11-29-2012, 08:37 PM
bimmerfest Supporting Member
Location: in the sticks you piney
Join Date: Nov 2007
Mein Auto: rescued bmw's
can you see the freeze frame data with the code(s)?? obd2 protocol requires it to be accessible on certain code reading tools (if it has the ability to clear them, ff data s/b atainable) this will tell you what the parameters are, whether the dme (pcm) is trying to add fuel or take it away.
if you are getting such poor mileage, i do not see that as a lean condition. however, if the fuel trim is going negative, the pcm is trying to lean the car out as there is too much fuel getting into the combustion chamber.
if the fuel trim numbers are going positive, then it is reading more air than is actually getting into the combustion chambers.
fuel trim is in long term (ltft) and short term (stft), and if it is going negative, check your fuel pressure, fuel pressure regulator (pulse width modulated fuel pump on this one?), injectors (possible leaky injectors) and evap system. this is where the fuel would be coming from.
you should also check the injector pulse width as well. see if your code tool will be able to give you live data. (check the speed by watching rpm, and when steady, snap the throttle and see how fast the numbers respond...the faster the better....)
if real fast, you should be able to check the injector pulse in ms.
if the fuel trim is going positive, then check all of the usual suspects for vac leaks. a smoke machine works well for this, and if you have a neighborhood dj that has a 'fog machine', it would work. (you just have to work on the logistics of getting the smoke channeled intothe intake....) and low pressure (1psi) is all you need. watch for the smoke to show = find leak. fix accordingly.
*edir* hit a wrong buttonon my laptop & it all went wonky....sorry.
if no smoke machine then use a vacuum guage.
if vac looks good, in range and no indications of issues, then i would suspect a faulty maf. @ idle these things will be lower voltage output, and go higher on an analog design. not sure if bmw uses a hz system like fords or not, have not scoped a newer 7 series yet...
regardless, if running rich due to increased fuel trim, the pcm is controlling the amounts. this could be caused by a faulty sensor (usually not an o2 at this point...) or faulty pcm.
i hope this helps shed some light on the subject. this is pretty much basic diagnostic proceedure on obd2 vehicles (even pre odb2), which requires some special equipment and knowhow to use it. however, the know how will allow you to use more readily available equipment to get a better idea as to what is going on without resorting to throwing parts at it.
oh yeah, and what were the readings?? that would be a huge piece of the puzzle....
hc= misfire indicator
co= rich indicator
co2 = efficiency indicator
o2= cat efficiency indicator
nox= high combuston temps (usually *never* with high co)
Last edited by drivinfaster; 11-29-2012 at 08:50 PM.