Originally Posted by Bad Noodle
^ No, I removed the ground from the maf tube as I didn't want to ground the maf in ways it wasn't intended. I'm afraid it might fry something in the ecu and cause a world of diagnostic hurt down the road.
Your corrections in all ranges should be +-5%. That's the common accepted value for a good maf scaling. However, most intakes can not achieve this without additional corrections. This is because the flow pattern past the maf changes with rpm and manifold relative pressure. This is where your MRP correction table comes in. If on flat ground and regular cruise your maf sees 10 g/s and the car is processing 10g/s, once you hit a hill and the manifold vacuum changes for the same rpm, your maf might see 8 g/s and car might be processing 12 g/s leading to a +4 correction. Then in that table you might enter +4 for the rpm and vacuum pressure. So you're making a preemptive correction for the condition then next time you see the same conditions your corrections will be 0 as the +4 g/s needed will come from the MRP table rather then your LTFT. Each intake needs a different scaling for this table as the geometry of the piping varies between intakes.
For my setup, I have this table zeroed out for now. When I tune an intake I first try and get the corrections centered around 0 using the maf scaling, then reduce the corrections using the MRP table. You know something is wrong when you get +-large corrections for the same MRP table cell. That means there is no consistency in the airflow readings for that condition. That means there is a physical problem with your setup, bad sensor, bad signals, or something else.
For my problem, it turned out to be bad signals as well as a bad sensor as far as I can tell so far. The bad signals (maf voltage reading) were coming from the maf sensor using the maf tube as a ground, which grounded intermittently as the engine moved around. Then a bad sensor was causing random large corrections intermittently. Once I ensured the maf has grounded properly and there was no electrical interference, the only thing left that could be causing the problems was bad sensors. So I started swamping out sensors and a new maf seemed to make the difference.
Grounding the maf like you did will take care of the random grounding issue and ensure a clean signal. The only trouble I see with the fix is that the OEM setup was not meant to run like that. Pin 4 on the maf is what gets grounded and the corresponding terminal on the plug does not have a direct route to the chassis ground. It gets grounded through the ecu, which means the ecu is monitoring that pin as a reference signal So if you ground the intake to the chassis directly, you are circumventing this operation without knowing what the end results will be. One thing is certain is that there is voltage running from the ecu, to the intake, then to the chassis ground that is not seen on an oem setup. Since this problem hasn't been investigated before, no one really knows what the consequences of this will be.
First, I think I didn't make my info clear with regard to what "grounding" I did. My ground strap from the AEM pipe to chassis grounds ONLY the pipe. The MAF sensor's ground/plate is perfectly insulated from that ground, and still follows 100% of the OEM circuitry.
This means my MAF sensor is, if anything, more free from spurious EMF than OEM now for two reasons. One, the "ground" is free of the big "antenna" of the pipe, and two, the MAF's ground has a high-freq by-pass capacitor now (created by the mica etc) that strips any remaining spurious radiation. A cleaner signal is a better signal in any case.
The end result is a significant change in my LTFT since removing the "normal" connection of the MAF's ground to the intake pipe, and then grounding the pipe to chassis. It is not drastic in terms of numbers, but in the makeup of those numbers. Where I got negative trims I now get positive, for example, not LARGER numbers. But my trims never get as high as 5%, with D always around 1% or closer. My fueling cannot afford 5%.
Just imagine this: ZERO change in circuitry, but a marked and constant change in fuel trims... that has, of course!, improved the driveability of my AEM CAI-equipped STi.
This is where I should admit I wasn't aware of the MRP table you reference.
Thank you for your input and explanation. This must explain some things I need to know.
FWIW, and my anecdotal experience may not convince, but there has been a change in how the car drives since my little MAF mod that is hard to voice. The words are smoother and silkier throttle. As I put on a couple hundred miles in the mountains this past weekend I was constantly aware of this, but found putting it into words difficult. I can now, but here isn't the place.
IMO, adding a ground to your MAF tube... given that your MAF sensor is electrically independent from it... can only be good.