Fuel pressure too high (regulator stuck closed)
The regulator is stuck closed or the return is restricted, so rail pressure runs high. Every pulse delivers more fuel than commanded, the same share in every cell, and the trims come down evenly. The MAF agrees and the coolant reading is normal.
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.
Injector flow goes with the square root of pressure, so a quarter more pressure means roughly a tenth more fuel per pulse, the same share in every cell.
MAF agrees with the speed density estimate everywhere Must be present
Rule: Airflow check between 95 and 105 percent in all four cells.
The MAF agrees with the estimate, so the air is measured correctly and the extra fuel comes from the fuel side.
ECT frozen at one low value Must be absent
Rule: ECT the same (within 1 C) in all four cells and no higher than 55 C.
It belongs to a neighbouring fault.
Why the numbers move
Flow through an injector goes with the square root of the pressure across it, so a rise of a quarter in pressure delivers roughly a tenth more fuel for the same pulse. That excess is the same share at every load, so the negative correction is flat across the cells.
Two other faults also pull the trims down evenly. A MAF that over reports gives it away in the airflow check; an ECT stuck cold gives it away in the coolant reading. High fuel pressure leaves both honest.
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 (%) | -10.6 | -10.4 | -10.4 | open loop |
| Airflow check (%) | 100 | 99 | 99 | 100 |
| MAF (g/s) | 3.02 | 7.61 | 14.09 | 83.89 |
| 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
- MAF over reporting. 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. Read the coolant temperature first: a frozen cold reading explains the whole correction.
- Leaking injector. Both mean extra fuel. High pressure adds the same share at every load; a drip matters at idle and fades by cruise.
On a real car
- Measure fuel pressure at idle and under load against the specification.
- Check the return line and regulator.
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 uses one constant pressure rise. A regulator that sticks intermittently would come and go.
The full model is on how the model works.
An example case
Case C-FS-0, clean tier. Open it in projection mode.
Case C-FS-0
- Engine
- 2.0 L inline four, narrowband upstream sensor
- Customer says
- Black smoke on a cold start
- 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
- 742 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 21.2 %
- MAP
- 30.3 kPa
- MAF
- 3.06 g/s
- ECT
- 89 C / 192 F
- IAT
- 37 C / 99 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- -0.9 % / -8.7 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 742 | 2493 | 2103 | 5014 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 30.3 | 22.1 | 41.2 | 97.2 |
| MAF (g/s) | 3.06 | 7.66 | 13.90 | 84.79 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 101 | 100 | 100 | 101 |
| Calculated load (%) | 21.2 | 15.8 | 33.9 | 86.8 |
| IAT | 37 C / 99 F | 35 C / 95 F | 30 C / 86 F | 29 C / 84 F |
| ECT | 89 C / 192 F | 91 C / 196 F | 91 C / 196 F | 90 C / 194 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | -0.9 | -1.7 | -1.1 | 0.0 |
| LTFT bank 1 (%) | -8.7 | -8.6 | -8.6 | -8.6 |
| Total trim bank 1 (%) worked out | -9.6 | -10.3 | -9.7 | -8.6 |
| O2 sensor B1S1 | switching, avg 0.44 V | switching, avg 0.45 V | switching, avg 0.45 V | 0.90 V steady |
| O2 sensor B1S2 (V) | 0.65 | 0.66 | 0.65 | 0.91 |
| Injector pulse bank 1 (ms) | 4.15 | 3.25 | 6.30 | 18.23 |
| 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.