Fuel, Ignition, and Emission Control Systems
Domain focus
This domain tests fuel delivery, carburetor circuits, electronic fuel injection, ignition, and emission controls. You need to diagnose by circuit and sensor rather than replacing parts, and you must know when a symptom is rich, lean, or ignition-related.
Core rules
- Carburetor circuits overlap by throttle opening: pilot and fuel screw at idle and low opening, slide needle and needle jet at midrange, main jet at high load, and enricher or accelerator pump for cold starts and transitions.
- EFI uses sensors such as throttle position, manifold pressure, intake air temperature, coolant temperature, crankshaft position, and oxygen content. Fuel pressure and volume, injector pulse, and intake sealing are basic checks.
- Ignition systems include CDI, TCI, coils, plug caps, and spark plugs. Use the specified plug gap and resistance; a fouled or wrong heat-range plug can mimic fuel faults.
- Emissions controls may include crankcase ventilation, evaporative canister, secondary air injection, catalytic converter, and oxygen sensor. Do not disable them; tampering is illegal and can damage the catalyst.
- Diagnose rich or lean by throttle position, engine temperature, and load. A vacuum leak leans the mixture; a stuck enricher or high float level richens it.
- After EFI or carburetor work, reset the throttle position sensor or idle learned values where specified, then verify with a road test.
Worked example
A carbureted 600 cc four runs black plugs and 7–8% CO at idle, but CO falls near normal at 3,000 rpm. The rich condition is concentrated in the idle and pilot circuit, which supplies fuel below roughly one-quarter throttle. The technician verifies the enricher plunger is fully seated, checks float level, confirms the pilot jet is stock, and counts the fuel-screw turns before adjusting. A dirty air filter or high float level would richen the whole range; a sticking enricher or oversized pilot jet fits the idle-only pattern. After correction, the idle mixture is reset and the plugs are re-read after a road test.
Traps
Adjusting idle speed instead of mixture; tuning by ear without a tachometer; replacing an oxygen sensor for a rich code when the sensor is only reporting; assuming EFI has no adjustments; checking fuel pressure without checking volume; using leaded fuel with a catalyst; and ignoring intake leaks or a sticking enricher.