Fuel trim practice: questions and answers
Frequently asked questions
What is TrimCase?
A practice bank of fuel trim and scan data cases for engine performance students, instructors and technicians moving into driveability work. Each case shows a freeze frame, a live data grid across four load cells and a trim chart; you name the fault and the cues that point to it, and the grade says what you missed.
Where do the numbers come from?
From one engine model written out in full on the model page: speed density air, MAF metering, expected fuel, and trim as the correction closed loop finds. A fault changes one physical quantity, such as a leak orifice or a rail pressure, and every reading follows from it. No data is copied from a real vehicle.
Is this a diagnostic procedure for my car?
No. It is a training aid built on a simulated engine. A real vehicle can behave differently, and the service information for that vehicle, with the physical tests it calls for, is what a real diagnosis rests on.
Has the model been reviewed by a master technician?
Not yet. The model and its assumptions are published on the model page so that a reviewer can check every step, and an independent review by an experienced driveability technician is planned. Corrections are welcome through the support address in the footer.
What is free?
The daily drill of 5 cases, practice on the unmetered air family, the model page, every fault page, the diagnostic guide and exporting your progress. None of it needs an account.
Can I use it in class?
Yes. Projection mode puts one case on the screen with the answer hidden until you reveal it, and prints a case sheet with the answer on its own page. Every case has a code, so students can open the same case on their own devices. The timed exam runs 20 cases.
Why are there no make or model names?
The engine families are generic on purpose: an inline four, a V6 and a V8, with narrowband and wideband sensors. The point is the reasoning, which carries across makes.
Where is my progress stored?
In your browser only. Nothing is uploaded. Export it as a JSON file from the progress page to keep a copy or move it.
Is it affiliated with a certification body?
No. TrimCase is not affiliated with ASE, the ASE Education Foundation, or any certification or accrediting body. A good score here is a self assessment, not a credential.