MAF sensor sometimes reads high
 

I have a stock 2012 86 with a replaced short block (yup, bearings failed after valve spring recall). This was more than a year ago, so there's no warranty on the work anymore.

Recently, I've been logging a lot with tactrix op2, and saw some very strange fuel trim data. Sometimes the fuel trims (sums of LTFT and STFT) are negative, between -10% and -30%. When starting the car, e.g. it's often around -20% or so when idling. Now and then, the negative trims will disappear, and stay away for the rest of the trip. It also goes the other way (starts with near zero trims, then go negative). These state changes are very quick, almost like a switch is flipped.

I've done a lot to eliminate causes. None of these made any difference:
- Blanked off PCV at intake (in case it was a bad piston ring).
- Blanked off EVAP line coming from the back of the car (in case it was the EVAP solenoid leaking).
- Cleaned MAF sensor.
- Tried a brand new MAF sensor.
- Cleaned AFR sensor.
- Checked if it happens when only DI is used, or only PI, in case it was an injector. Yup, happens for both cases.
- Checked for intake leaks
- Cleaned throttle body
- Replaced air filter.

Then I looked at logs in more detail, comparing 2 warm idles
- in the same trip, similar environment,
- both with A/C off,
- both at 750rpm,
- using DI only in both cases,
- one case with zero trims and one with -30%.
- The MAP readings are the same in both cases, and IAT was very similar. For the same RPM, I would expect the MAF reading to also be similar (it would have been, if speed density was used to estimate it..).
- Also, the fuel pressure is the same and the DI pulse widths are the same. So the fuel delivered does not change, but fuel trim goes very negative in order to achieve that.
- I looked at the MAF voltages seen by the ECU and indeed it's about 0.1V higher where the fuel trim is negative.
- I don't see obvious issues with other sensor readings in the logs.

So, I suspect a sensor ground leak in the harness somewhere. The fact that it comes and goes make me think it could be due to heat and moisture, creating a path to another ground, depending on the environment. After fiddling with the harness a bit (cleaned some connectors), the issue went away for almost a month, but now it's coming back. Does anyone have experience with this kind of thing, what's the best way to find the cause? I don't want to spend huge amounts of money on a new harness.

I was recently tuning with my tuner and my MAF was doing weird things. The connector was routed weird after the shop removed my Sprintek SC. I noticed this and made sure the connector wire was straight an snuggly put on. Ever snice then the MAF has been fine. My car is a 2013 but outside of the re-call.