P0170,P0173 fuel trim blues

[PingSpike]

Okay, I'm tired of randomly throwing money at these fault codes I keep throwing.

Car info. 1997 540/6 96,000 miles

Started out with CEL fault code P0173, fuel trim bank 2, car has 96000 miles so I replaced both pre cat O2 sensors & spark plugs, reset ECU, drove car about 50 miles threw the same P0173 fault, figured the new O2 on bank 2 was faulty, swapped sensors from bank 1 to 2, drove car about 100 miles threw CEL fault, code read it and now both fuel trim faults for both banks were thrown.:devillook

Next step: replaced MAF sensor drove car about 40 miles,(noticed much better throttle response after replaceing MAF) threw same CEL fuel trim faults bank 1 & 2.

Took car in to have leak down test to check for an intake leak & check for loose or bad vacuum hoses and add fuel injector cleaner, change fuel filter, air filter and replace all coil boots.

Diagnosis: car passed with flying colors, tight as a drum, no intake leaks and all my vacuum hoses were fine and attached porperly *sigh*

Mech. decided to flash/Burn in ECU with newly dated factory software (car still had origonal 1997 software) with higher fuel trim allowances (old software had allowance of 12%, newer software has an allowance of 16%). Drove car for 3 weeks total of about 200 to 300 miles, thought my problem was solved, bang! threw the same fuel trim faults:(

This car is showing no signs of misfires, the car idles great, never stalls, smooth when throttling up, runs freakin strong, just keep throwing these 2 fuel trim faults.

If anybody here can point me in the right direction I'd be much appreciated. The only things I have not replaced yet that have to do with "fuel" are the post cat O2 sensors, the fuel pump, & fuel regulator.

I assume with these faults, I am looking for something that has to due with "fuel" correct??. :confused

Like I said the car is running great, once the CEL comes on, it goes out after 3 driving cycles, can drive the car for weeks without reoccurance of faults, so I assume these misfires are a slight random occurance, just very annoying.

Again any help would be appreciated:)

signed Mr. frustrated. hehe!

[thejlevie]

On important item that we don't know is which trim limit (lean or rich) is being hit. A DIS or similar can pull that out of the ECU and it will help identify possible causes.

Are you sure that the O2 sensors aren't cross-connected now?

Did they do a smoke test to check for intake leaks? A visual inspection can miss a leak that a smoke test will instantly reveal.

As far as I know, the ECU only trims the mixture based on the output of the pre-cat O2 sensors. Those are new (are they OEM sensors?) so they should not be the problem unless they are cross connected. However, other problems with the engines sensors and/or controls can cause a lean or rich condition which will be reflected in the pre-cat sensor's output that will cause the ECU to run up against the trim limit.

[PingSpike]

thejlevie, thanks for the response, dude:buttrock


On important item that we don't know is which trim limit (lean or rich) is being hit. A DIS or similar can pull that out of the ECU and it will help identify possible causes.

both banks are running lean:


"current type of fault:
Mixture at the oxygen sensor is
too lean. Oxygen-sensor control
cannot make mixture any richer,
because control limit has already
been reached.
The fault is not currently present.
detected 5 times"


Are you sure that the O2 sensors aren't cross-connected now?

Hmmm!?!...I'm assuming not but not sure, what we did was swaped sensors ran the car and instead of swaping back, just replaced BOTH sensors with new different ones because it threw lean fuel trim for both banks, thinking both O2's might have been faulty.

What conditions would be caused if these sensors were "cross connected"??

How is it that the O2's become cross connected??

Like I said the car is running strong, would I be correct to assume that if they were cross connected the car would run terrible or the faults would constantly be thrown??


(are they OEM sensors?)

Yes, they are...


Did they do a smoke test to check for intake leaks? A visual inspection can miss a leak that a smoke test will instantly reveal

Yes, a smoke test was done, car is tight as a drum

Again thanks for the reply:)

[killcrap]

have you checked your exhaust for leaks? if they are lean, then you probably have a misfire or false air. do a 5 gas reading at idle, 1500rpm, 2500rpm and post them on here.

[PingSpike]

have you checked your exhaust for leaks? if they are lean, then you probably have a misfire or false air. do a 5 gas reading at idle, 1500rpm, 2500rpm and post them on here.

killcrap, thnx. for the input...

I thought about a exhaust leak, but wouldn't that be revealed when doing a intakeleak\smoke test?

I assuming doing a "5 gas reading" means taking the car in for diagnostics??, what is this test and how is it done?

I do notice that the faults have been thrown when throttling up in 2nd & 3rd gear @ about 2000 to 3000 rpms

Again thnx for the help:buttrock

[thejlevie]

Running lean sounds like an intake leak, bad MAF or fuel delivery problem. Since a smoke test was done with negative results and the MAF has been replaced, a fuel delivery problem (low fuel rail pressure or clogged injectors) would seem to be the next suspect. I'd say to check the pressure at the fuel rail and see it is correct before considering the injectors. Whle it possible for all (or most) of the injectors for both banks to become clogged, a more likely case is that the pressure is too low.

Cross connected sensors means that you have the sensor for bank 1 connected to the circuit for bank 2 and vice versa. It wasn't clear to me from your question what you'd done with the sensors, but the later comments suggest this isn't the case.

[txminime]

Are you running a CAI or the stock air box? I ran into a fuel trim problem that never seems to go away regardless of what I do. Finally decided to throw the stock air box back in just for kicks and the code went away for good.

[PingSpike]

I'd say to check the pressure at the fuel rail and see it is correct before considering the injectors. Whle it possible for all (or most) of the injectors for both banks to become clogged, a more likely case is that the pressure is too low.

What is the diagnosis if the pressure is to low at the fuel rail?

Am I looking at a new fuel pump or fuel regulator?...Or something else?

What test is done to prove or disprove this?

If some (or all) fuel injectors are clogged, do they need replaced?, If fuel injectors are clogged, wouldn't the car run like crap?.

Again the car seems to be running great...

Just looking to narrow this down as much as possiible before throwing more new parts into the car...

Thnx again, thejlevie...:)


Are you running a CAI or the stock air box? I ran into a fuel trim problem that never seems to go away regardless of what I do. Finally decided to throw the stock air box back in just for kicks and the code went away for good.

Nope, I'm running the stock air box...The car is completly stock

Thnx for the input...:cool

[thejlevie]

Low pressure at the fuel rail could be either pressure regulator or the pump. I believe that FPR is the cheaper of the two and that's what I'd replace first if the test showed inadaquate fuel pressure.

[killcrap]

Heres an explaination on how your m62 motor adjusts for ideal mixtures, and common problems that cause the CEL to come on.
http://img293.imageshack.us/img293/7104/13gy1.jpg
http://img293.imageshack.us/img293/9163/25kd.jpg
http://img293.imageshack.us/img293/2097/38id1.jpg

[PingSpike]

killcrap....Thats freakin beautiful, man!, thank you very much for posting that. Thats the complete breakdown & explanations I've been looking for...along with the fault codes.

You rock!...:cool

hmmm!..Mass Air Meter = MAF, never heard it called that before



thejlevie...thanks for your valuable input. Looks like a fuel pressure regulator is my next venture.

Will update the car and post results

[killcrap]

on bmw's some are called Air Mass Meters. but SAE calls it MAF= Mass Air Flow