Lean Idle with Bosch EV14 550cc Injectors on Stock 7MGTE ECU - Injector Latency Mismatch

Fuel Systemintermediate✓ OP Confirmed

Problem

After installing Bosch EV14 550cc injectors with a Lexus AFM on a stock 7MGTE ECU, the engine idles extremely lean (17-21 AFR) when warmed up, bogs and nearly dies when given throttle, and the injectors produce abnormally loud ticking audible inside the cabin. Cold start idle is normal (14.7 AFR) but AFRs climb lean as the engine warms and idle RPM drops below ~700.

Extremely lean AFR at warm idle (17-21:1), climbing from near-stoich into the 20sEngine bogs and nearly stalls when given throttle from idleAbnormally loud injector ticking audible inside cabin with doors closed and windows upInjector ticking gets louder as engine warms upCold start idle AFR is normal (14.7:1) but degrades as engine reaches operating temperatureFuel visible spitting from exhaust despite lean AFR readingsRough idle when warmed up

Root Cause

Modern Bosch EV14 injectors have significantly different electrical latency characteristics compared to the OEM Denso injectors the stock 7MGTE TCCS ECU was designed to control. At 12V, the EV14 latency is 1175 microseconds versus 800 microseconds for the OEM 440cc injectors. This latency mismatch means the ECU's injector pulse width is only correct at a single operating point, causing wildly varying fuel trim across the RPM/load range. The stock ECU's closed-loop fuel trim can only correct approximately ±20%, which is insufficient to compensate for the latency difference, especially at low RPM idle where pulse widths are shortest and the latency error is proportionally largest.

Confidence

Moderate confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted September 2023 on SupraForums.com