86’ corvette MAF mystery issue code 33
1986 Corvette C4 L98 TPI automatic
Getting persistent Code 33 and unplugging the MAF causes absolutely no idle/drivability change at all. Engine still idles smoothly around 600 RPM and runs surprisingly well otherwise.
Things already checked/tested
- Timing properly set to 6° BTDC with EST bypass disconnected
- Engine mechanically seems healthy
- Stable idle/oil pressure/temp
- No major vacuum leak symptoms
- MAF relay and burn-off relay both appear to function correctly
- Grounds tested good with continuity to chassis
- Connector power/ground seems present
Connector readings with engine running:
- Purple wire: ~14.4V
- Both black wires: ground
- Green middle wire fluctuates around ~3.5V idle and rises toward ~5V with revs
Original MAF behaved this way and replacement MAF behaves IDENTICALLY. Both sensors produce:
- Code 33
- no idle change when unplugged.
I have a Delphi AF10320 on the way, but I’m trying to determine whether this points more toward a signal wire issue, ECM/PROM issue, ECM permanently in fallback mode, or a known 1986 PROM/calibration problem
Any help from someone familiar with early GM/Bosch hot-wire MAF systems would be greatly appreciated.