rich running

[ss2115]

For a long time my 530i Touring would return around 13.8 to 14.5 l/100k around town and down to about 8 l/100k or better freeway driving.
For a while now though its seems to be stuck on 15.1 all the time and on the last couple of freeway runs only dropped to about 14.4
The exhaust pipe is very sooty black.

However, the engine performance is great - definitely not in limp mode and does everything asked of it superbly.
No missing or stumble, starts immediately, pulls well. No smoke black or white.

Reading the error codes shows nothing like I might have expected - ie: O2 sensors in range, no MAF codes. (The MAF was new about 2 years ago anyway).

Any ideas?

[edjack]

Pre-cat sensors may be old enough to respond too slowly. How many miles on them?

[ss2115]

Pre-cat sensors may be old enough to respond too slowly. How many miles on them?

I cannot say with any certainty.
I bought the car from the first owner at 115,000klms two years ago. I don't have any info as to its full service history unfortunately. Are the O2 sensors marked in any respect as to their date of manufacturer? Your talking about the two that their connectors are along the top of the two exhaust manifolds in the engine bay. They don't look as if they have ever been disturbed but thats not a definitive answer for sure.

[edjack]

And how many klms on them now? And what are your short term fuel trim numbers?

[ss2115]

And how many klms on them now? And what are your short term fuel trim numbers?

Cars now at 142,000klms. Didn't look at trim numbers. Need to plug the PC in again. Good idea though. Thanks.

[14W]

Do you still have an original mass airflow sensor?And in what kind of condition are your vacuum lines? Faulty maf can cause all sorts of symptoms but high mileage is common,because car stays in open loop or at least ignore the data from the sensor(runs fine but rich) if the ecu detects any sort of issue with maf. Also sometimes it will illuminate an check engine light other times it will not. Mine 520i doesn't. You can try to unplug the maf connector while your car is running at operating temp. If your idling will improve or stay the same the maf may have a problem. But if the car will stumble and try to stall or run worse than your maf is probably ok.

[ss2115]

Do you still have an original mass airflow sensor?And in what kind of condition are your vacuum lines? Faulty maf can cause all sorts of symptoms but high mileage is common,because car stays in open loop or at least ignore the data from the sensor(runs fine but rich) if the ecu detects any sort of issue with maf. Also sometimes it will illuminate an check engine light other times it will not. Mine 520i doesn't. You can try to unplug the maf connector while your car is running at operating temp. If your idling will improve or stay the same the maf may have a problem. But if the car will stumble and try to stall or run worse than your maf is probably ok.

Hi 14W.

when I first purchased the car, the cruise control wouldn't work. The car drove well and from what I remember, the fuel consumption on a trip was okay as flew interstate to pick it up and drove it 1000klms home.
It took a while to find but it was determined to be the MAF and it was replaced, and magically the cruise control worked again.
This was about 2 - 2.5 years ago.
*I did a lot of reading and its amazing how many things the MAF can effect.

At this time, there is no error code and the cruise control is working. However, I will do the unplug test for sure.
No vacuum leaks, not irregular idle - smooth as any well tuned BMW 6 cylinder always is - no stumbles or misbehavior.

[ss2115]

Hi again 14W.
I think you made a good call. Engine warmed up after 20 minute drive, unplugged the MAF and the engine did a tiny single stumble and returned to exactly as it was before unplugging. Plugged it back in and same thing - tiny little stumble and then back to exactly the same idle.
Looks like I need a new MAF. Again.
At least its an easy job unlike an O2 Sensor or some obscure sensor up under the manifold or back of the engine etc.
Thanks for your great suggestion.

Wonder why it didn't show an error code though?

[14W]

Hi ,no problem. Yeah diagnosing faulty maf on old cars can be a lot of fun for sure, especially when there in no cel light or error code.If there was only tiny stumble and back to the previous state I would say that unfortunately you need a new maf, which sucks considering the price of it. May I ask is your current maf oem or china maf ? Because bmw generally hates all china sensors, especially maf which should contain platinum wire. Sometimes they work for a few months or year and then they die on you. But new oem maf is so expensive and with used oem from junkyard you never know what you get.

I think that no code thing is due to the problem that the faulty sensor still gives valid (but incorrect) output within the ecu's tolerance check parameters so the ecu doesn't recognise the reading as faulty and uses the voltage output from faulty sensor instead to switch from closed to open loop and throw a code .Also the ecu itself was programmed in the times when computer science was nowhere near today. But bmw for sure made a great job with their ecus and efi systems in those days in my opinon.

[ss2115]

Hi ,no problem. Yeah diagnosing faulty maf on old cars can be a lot of fun for sure, especially when there in no cel light or error code.If there was only tiny stumble and back to the previous state I would say that unfortunately you need a new maf, which sucks considering the price of it. May I ask is your current maf oem or china maf ? Because bmw generally hates all china sensors, especially maf which should contain platinum wire. Sometimes they work for a few months or year and then they die on you. But new oem maf is so expensive and with used oem from junkyard you never know what you get.

I think that no code thing is due to the problem that the faulty sensor still gives valid (but incorrect) output within the ecu's tolerance check parameters so the ecu doesn't recognise the reading as faulty and uses the voltage output from faulty sensor instead to switch from closed to open loop and throw a code .Also the ecu itself was programmed in the times when computer science was nowhere near today. But bmw for sure made a great job with their ecus and efi systems in those days in my opinon.

Hi. It was a VDO MAF I installed a couple of years ago, but in those days I purchased it from BMA in CA. The exchange rates were almost 1:1 and so US prices were generally half of what we have to pay here. For many years I purchased parts for my E34 and then the E39 from Patrick at BMA. He would always offer the OEM and also quality aftermarket alternatives.

I've ordered another VDO locally as I can get it before the weekend and the price is about the same as from BMA when I take in the shipping costs as well, but they can't get it here in time for this weekend of course.

Thanks again for pointing me in the right direction.

[ss2115]

Unfortunately, $280 later, it wasn't the MAF.
New VDO MAF installed on Sunday and all trims etc returned to default values. Been driving around, including today almost 30klms of 100k/h freeway on cruise control, and the reading only changed from 15.3 to 15.2 l/100k.
Next idea is to plug the laptop in and go for a drive watching the O2 sensors cycling to 1 volt and back and see if anything shows there. However, I'm skeptical because old, worn, non working O2 sensors usually throw an error code that they are out of range or out of spec has been my experience in the past.
I'd appreciate any other ideas or past similar experiences please.