HIGH ENGINE TEMP/OVERHEAT CHECK FOR BLOCKED PASSAGE

VEHICLES AFFECTED: 1980-1984 Series 10 Thru 60 Equipped With Mark IV 366, 427, 454 Engines

When diagnosing for a possible high engine temperature or overheat condition on Mark IV engines, refer to the Shop Manual Engine Cooling Diagnosis Section. If the condition cannot be resolved by normal diagnosis procedures, check the cylinder head coolant passages for possible blocked cavities.

To make this inspection, it is not necessary to remove the cylinder head from the engine. Only the removal of the intake manifold is required.

Casting flash on a Mark IV head may leave a barrier or envelope through the middle of the head at the core parting line.

With the intake manifold removed, inspect the coolant passages at both ends of the cylinder head. If the passages are not clear and excessive casting flash or walls are observed, replace the cylinder head. This condition may be found on replacement cylinder heads so be sure to inspect the new cylinder head prior to assembly.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.