Lean Idle / Rough Idle on 7MGTE - Vacuum Leak at Intake Manifold Bracket Bolt + ECU Coolant Temp Sensor

Engineintermediate✓ OP Confirmed

Problem

1988 7MGTE turbo running lean at idle (17-18 AFR on wideband) with rough/stuttering idle that worsened after engine was warm. Issue began after a shop replaced engine gaskets (valve cover, camshaft, crankshaft seals, and oil pump). AFR was fine under boost and while driving, only lean at idle.

Air/fuel ratio reading 17-18 at idle (lean)Rough/stuttering idleRPM slowly drops after startup then begins stutteringWorse on hot restart (within 1-2 hours of shutdown), hovering around 500 RPM with bouncingFuel smell at rear of carIdle smooths out with A/C onRev bouncing/inability to hold steady RPM in 1750-2500 range

Root Cause

Multiple causes: (1) A broken/backed-out bolt holding the vacuum bracket on the side of the intake manifold was allowing unmetered air into the intake, causing a significant vacuum leak that only affected idle (not driving/boost). (2) A faulty ECU coolant temperature sensor was causing incorrect fueling at idle. (3) The TPS, even when set to factory TSRM specs, was not responsive enough and caused rev bouncing in the 1750-2500 RPM range.

Confidence

Moderate confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted August 2019 on SupraForums.com