I just ran a compression check 200 EFI on my 1998. #OG607556. The motor was cold, the throttle was not at WOT and all the plugs were removed. After reading this thread, I see that this was not the correct procedure. However, the readings were 130 to 134 on all six cylinders. My question is, how would the numbers change if I would have done it the correct way?
Thanks, Tommy
likely none with numbers that good , that engine been modified ???
.................................................. ...the scariest thing in life is the unknown ...................................
Honestly can't answer the question accurately, as the Service Manuals clearly indicate the CORRECT method for Compression Testing.
You would have to test your engine, back-to-back, switching to a second fully charged, properly rated battery for the second compression test (in order to accurately answer the question).
While your engine might have very little difference, I've seen several that were more than 75 psi increase just by opening the throttle plate. So yes, it can make a significant difference in some cases.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 48 years (learn something new every day).
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No, stock motor. I took it to a Mercury mechanic last spring for injector service and when he started the motor, water was spraying out of the head gaskets. He pulled the heads (first time it's been apart) he told me that the cylinders showed hardly any wear. He was surprised how heathy the motor looked. At that time, his compression numbers were 128 to 130.
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