. Which is a roundabout way of saying you are just cheating the DME and not solving the issue. I'd suggest that the way forward is to fit a good known MAF, check your fuel trims and address any fault codes. Fuel filter and pump been changed recently?

[MarcoZandrini]

Plug the MAF back in, clear the codes to and see what comes up. Most likely you have vacuum leaks.

[95BMWIC]

If you disconnect the MAF, the DME doesn't know how much air is being drawn into the engine. So it reverts to a stored map. As I understand it, that map is for a warm engine so it probably doesn't like cold starting a lot. Which is a roundabout way of saying you are just cheating the DME and not solving the issue. I'd suggest that the way forward is to fit a good known MAF, check your fuel trims and address any fault codes. Fuel filter and pump been changed recently?

See above in my first posting - Have brand new MAF and (ALSO) switched with another known good E46. I don't get a code on Fuel Trims when MAF is unplugged - not sure how to get actual numbers of fuel trim? - I have a Scwaben "pro" code scanner with BMW software loaded.

I get 1 code - MAF not reading

No problems starting in cold weather? -4 Celsius here in midwest yesterday morning - started right up.

I'm aware that the default mode is rich. However I still get pretty good gas mileage - averaging 24 mph..

ADDED: - Fuel Filter AND Fuel Pump have been changed within the year. Fuel flow issue?

[95BMWIC]

Plug the MAF back in, clear the codes to and see what comes up. Most likely you have vacuum leaks.

Plug MAF in and car runs like crap - trouble idling - cannot drive very far at all - several codes - fuel trim and misfires.

Smoke machine indicates No vacuum leaks.

Clear codes and unplug MAF - Runs great - Have driven it without MAF for many miles - 1 code - MAF error..?

[95BMWIC]

With MAF

IMG_20190417_151808310.jpg

IMG_20190417_151848721.jpg

[95BMWIC]

I get that this might be a weird one..

Ive drove this on long trips this summer - drove it to work - no issues with it unplugged.. 4,000 miles or so

And obviously - that's what I will continue to do...

However I wish I knew why it would run so bad with a MAF signal - Bad DME? Why would the defualt map be so much better than the actual signal from the MAF sensor? Something overriding the fuel trim besides MAF? Bad o2?

[bluptgm3]

My inclination is bad O2 sensors judging from codes.


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[whiskychaser]

The pre-cat O2 sensors tell the DME if the mixture is lean or rich and the DME adjusts the injector times accordingly. The codes mean that engine has hit the limit of what the DME can do. So it throws its hands up and says 'over to you pal - you sort it'. I'd suggest your misfire codes are as a direct result and imagine your CEL comes on too. Ideally, you want to be able to see live data for the trims, air flow and O2 sensors.

Edit: I wouldn't change the O2 sensor until you know for sure they are at fault

[nonturbodan]

02 sensors are operating regardless of MAF input. If they were malfunctioning you would have the same problem without the MAF signal. Removing the MAF input makes the DME to revert to alpha N load calculation, based off of throttle angle. Its not lean or rich, its just estimating load based off of a default map. The oxygen sensors are more crucial here since there is no direct measurement of airflow into the engine. The closed loop feedback is critical to prevent damage to the cats while operating in this mode.

My first suspicion is still a major air leak, I know you have checked the intake but clearly you still have a problem. Could be a bad connection or wiring issue at the MAF connection or in the harness too. I know there is a fuse in the ebox for the crank sensor, there may also be a fuse for the MAF too. Easy enough to check.

[95BMWIC]

Update.. still driving car - nothing changed - unplugged MAF - 230,000 miles now on odometer.. 24 mpg average.. runs great after wamup - Still no MAF plugged in - Note to Self - Really need to try 2 new o2 sensors..

In the last 3 years - Vanos seals done, oil filter seal, PCV tubing changed.. oil dipstick o ring - intake boots changed.. new fuel filter, new fuel pump.. DISA valve aftermarket, runs a little rough when cold - but once warm - no issues..