OK, here is a story. It may apply here or it may not.
I thought about just writing a "fix" post but it seems to apply here.
My fun repair of the week was a 1998 E430. Fun repairs are those that breed fear and when completed bring the ultimate of satisfaction (very often with little monetary gain). The ticket said that the car lacked power and the check engine light was on and shifted poorly.
Upon driving the thing, the light wasn't on and mostly it drove fine. it did have some hesitation but the engine was warm when I started. Occasionally it would just quit accelerating under full throttle. So.. first step lets see what codes are stored. After a short test I had NO codes indicated for ME (engine management). There were codes in many control units ASR, BAS, SAM, ICM, ESLM (electric seat left and right I forget the acronym) and ACC.
I tested the ME anyway and there were stored codes for EGR, O2 sensors both sides, and P0170 and 173 adaptation codes. Based on the O2 codes and the adaption codes I checked actual values for adaptation. Lower partial was at 1.02, idle was at about .5ms. The 1.02 value was terrible for diagnostics as it is almost perfect. The .5ms is a significant mixture correction (adding fuel) but well within margins. The O2 sensors were slow but functioning well. It didn't make much sense.
The other modules all had CAN communications errors. The ASR didn't see the ME and the BAS didn't see the ASR and all the other modules had CAN errors.
Here is where the fear comes in. In the road tests the lack of power seems very easy to explain with a protective strategy based upon ASR / ME communications. Either a throttle position control or a timing control. This could be real trouble......After clearing codes and repeated driving, the network codes (CAN) did not reappear but the running was getting worse not better. The EGR codes reappeared and so did the adaptation codes while adaptation values maintained right around one 1.0(what can this be?). I had put aside my biggest fear by this time (an intermittant CAN short or control unit pulling the network down).
When in doubt try a AMM (air mass meter) which I would have by this time but I didn''t have a V8 one in stock.... so I get one coming for next day and we decide to remove the EGR pipe for cleaning while waiting for the next diagnostic step (the substitution of new AMM). My assistant is pulling the pipe when about 15 minutes later I hear one of those "How about that" screams. The first step to getting to the EGR pipe had been the removal of the formed hard plastic/flexible rubber intake (from the throttle assy to the AMM) pipe and there it is..... the flexible section is plastic clamped to the hard plastic and it ISN'T. There is a 3/4 inch gap at the bottom (the top is together or the repair would have been obvious). This pipe is one piece 30-40 inches long. The union of rubber to hard plastic is not servicable within MB procedures... the whole thing is on order,,, BUT a couple pieces of duct tape and some 5 in diameter tie wraps makes a virtual permanent repair and instant gratification to the diagnostician. The AMM came in next day (yesterday) and will join the others in stock (four total - 104HFM, 104ME, 112ME, and now 113ME).
There are two points to this story (maybe three). First is that directly, all my fault codes were leading me wrong. This is more often the case than otherwise. The majority of codes have no use for the symptom at test. Second is that this air pipe must be watched.. the car had recently had its pan resealed (at another facility) and the driveability came with the repair. and thirdly a functioning AMM does not directly indicate whether it is volumtrically efficient or correct. This one was in effect in the trunk, but it measured the air going across it (none) correctly and fourthly the engine management does an amazing job of taking care of itself. At any constant speed the car ran flawlessly by using its other inputs.
Bosch Master, ASE Master, L1
33 years MB technician