TrimCase
Fuel trim case drills

Fuel trim diagnosis practice: read the trims, name the fault

Graded fuel trim and scan data cases for engine performance students and instructors: read the freeze frame and the live data grid, name the fault and the cells that point to it, and see what you missed.

Start today's 5 casesRead the diagnostic guide

What a case asks you to see

A 2.0 l inline four, narrowband upstream sensor. Total trim is +16.2 percent at idle and +2.7 percent at cruise, and at idle the MAF reads only 85 percent of what the displacement, rpm and MAP say the engine is drawing. Positive at idle, near zero at cruise, and air the MAF never saw: that is unmetered air, not fuel delivery.

Example: a vacuum leakBank 1: Idle +16.2 percent, 2500 +4.9 percent, Cruise +2.7 percent. The shaded band is plus or minus 5 percent, the usual healthy range.-20-100+10+20Idle2500CruiseTotal trim, percent. Airflow rises left to right.
Bank 1Shaded: plus or minus 5 percent

Read the vacuum leak fingerprint, or open this case in projection mode (code C-UA-0).

How a drill works

  1. Read the complaint, the freeze frame and the live data across four cells: idle, 2,500 rpm with no load, cruise and a WOT snapshot.
  2. Pick the fault. Tick the cues you used, such as trims positive at idle only, or MAF short of the estimate.
  3. The grade names what you missed and why it matters, and links the fault page with its fingerprint table.

New to trims? Start with long term vs short term fuel trim and positive fuel trim at idle, then the diagnostic order the drills reward.

Every number comes from one engine model written out on the model page: air mass from the MAF, expected fuel, trim as the correction. So the trims, the airflow, the sensors and the coolant always agree, and two faults in one engine combine the way physics says they should.

Thirteen faults and a healthy control

Unmetered air

Air or vapour that reaches the cylinders without passing the MAF: a vacuum leak, or a purge valve stuck open.

Air metering

The MAF reading low or high, and an exhaust restriction that caps how much air the engine can breathe.

Fuel supply

Fuel pressure too low under load, too high everywhere, and an injector that drips when it should be shut.

Exhaust and sensors

Things that make the upstream sensor lie: an exhaust leak ahead of it, a biased sensor, a misfiring cylinder.

Coolant temperature

A coolant sensor stuck at a cold reading, and a thermostat stuck open that never lets the engine warm fully.

Plus the healthy control, because "nothing is wrong with the fuel control" is a diagnosis too.

For students and for class

Daily drillFive cases a day, the same for everyone.Practice by familyEight cases from one family, clean, noisy or two faults.Timed examTwenty cases, forty minutes, review at the end.Projection modeOne case on the big screen, reveal on click, printable sheet.

Four engine families

TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle. No make or model is depicted.