A turbocharger's center housing is a thermal disaster zone. The turbine wheel spins in exhaust gas that can hit 1,800°F (980°C) on a hard-driven gasoline engine, and that heat conducts straight down the shaft into the bearing housing. The only things keeping the bearings alive are a constant flow of pressurized engine oil and (on modern units) a coolant jacket wrapped around the center section.

Shut the engine off after a hard pull and you create a catastrophic situation called heat soak. Oil flow stops instantly. Coolant flow stops instantly. But the turbine housing is still glowing hot, and that heat now radiates inward with nowhere to go. The oil trapped inside the bearing housing — sitting on the shaft, in the journal bearings, in the oil drain passages — gets cooked.

This is coking: oil thermally decomposing into hard carbon deposits. Once oil exceeds roughly 400°F (205°C), the base stock breaks down. Synthetic oils tolerate higher temps (often 450°F+) before coking, which is why turbo manufacturers universally specify full synthetic. Conventional oil in a hot turbo will leave varnish in days, hard carbon in weeks.

The damage cascade: coked oil narrows the oil feed line, restricting flow on the next cold start. Reduced flow means hotter bearings, which cokes more oil, which restricts flow further. Eventually the shaft seizes, or the carbon abrades the journal bearings until shaft play lets the compressor wheel contact the housing.

Real example: Subaru's EJ257 (WRX STI) became infamous for "turbo banjo bolt" failures. The oil feed line used a banjo fitting with an internal screen designed to catch debris. After thousands of hot shutdowns, coked oil plugged the screen, starved the IHI VF-series turbo, and grenaded bearings around 60-80k miles. Subaru eventually issued service bulletins recommending screen removal and synthetic oil only.

Cooldown rule of thumb: after sustained boost (highway pulls, towing, track use), idle 60-90 seconds before shutdown. Why this works: at idle, oil flow continues at roughly 10-15 psi while exhaust gas temperature drops fast — typically from 1,500°F under load to 600-800°F within 30 seconds. That single minute drops peak soak temperature by hundreds of degrees.

Water-cooled center housings (standard since the late 1990s) help enormously. After shutdown, residual coolant in the jacket thermosiphons — heated coolant rises out of the housing, cooler coolant flows in from the radiator, all without the water pump running. Some performance cars (Porsche 911 Turbo, Nissan GT-R) add an electric auxiliary coolant pump that runs for several minutes post-shutdown specifically to flush turbo heat.