Technical Bulletins

Engine Hydraulic Lock

The AERA Technical Committee offers the following information to consider while diagnosing engine drive ability complaints or engine failure. In some instances the cause of either one of the previously mentioned conditions may be the result of one or more cylinders that have hydraulically locked during engine start-up. In severe situations the engine may suffer catastrophic engine damage.

One possible cause of the above conditions may be fuel or coolant entering the combustion chamber while an intake valve is open. The cause of unwarranted fuel may be resulting from an injector sticking in the open position. The injector may be an individual cylinder or a throttle body type. The cause of coolant entering the intake plenum may be a passage created by a crack or erosion in the water-cooled portion of the manifold. Coolant passages are sometimes located in both lower and upper manifolds and may include the throttle body and egr areas.

Fluid does not compress and depending upon the amount of fluid on top of the piston during cranking, something has to give. It’s generally the weakest component in the given circumstances. In some instances, a connecting rod may bend slightly without causing engine failure. The only symptom noticed while the engine is running may be a lack of power or loss of coolant. The shorter connecting rod reduced the compression within that cylinder preventing the full power from that cylinder.

To test for this condition on engines that run, do a cranking compression test and be suspect of any cylinder that has a value of 25% or less than the highest cylinder.

The AERA Technical Committee