Ford Mustang Compression Test
I'm planning to perform a compression test on a 95 Ford Mustang with the 5.0 liter engine and I'm using manual #36051 for Ford Mustangs 1994-2004.
In Chapter 1, page 1-22, section 30.4, it says you should wait until the engine is completely cool before removing the spark plugs because some models have aluminum cylinder heads that may become damaged if you remove the spark plugs while hot.
In Chapter 2 Part C, page 2C-8, section 3, it says the the engine should be at operating temperature when performing the compression test.
So, my concern is removing the spark plugs while the engine is hot and causing damage to the heads. What do you recommend as the proper course of action in this case?
The heads on your 5.0L are cast iron. The aluminum heads on the modular engines (4.6L OHC, etc.) are aluminum and are notorious for spark plug removal problems. You shouldn't have any trouble removing the plugs from a 5.0L when warm.
Compression tests are always more accurate on a warm engine, as the pistons and rings are seating against the cylinder walls as they should be. But all things being equal, a cold compression test will reveal problems too. The most important variable is compression variation between cylinders, or a dramatic difference between a dry and wet compression reading on any given cylinder.