Restricted exhaust
A plugged converter or crushed pipe limits how much exhaust can leave. Trims stay quiet because the MAF measures the reduced air honestly, but at WOT the engine draws far less air than its displacement and MAP say it should.
Fingerprint
Trims close to zero in every closed loop cell Must be present
Rule: Total trim (STFT + LTFT) within plus or minus 5 percent at idle, 2,500 rpm and cruise.
The MAF measures the air that actually gets in, so the fuel still matches it and the trims stay quiet.
MAF short of the estimate at WOT only Must be present
Rule: Airflow check at or below 88 percent at WOT while cruise is at least 95 percent.
At WOT the engine cannot push its exhaust out, so it draws far less air than its displacement and MAP say it should. At light load the restriction hardly shows.
Upstream sensor reads lean at WOT Must be absent
Rule: At WOT the upstream sensor reads below 0.45 V (narrowband) or above lambda 1.00 (wideband).
It belongs to a neighbouring fault.
Why the numbers move
Exhaust back pressure leaves more burned gas in each cylinder, which lowers volumetric efficiency. The restriction's back pressure rises with the square of the flow, so at idle and light load it barely matters and at WOT it dominates. Off WOT the throttle simply opens a little more to hold the same airflow, so MAP creeps up; at WOT there is no more throttle to open and the airflow falls.
Fuel control is unaffected because the air that does get in is measured, so the trims stay near zero. The fingerprint is on the air side only: an airflow check close to normal at light load and well short at WOT.
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 (%) | 0.0 | 0.0 | 0.0 | open loop |
| Airflow check (%) | 100 | 98 | 98 | 74 |
| MAF (g/s) | 3.02 | 7.61 | 14.09 | 62.27 |
| MAP (kPa) | 30.0 | 22.2 | 42.7 | 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
- MAF under reporting. 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.
- No fault (healthy control). The trims are healthy, but WOT airflow falls far short of what the displacement and MAP should draw.
On a real car
- Measure exhaust back pressure or run a vacuum test at a held rpm, as the service information describes.
- Compare MAF at WOT against the expected value for the engine.
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 the restriction to both banks together, as a restriction after the pipes join would.
The full model is on how the model works.
An example case
Case C-AM-0, clean tier. Open it in projection mode.
Case C-AM-0
- 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
- 762 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 20.8 %
- MAP
- 30.1 kPa
- MAF
- 3.09 g/s
- ECT
- 90 C / 194 F
- IAT
- 38 C / 100 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- -0.3 % / 0.0 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 762 | 2506 | 2106 | 4990 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 30.1 | 21.6 | 42.8 | 97.0 |
| MAF (g/s) | 3.09 | 7.41 | 14.12 | 61.92 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 100 | 98 | 97 | 74 |
| Calculated load (%) | 20.8 | 15.2 | 34.4 | 63.7 |
| IAT | 38 C / 100 F | 35 C / 95 F | 30 C / 86 F | 29 C / 84 F |
| ECT | 90 C / 194 F | 90 C / 194 F | 91 C / 196 F | 90 C / 194 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | -0.3 | +0.4 | +0.6 | 0.0 |
| LTFT bank 1 (%) | 0.0 | 0.0 | 0.0 | 0.0 |
| Total trim bank 1 (%) worked out | -0.3 | +0.4 | +0.6 | 0.0 |
| O2 sensor B1S1 | switching, avg 0.46 V | switching, avg 0.46 V | switching, avg 0.44 V | 0.89 V steady |
| O2 sensor B1S2 (V) | 0.66 | 0.65 | 0.66 | 0.91 |
| Injector pulse bank 1 (ms) | 4.52 | 3.45 | 7.08 | 14.76 |
| 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.