MAF under reporting
A contaminated or failing MAF reports less air than the engine draws. The trims add fuel, more as flow rises, the MAF falls further below the speed density estimate as flow rises, and WOT airflow is well short.
Fingerprint
Trims grow more positive as airflow rises Must be present
Rule: Cruise total trim at least +4 percent and at least 3 points above idle.
A contaminated sensing element under reads more as flow rises, so the module has to add more fuel as airflow rises.
MAF short of the estimate, and more so as flow rises Must be present
Rule: Airflow check at or below 94 percent at cruise, and at WOT at least 8 points below idle.
The MAF falls further below the speed density estimate as flow rises. That is the sensor reading low, not air going missing.
Why the numbers move
The module meters fuel from the MAF reading. If the sensor reads a fraction low, the base fuel is that fraction short and closed loop adds it back. In this model a contaminated element loses more of its signal as flow rises, so the shortfall and the correction both grow with airflow.
The airflow check is the tell. The engine's actual airflow is set by displacement, rpm, manifold pressure and temperature; the MAF should match it. A leak makes the MAF short because air goes around it; a failing MAF is short because it reads low, and the gap keeps growing all the way to WOT, where a leak would have closed up.
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 (%) | +5.2 | +8.4 | +11.8 | open loop |
| Airflow check (%) | 95 | 91 | 89 | 74 |
| MAF (g/s) | 2.87 | 7.01 | 12.59 | 62.08 |
| 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 | 0.12 V steady |
| Downstream B1S2 (V) | 0.66 | 0.66 | 0.66 | 0.13 |
| ECT (C) | 90 | 90 | 90 | 90 |
| Misfire per 1,000 revs | 0 | 0 | 0 | 0 |
Easy to confuse with
- Vacuum leak. Both leave the MAF short. A leak is a fixed mass, biggest at idle; a low reading MAF errs more as flow rises, so its trims grow with airflow and WOT airflow falls well short.
- Restricted fuel filter or weak pump. Both push the trims up as airflow rises. Check the MAF against the estimate: short and getting shorter means the sensor; honest air with a lean WOT means the fuel side.
- Restricted exhaust. Both leave WOT airflow short. A low reading MAF also drives the trims up with airflow; a restriction keeps the trims quiet because the MAF measures the reduced air honestly.
On a real car
- Compare MAF grams per second against a calculated value at a steady 2,500 rpm and during a WOT run, using the service information's figures.
- Inspect the sensing element and the air filter housing for contamination or leaks after 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
Some failing sensors under report more at low flow instead. The fingerprint here is the high flow pattern; the low flow version would look closer to a vacuum leak without the WOT recovery.
The full model is on how the model works.
An example case
Case C-AM-9, clean tier. Open it in projection mode.
Case C-AM-9
- 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
- 750 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 19.9 %
- MAP
- 30.1 kPa
- MAF
- 2.90 g/s
- ECT
- 90 C / 194 F
- IAT
- 38 C / 100 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- 0.0 % / +4.6 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 750 | 2498 | 2092 | 5001 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 30.1 | 21.9 | 42.0 | 97.0 |
| MAF (g/s) | 2.90 | 6.94 | 12.63 | 63.10 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 96 | 91 | 89 | 75 |
| Calculated load (%) | 19.9 | 14.3 | 31.0 | 64.8 |
| IAT | 38 C / 100 F | 35 C / 95 F | 30 C / 86 F | 29 C / 84 F |
| ECT | 90 C / 194 F | 91 C / 196 F | 90 C / 194 F | 90 C / 194 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | 0.0 | +1.2 | +0.2 | 0.0 |
| LTFT bank 1 (%) | +4.6 | +7.5 | +10.5 | +10.5 |
| Total trim bank 1 (%) worked out | +4.6 | +8.7 | +10.7 | +10.5 |
| O2 sensor B1S1 | switching, avg 0.45 V | switching, avg 0.45 V | switching, avg 0.45 V | 0.23 V steady |
| O2 sensor B1S2 (V) | 0.66 | 0.66 | 0.65 | 0.27 |
| Injector pulse bank 1 (ms) | 4.51 | 3.50 | 7.04 | 16.48 |
| 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.