1JZ VVTI Running Rich Under Boost - Factory ECU Replaced with Standalone

Fuel Systemadvanced✓ OP Confirmed

Problem

1JZ VVTI ETCS-i swap (from JZX110/JZS171) into MK3 Supra running extremely rich under boost/WOT while idling and cruising normally at 14.7 AFR. Car uses stock auto ECU in a manual swap with R154 transmission.

AFR drops below 10:1 (wideband floor) under boost or WOTEngine breaks up under hard accelerationCarbon-fouled spark plugsNormal idle (~14.7 AFR) and cruise AFRsRich condition occurs at any RPM under heavy throttle, not RPM-dependentLight throttle allows revving to 6k at 10.5-11 AFR but WOT immediately drops below 10:1

Root Cause

Factory automatic ECU (stock JZX110/JZS171 auto ECU used in manual R154 swap) was not fueling correctly under boost despite appearing visually fine internally. The exact failure mode was never isolated, but possibilities include internally degraded capacitors that appeared fine on visual inspection, or ECU incompatibility/malfunction with the swapped configuration. Multiple community members with identical setups (1JZ VVTI from JZS171) reported the same unresolvable rich condition on the factory ECU.

Confidence

Moderate confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted October 2023 on SupraForums.com