# TrimCase

> 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.

TrimCase is a browser based practice bank for reading automotive fuel trim and scan data. Every case comes from one published engine model, so trims, airflow, sensors and coolant always agree. No account, nothing uploaded. Last reviewed 2026-09-26.

## Practice

- [Home](https://trimcase-web.pages.dev/): What TrimCase is, an example case and the fault families.
- [Daily drill](https://trimcase-web.pages.dev/drill/): Five fuel trim cases a day, the same for everyone, graded on the cues as well as the answer. Free.
- [Practice by fault family](https://trimcase-web.pages.dev/practice/): Eight graded cases from one fault family, clean, noisy or two faults.
- [Timed exam](https://trimcase-web.pages.dev/exam/): Twenty cases in forty minutes across every family and tier, reviewed at the end.
- [Projection mode](https://trimcase-web.pages.dev/projection/): One case on a classroom screen, reveal on click, printable case sheet.
- [Progress and export](https://trimcase-web.pages.dev/progress/): The case record kept in this browser, with JSON export and import.

## Guides

- [Fuel trim diagnosis, step by step](https://trimcase-web.pages.dev/guide/): The diagnostic order of operations the drills reward, and the guide index.
- [Positive fuel trim at idle: the causes, and how to tell them apart](https://trimcase-web.pages.dev/guide/positive-fuel-trim-at-idle/): Fuel trim high at idle but near zero at cruise usually means a vacuum leak or an exhaust leak. How to tell them apart with the MAF check and the banks.
- [Negative fuel trim: the causes, and how to check each one](https://trimcase-web.pages.dev/guide/negative-fuel-trim-causes/): Negative fuel trim means the computer is pulling fuel. The usual causes, from a leaking injector to high fuel pressure, and the checks that tell them apart.
- [Long term vs short term fuel trim, explained](https://trimcase-web.pages.dev/guide/long-term-vs-short-term-fuel-trim/): Short term fuel trim is the instant correction; long term fuel trim is the learned one. How each behaves, why you add them, and how to read them per bank.
- [MAF grams per second per litre: the rule of thumb, and a better check](https://trimcase-web.pages.dev/guide/maf-grams-per-second-per-litre/): A healthy engine at idle reads roughly 1 g/s of MAF per litre. Why that is only a first look, and how to check airflow properly with rpm, MAP and IAT.
- [Bank 1 vs bank 2 fuel trim: what a split between the banks means](https://trimcase-web.pages.dev/guide/fuel-trim-bank-1-vs-bank-2/): When one bank's fuel trim moves and the other stays near zero, the cause belongs to that bank. What bank 1 means, and how to read a split on a V engine.
- [How to practise fuel trim diagnosis without a car](https://trimcase-web.pages.dev/guide/fuel-trim-diagnosis-practice/): Practise fuel trim diagnosis from scan data cases: read every load cell, name the fault and the cues, and get graded on the reasoning. Free, in your browser.

## Reference

- [How fuel trim works: the engine model](https://trimcase-web.pages.dev/model/): Every equation the cases are generated from, and what each fault changes.
- [Fault fingerprints](https://trimcase-web.pages.dev/faults/): Every fault and the healthy control, with the cues that tell each apart.
- [No fault (healthy control)](https://trimcase-web.pages.dev/faults/no-fault/): What healthy fuel trim data looks like: trims near zero in every cell, the MAF agreeing with the engine, warm coolant, a steady downstream sensor.
- [Vacuum leak](https://trimcase-web.pages.dev/faults/vacuum-leak/): A vacuum leak adds fuel trim at idle that fades by cruise, and the MAF reads short of what the engine draws at idle. The fingerprint, numbers and checks.
- [EVAP purge valve stuck open](https://trimcase-web.pages.dev/faults/purge-stuck-open/): A purge valve stuck open pulls fuel trim negative at idle with the MAF short of the estimate. How to spot it on scan data and confirm it on the car.
- [MAF under reporting](https://trimcase-web.pages.dev/faults/maf-under/): A MAF reading low makes fuel trim rise with airflow and falls further short of the engine's real airflow as flow rises, most of all at WOT.
- [MAF over reporting](https://trimcase-web.pages.dev/faults/maf-over/): A MAF reading high pulls fuel trim negative by the same amount in every cell, with the MAF above the speed density estimate everywhere.
- [Restricted exhaust](https://trimcase-web.pages.dev/faults/restricted-exhaust/): A restricted exhaust leaves fuel trim normal but caps airflow at wide open throttle. How to tell it from a MAF reading low on scan data.
- [Restricted fuel filter or weak pump](https://trimcase-web.pages.dev/faults/fuel-delivery/): A weak fuel pump or clogged filter shows fuel trim rising with demand and a lean upstream sensor at WOT while the MAF agrees with the engine.
- [Fuel pressure too high (regulator stuck closed)](https://trimcase-web.pages.dev/faults/high-fuel-pressure/): Fuel pressure too high pulls fuel trim negative in every cell while the MAF and coolant read normal. The fingerprint and how to confirm it.
- [Leaking injector](https://trimcase-web.pages.dev/faults/leaking-injector/): A leaking injector pulls fuel trim negative at idle, near zero at cruise, on one bank of a V engine, with the MAF agreeing with the engine.
- [Exhaust leak upstream of the upstream sensor](https://trimcase-web.pages.dev/faults/exhaust-leak/): An exhaust leak ahead of the upstream sensor adds fuel trim at idle on one bank while the MAF stays honest. How to tell it from a vacuum leak.
- [Upstream O2 or A/F sensor biased lean](https://trimcase-web.pages.dev/faults/biased-sensor/): An upstream sensor biased lean adds the same fuel trim in every cell on its bank, and the downstream sensor reads rich. The fingerprint and checks.
- [Single cylinder misfire](https://trimcase-web.pages.dev/faults/misfire/): A single cylinder misfire makes fuel trim go positive because unburned oxygen fools the upstream sensor. Misfire counts and a rich downstream name it.
- [ECT sensor stuck cold](https://trimcase-web.pages.dev/faults/ect-stuck-cold/): A coolant sensor stuck cold adds warm up fuel the trims take back out: flat negative fuel trim and one frozen low coolant reading on a warm engine.
- [Thermostat stuck open](https://trimcase-web.pages.dev/faults/thermostat-open/): A thermostat stuck open keeps the coolant below its regulating temperature and falling at road speed, with small negative trims from warm up fuel.

## Help and policies

- [FAQ](https://trimcase-web.pages.dev/faq/): Where the numbers come from, what is free, classroom use, and what the site is not.
- [Privacy](https://trimcase-web.pages.dev/privacy/): Nothing you answer is uploaded; progress stays in your browser.
- [Terms of use](https://trimcase-web.pages.dev/terms/): The limits of what a simulated case can be relied on for.

## Pricing

Everything on the site is free today, and paid plans are not turned on. Free forever, whatever happens later:

- The daily drill of 5 cases, the same for everyone and new every day
- Practice on the unmetered air family, clean and noisy tiers
- The model page, which writes out every equation the cases are generated from
- Every fault page, with its fingerprint table and example chart
- The diagnostic guide and the FAQ
- Export and import of your progress file as JSON

## What it is not

- TrimCase is not affiliated with ASE, the ASE Education Foundation, or any certification or accrediting body.
- TrimCase is a training aid built on a simulated engine model. It is not a diagnostic instruction for any real vehicle.
- No make, model, engine code or calibration is depicted, and no data is copied from a real vehicle.
- The model has not yet been reviewed by an independent master technician; its assumptions are published on the model page so anyone can check them.
- A score is a self assessment, not a credential.

## Optional

- [llms-full.txt](https://trimcase-web.pages.dev/llms-full.txt): the full text of the guides, the diagnostic order, the engine model, every fault fingerprint and the FAQ in one Markdown file.
- Support: socialreminderinfo@gmail.com