Fuel Pressure¶
The pressure across the injector sets how fast fuel flows. It rises and falls with the square root of pressure — and if it drops too low, the mixture goes lean and closed-loop bows out.
What it is¶
An injector's flow depends on the pressure difference pushing fuel through it. From Bernoulli's relation, flow scales with the square root of pressure:
So doubling pressure doesn't double flow — it multiplies it by about 1.41. The ECU assumes a reference pressure when it computes injector pulse widths; if actual pressure differs, real flow differs.
Why it matters¶
- Low rail pressure means each pulse delivers less fuel than the ECU expects, so the engine runs lean. Dead time also grows as pressure falls, compounding the error at idle.
- If pressure drops below what the system needs, closed-loop trim disables — it won't try to correct a mixture error caused by failing hardware.
- Diesel is the extreme case: common-rail injection needs very high pressure (see Diesel Fuelling) just to atomize and ignite the fuel at all.
How to tune it¶
Keep rail pressure at or above the system's reference. If you raise pressure deliberately (for more injector headroom), remember flow only grows with its square root — a big pressure bump buys a modest flow gain.
Watch out
A sudden lean condition under high demand, with injectors not maxed, often means the fuel system can't hold pressure. Check the fuel pressure sensor readout before re-tuning fuel tables.
In the game¶
Rail pressure is reported by the fuel pressure sensor. Watch it hold steady under load; a drop there explains a lean readout that the tables don't.