MAF over reporting
The MAF reports more air than the engine draws, so the module meters too much fuel and closed loop takes the same share back out everywhere. The MAF reads above the speed density estimate in every cell.
Fingerprint
Trims negative by about the same amount in every cell Must be present
Rule: Every closed loop total trim at or below -5 percent, with no more than 3 points between them.
The module believes there is more air than there is, meters fuel for it, and then takes the same share back out in every cell.
MAF above the estimate everywhere Must be present
Rule: Airflow check at least 105 percent in all four cells.
The MAF reads above what the displacement, rpm and MAP can physically draw.
Why the numbers move
Over reporting by a fixed fraction means base fuel is high by that fraction at every load, so the negative correction is the same size at idle, at 2,500 rpm and at cruise.
An engine cannot draw more air than its displacement, rpm and manifold pressure allow. A MAF reading above the estimate everywhere is the sensor, not the engine.
The reference numbers
Computed by the model at build time: the fault at its clean reference size on a 2.0 l inline four, narrowband upstream sensor, with no noise.
| Reading | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Total trim bank 1 (%) | -9.1 | -9.0 | -9.0 | open loop |
| Airflow check (%) | 110 | 109 | 109 | 110 |
| MAF (g/s) | 3.33 | 8.37 | 15.50 | 92.28 |
| MAP (kPa) | 30.0 | 22.0 | 42.0 | 97.0 |
| Upstream B1S1 | switching, avg 0.45 V | switching, avg 0.45 V | switching, avg 0.45 V | 0.90 V steady |
| Downstream B1S2 (V) | 0.66 | 0.66 | 0.66 | 0.90 |
| ECT (C) | 90 | 90 | 90 | 90 |
| Misfire per 1,000 revs | 0 | 0 | 0 | 0 |
Easy to confuse with
- Fuel pressure too high (regulator stuck closed). Both pull the trims down evenly. The airflow check splits them: a high reading MAF sits above the estimate; high fuel pressure leaves the MAF honest.
- ECT sensor stuck cold. Both pull the trims down evenly. A frozen cold ECT explains it on its own; a high reading MAF shows above the speed density estimate.
On a real car
- Check the reading against a calculated value with the engine warm and steady.
- Look for an air filter or intake duct problem that disturbs the flow across the sensor.
TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle. Follow the service information for the vehicle in front of you.
Where the model is simpler than an engine
The model applies one uniform over report. A real sensor's error usually varies somewhat with flow.
The full model is on how the model works.
An example case
Case C-AM-B, clean tier. Open it in projection mode.
Case C-AM-B
- Engine
- 2.0 L inline four, narrowband upstream sensor
- Customer says
- Engine seems down on power
- Check engine light
- Off
- Tier
- Single fault, clean
Warm engine, sea level, 100 kPa barometric pressure. Simulated data for training, not from a real vehicle.
Freeze frame
No code stored. Snapshot saved by the technician at warm idle.
- Codes
- None stored
- Captured at
- Idle
- Engine speed
- 743 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 22.3 %
- MAP
- 29.9 kPa
- MAF
- 3.23 g/s
- ECT
- 90 C / 194 F
- IAT
- 38 C / 100 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- -2.0 % / -8.0 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 743 | 2501 | 2098 | 4997 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 29.9 | 21.9 | 41.9 | 97.1 |
| MAF (g/s) | 3.23 | 8.37 | 15.19 | 91.69 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 108 | 110 | 108 | 109 |
| Calculated load (%) | 22.3 | 17.2 | 37.2 | 94.2 |
| IAT | 38 C / 100 F | 35 C / 95 F | 31 C / 88 F | 29 C / 84 F |
| ECT | 90 C / 194 F | 89 C / 192 F | 89 C / 192 F | 90 C / 194 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | -2.0 | -2.0 | -1.7 | 0.0 |
| LTFT bank 1 (%) | -8.0 | -7.9 | -7.9 | -7.9 |
| Total trim bank 1 (%) worked out | -10.0 | -9.9 | -9.6 | -7.9 |
| O2 sensor B1S1 | switching, avg 0.45 V | switching, avg 0.46 V | switching, avg 0.45 V | 0.91 V steady |
| O2 sensor B1S2 (V) | 0.67 | 0.66 | 0.67 | 0.88 |
| Injector pulse bank 1 (ms) | 4.45 | 3.49 | 6.96 | 19.96 |
| EVAP purge command (%) | 0 | 20 | 35 | 0 |
| Misfire count per 1,000 revs | 0 | 0 | 0 | 0 |
Rows marked worked out are arithmetic on the rows above them, done for you: total trim is STFT plus LTFT, and the airflow check divides the MAF by the speed density estimate on the model page.