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?