Failure mode - Ford 6.0L Power Stroke

Degas bottle / coolant reservoir crack, or failed cap

Reliability tier: verified - verify before ordering, see /disclosure/

Symptoms

coolant seepage and staining around the reservoir at the top rear of the engine
slow, unexplained coolant loss; coolant smell
coolant overflow from the degas bottle
cap not holding pressure; false overheating or coolant-push symptoms that mimic head gasket / EGR cooler failure

Root cause

The plastic degas bottle and its pressure cap age and crack from heat cycling and pressure. A failing cap alone can cause symptoms that look like far more serious problems, so it should be ruled out early. Note that degas-bottle overflow is often a SYMPTOM of an EGR-cooler or head-gasket failure pressurising the system, not a fault of the bottle itself.

Diagnosis

pressure-test the cap and the bottle
inspect for hairline cracks and coolant staining
rule this out before diagnosing head gasket or EGR cooler failure

Fixes

replace the degas bottle and cap (inexpensive); do it during any other cooling-system work ()

degas-bottle (part category - specific model not chosen)
degas-bottle-cap (part category - specific model not chosen)

Prevalence

common