818 Engine Drivability Problem

[Dave 53]

I just lengthened my air intake pipe. The car ran fine before, but now the engine "stumbles."

It seems fine under load while accelerating up to red line, but cruising on level road at partial throttle, it stumbles. Wish I had better way words to describe it. Going down a hill that needed some throttle just to keep going, it seemed worst. I'm feeling it around 2,300 rpm. As I increase revs, like take it up slowly to 6,000 rpm in second gear, no load, the stumbling seems to go away.

I did two things since it ran fine. First, I physically lengthened the air intake pipe.

Second, I lengthened the MAF sensor wires. At first, I was getting a P0102 code (MAF circuit low). I figured out that one of the wires has a ground shield and not knowing that, I soldered the shield and data wire together. After fixing that including shielding the wire I spliced in, the code went away. (Note, the O2 sensor data wire also has a shield)

I don't see how lengthening the air intake pipe or MAF sensor wires would cause this.

I reviewed a data log and no engine parameters jumped out at me. Everything looks normal to the best that I can analyze the sea of number on the spread sheet.

Help. What should I check?

[Sgt.Gator]

Looking at the pics in your other MAF thread in the general discussion (Hint this should be in that thread IMHO ) ....I'll take a guess that the maf is too close to the filter and the filter is now getting turbulent air from the new location at the scoop.
If possible, try re-locating the MAF sensor further from the filter where there's a few inches of straight pipe on both sides of the sensor. Or move the filter up away from the scoop, or tape the scoop closed, even if it's just temporary to rule out a wiring issue.
You can also try using a honeycomb maf straightener. Google search "maf air straightener". They are just honeycomb "filters" you install.

This guy has has been making filter plus adaptors that includes a housing along with the honeycomb filter. He makes them in a bunch of intake pipe sizes to fit Subaru mafs. I think I still have one buried in my spare parts storage. What size is your intake pipe?

2.5 OD MAF Mass Air Flow Housing Air Straightener for 02-07 Subaru WRX STI

[Dave 53]

Looking at the pics in your other MAF thread in the general discussion (Hint this should be in that thread IMHO ) ....I'll take a guess that the maf is too close to the filter and the filter is now getting turbulent air from the new location at the scoop.
If possible, try re-locating the MAF sensor further from the filter where there's a few inches of straight pipe on both sides of the sensor. Or move the filter up away from the scoop, or tape the scoop closed, even if it's just temporary to rule out a wiring issue.
You can also try using a honeycomb maf straightener. Google search "maf air straightener". They are just honeycomb "filters" you install.

This guy has has been making filter plus adaptors that includes a housing along with the honeycomb filter. He makes them in a bunch of intake pipe sizes to fit Subaru mafs. I think I still have one buried in my spare parts storage. What size is your intake pipe?

2.5 OD MAF Mass Air Flow Housing Air Straightener for 02-07 Subaru WRX STI

When I first posted the drivability issue, I didn't expect the MAF was still the issue and it is. Agreed, taking this back to the original MAF post.

[Dave 53]

Deleted

This thread moved BACK to the "818 MAF sensor failure and track days" post.