Leaking injector
One injector drips while it should be shut. The trims on its bank go negative at idle and come back towards zero by cruise, and the MAF agrees with the estimate. On a V engine only one bank is affected.
Fingerprint
Trims negative at idle, near zero at cruise Must be present
Rule: Idle total trim at or below -6 percent and at least 5 points below cruise.
The injector drips while it is shut. At idle it is shut almost all the time, so the drip is a big share of idle fuel; at cruise it is open longer and the drip hardly matters.
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 no unmetered air or vapour is involved: the extra fuel is inside the injector.
Only one bank is affected Supports it on a V engine
Rule: On a dual bank engine, the two banks' total trims differ by at least 5 points in some cell.
Only the bank with the dripping injector has to pull fuel out.
Why the numbers move
A dripping injector leaks while it is closed. At idle each injector is shut for well over ninety percent of the cycle, so the drip is a large share of idle fuel; at cruise it is open longer and the drip becomes a small share. The negative correction therefore shrinks with load.
No air is involved, so the airflow check is normal, which separates it from a purge valve stuck open. The extra fuel lands on one bank only, so on a dual bank engine the other bank stays at zero.
The reference numbers
Computed by the model at build time: the fault at its clean reference size on a 3.5 l v6, dual bank, wideband air fuel sensors, with no noise. The fault sits on bank 2; bank 1 is healthy.
| Reading | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Total trim bank 1 (%) | 0.0 | 0.0 | 0.0 | open loop |
| Total trim bank 2 (%) | -12.0 | -4.1 | -2.7 | open loop |
| Airflow check (%) | 100 | 99 | 99 | 100 |
| MAF (g/s) | 4.59 | 12.72 | 19.10 | 146.81 |
| MAP (kPa) | 30.0 | 21.0 | 38.0 | 97.0 |
| Upstream B1S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.85 |
| Upstream B2S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.87 |
| Downstream B1S2 (V) | 0.66 | 0.66 | 0.66 | 0.90 |
| Downstream B2S2 (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
- EVAP purge valve stuck open. Both put the trims negative at idle. The airflow check splits them: the purge stream enters below the MAF, so it reads short at idle; an injector drip adds no air, so the MAF agrees.
- Fuel pressure too high (regulator stuck closed). 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
- Check rail pressure bleed down with the engine off.
- Balance or flow test the injectors on that bank as the service information describes.
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 a constant drip rate. Real leaks can worsen with heat, which is why hot restart complaints are common.
The full model is on how the model works.
An example case
Case C-FS-9, clean tier. Open it in projection mode.
Case C-FS-9
- Engine
- 3.5 L V6, dual bank, wideband air fuel sensors
- Customer says
- Long crank after a hot soak
- 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
- 652 rpm
- Vehicle speed
- 0 mph
- Calculated load
- 21.0 %
- MAP
- 30.3 kPa
- MAF
- 4.67 g/s
- ECT
- 89 C / 192 F
- IAT
- 38 C / 100 F
- Fuel system
- Closed loop
- STFT / LTFT bank 1
- -1.3 % / 0.0 %
- STFT / LTFT bank 2
- -1.0 % / -10.6 %
Live data by load cell
| PID | Idle | 2,500 rpm, no load | Cruise | WOT snapshot |
|---|---|---|---|---|
| Engine speed (rpm) | 652 | 2495 | 1801 | 5003 |
| Vehicle speed (mph) | 0 | 0 | 60 | 45 |
| MAP (kPa) | 30.3 | 20.9 | 37.8 | 97.1 |
| MAF (g/s) | 4.67 | 12.62 | 19.09 | 147.54 |
| Airflow check, MAF as % of speed density estimate (%) worked out | 101 | 99 | 100 | 100 |
| Calculated load (%) | 21.0 | 14.8 | 31.1 | 86.5 |
| IAT | 38 C / 100 F | 34 C / 93 F | 31 C / 88 F | 29 C / 84 F |
| ECT | 89 C / 192 F | 90 C / 194 F | 90 C / 194 F | 90 C / 194 F |
| Fuel system | Closed loop | Closed loop | Closed loop | Open loop |
| STFT bank 1 (%) | -1.3 | +0.4 | +0.2 | 0.0 |
| LTFT bank 1 (%) | 0.0 | 0.0 | 0.0 | 0.0 |
| Total trim bank 1 (%) worked out | -1.3 | +0.4 | +0.2 | 0.0 |
| STFT bank 2 (%) | -1.0 | +0.5 | -0.6 | 0.0 |
| LTFT bank 2 (%) | -10.6 | -3.7 | -2.4 | -2.4 |
| Total trim bank 2 (%) worked out | -11.6 | -3.2 | -3.0 | -2.4 |
| A/F sensor B1S1 | lambda 1.01 | lambda 0.99 | lambda 1.00 | lambda 0.84 |
| A/F sensor B2S1 | lambda 1.00 | lambda 1.00 | lambda 1.00 | lambda 0.87 |
| O2 sensor B1S2 (V) | 0.66 | 0.66 | 0.65 | 0.90 |
| O2 sensor B2S2 (V) | 0.67 | 0.67 | 0.65 | 0.90 |
| Injector pulse bank 1 (ms) | 4.45 | 3.31 | 6.27 | 19.48 |
| Injector pulse bank 2 (ms) | 4.00 | 3.20 | 6.11 | 19.02 |
| 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.