Surging Idle After Head Gasket Job - Intake Manifold Air Leak from Stripped Bolt and Leftover Gasket Material

Engineintermediate✓ OP Confirmed

Problem

After a head gasket replacement, the engine developed a surging/hunting idle that worsened as the engine warmed up or when settling back down from throttle input. The idle would fluctuate and the engine would sometimes nearly stall before recovering.

Idle surges/hunts between 700-900 RPM when warmIdle drops low like it's going to die then recovers after giving throttleHigh cold start idle around 1800 RPMEngine nearly stalls when releasing the gas then surges back upUnplugging AFM connector causes idle to stabilize at 1100 RPM with no surgeHigh warm idle around 1100-1200 RPM

Root Cause

A stripped intake manifold plenum bolt hole (previous helicoil repair had failed/crumbled) was allowing unmetered air ('pirate air') to be sucked into the intake manifold past the gasket. Additionally, the mechanic who performed the head gasket job had not properly scraped off old gasket material from the plenum and EGR inlet mating surfaces, further compromising the seal. The ECU could not compensate for this unmetered air because it was not passing through the AFM, causing erratic fuel trim and idle hunting.

Confidence

Moderate confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted January 2026 on SupraForums.com