Skip to content

FAQ

Common symptoms and where to look. Most problems trace back to one or two pages.

Why won't it start when cold?

A cold engine needs extra fuel and cranking time. Check, in order:

  • Cranking enrichment — the cold end of the curve may be too low (won't catch) or too high (floods). If it fires then drowns, reduce it.
  • The start sequence — ignition (E), hold crank (R), blip throttle (W). See Starting an Engine.
  • Coolant temperature — a wrong reading throws off all cold-start enrichment.

Why is it hunting at idle?

RPM swinging up and down at idle usually comes from one of:

  • An erratic idle VE cell — common with an aggressive cam, whose overlap makes idle airflow choppy. Stabilize fuelling first.
  • The idle control response being too aggressive.
  • A light crank/flywheel — less inertia means a busier idle the controller must manage.

Why is there no power up top?

If the engine flattens out near redline:

  • Injectors maxed — at 100% duty the mixture leans out, acting like a soft rev limit. Check duty cycle first.
  • Hitting the rev limiter — you may simply be at the weakest component's RPM ceiling.
  • Breathing — the head, cam, intake, and exhaust may not flow at high RPM.
  • Timing pulled by knock — if knock control is retarding, you'll lose top-end power.

Why does it run rich when warm?

Often an enrichment curve that doesn't fully taper out:

Why did it suddenly go lean under load?

If injectors aren't maxed, suspect fuel pressure sagging — check the fuel pressure sensor. Low pressure also disables closed-loop.

Why is it knocking?

Knock rises with load, heat, and advance, and falls with octane and richer mixtures. On a hot day, low octane, or more boost, pull timing or richen the mixture.