I have a 2000 502MPI (233 hours) that cranks for 5-10 seconds or more before it starts. Does it hot or cold. Fuel pump comes on and cycles when key is turned. When it fires, runs great, doesn't miss, no soot on transom, idles fine, no rich smell. When it fires I hear the hissing sound from arrestor. Any ideas before I drop money on a scan tool?

air / vacuum leak -

loose throttle body

IAC - scan engine and see what the IAC parameters are reporting.

Dirty flame arrestor and or internal dirty throttle body and throttle plate. Dirty in the IAC passage

has the throttle body gasket been replaced and if so ----- years ago Merc had a issue with it. the gasket.

How long have you owned this boat and what recent work has been done ?

Owned it 10 years, it is box stock. Hasn't been touched except for normal maintenance items. Just from reading I was leaning toward IAC...looks like time to buy a scanner.

check the vacuum line going to the fuel pressure regulator for leaking fuel inside.. might be loosing the charge on the fuel rail and its takes a few seconds to prime it back up.. pretty common.

thanks for the update and that would have been the vacuum leak and good post by downforce137