Mercruiser 7.4L Top-end Rebuild


This is a chronology of pulling and doing a valve job and general refurbishment of a 1991 Mercruiser 7.4 Liter (454 CI) boat engine. 

It all started when the operating temperature of the engine rose from the normal 155 degrees to about 180 degrees.  Diagnostic efforts ruled out basic water flow issues, and so a cylinder leak test was performed.  The leak test revealed excessive leakage from the middle cylinders on each side of the engine.  Much of the leakage appeared to be at the top-end of the engine.  Therefore, a re-build of the top-end (i.e. cylinder heads) was in order.  While the engine is apart the condition of the cylinders/pistons will also be examined to determine if a full re-build is needed.

Performing the leak test. Here is one of the cylinders that was showing about 50% leakage.  In total there were 3 good cylinders ( 10% leak), 1 fair (10-20% leak) and 4 poor (30-50% leak).

Removing fuel and ignition systems from the manifold area

Ready to take the intake manifold off

Intake manifold removed. Some rust along the exposed surface of the cylinder head, but not too bad looking.

At this point, I decided to pull the engine from the boat in order to do the engine work.  Access to the exhaust manifolds is extremely difficult with the engine in the boat, and I could easily check/replace other components as needed.  See the process of pulling the engine from the boat.  Click here to go to Pulling the Engine Page.