Why injectors stop behaving predictably at very short pulse widths — even when the math, data, and tune are technically correct.
Conceptual definition
Low-pulse nonlinearity describes the region where injector fuel delivery no longer scales proportionally with commanded pulse width.
At moderate and long pulse widths, injectors behave linearly — more on-time produces more fuel.
At very short pulse widths, that relationship breaks down. The injector may deliver less fuel than expected, inconsistently, or not at all.
This is a physical and electrical limitation of injector operation, not a tuning error.
Why it occurs
Injectors are not ideal on-off valves. At very short commands, several effects stack together.
Opening delay. Electrical latency consumes a larger percentage of the total pulse.
Partial pintle lift. The injector may never reach full opening before being commanded closed.
Fuel pressure effects. Higher differential pressure increases the force required to open the injector.
These effects are negligible at load — but dominate at idle and light throttle.
Why it matters
Low-pulse nonlinearity explains why engines can run clean at WOT but fall apart at idle.
The ECU may be commanding fuel correctly, yet the injector cannot deliver it consistently at short pulse widths.
This creates unstable AFR, idle hunting, and voltage-sensitive behavior even with correct injector data.
Once the injector operates above the nonlinear region, fueling stabilizes.
What it is not
Low-pulse nonlinearity is not incorrect injector flow rate.
It is not fixed by changing AFR targets, fuel trims, or VE tables.
It is not solved by raising base fuel pressure.
When this limit is exceeded, no amount of tuning can restore precise control at idle.
Failure modes
Unstable idle. AFR oscillates without a clear pattern.
Idle that improves with RPM. Injector operation exits the nonlinear region.
Oversized injector symptoms. Larger injectors worsen control despite low duty cycle.
Artificial fixes. Idle only stabilizes with excessive enrichment or timing tricks.
How SpeedNeeds uses it
SpeedNeeds treats low-pulse nonlinearity as a control limit, not a tuning failure.
Injector sizing guidance prioritizes idle and low-load resolution, not just peak horsepower capacity.
Systems that operate near known nonlinear regions are flagged as high risk for drivability issues.
Fuel calculations assume injectors can operate linearly in the intended operating range.
Caution and edge cases
Race engines. Systems that never idle meaningfully may ignore this limitation.
Advanced OEM injectors. Some designs extend linear behavior deeper into low pulse widths.
Ethanol fuels. Higher pressure requirements can worsen low-pulse behavior.
Closing clarity
Low-pulse nonlinearity explains why correct math can still fail at idle.
It is not a flaw in tuning — it is a limit in injector control.
This explainer exists so idle problems stop being chased — and start being designed around.