I have a loud vacuum noise on my starbaord motor that is extremely loud when the engine cover is open. Boat runs fine, but it is annoying. Motors are 500 EFI's (2001 Fastech) It sounds liek the air cleaner is loose, but it's not that. I did notice that the starboard motor runs at 155 degrees and the port motor runs about 185 degrees when warmed up. Is there an electronic module that controls the amount of vacuum that could be bad or adjusted?

dukenrock. 185 degrees is too warm. Have a Merc mechanic use a scan tool to diagnose the fault. If you're anywhere near Fort Lauderdale I'll be happy to use mine. As to the vacuum sounds, it's more than likely a faulty IAC (Idle air control) You'll find it on the intake manifold, engine side of the air cleaner. This valve opens on deceleration and at idle. When it opens, there is a hissing sounds that can be loud. It should go away at speed. If not, the scan tool will indicate if it's operating correctly. If not, it's about $80 and takes 10 minutes to replace. You do not need the HP blue ones, any late model Merc EFI big block IAC motor will be fine. It is round in shape, 2 screws hold it in place at about 7 and 2 on the clock. The actuator is a small plunger with a spring. They tend to go bad from either salt intrusion, corrosion or exhaust scarring of the plunger shaft. Hope this is helpful for you.

Steve David

Had exactly same problem although didn't temp variance--was IAC fault.

Got replacemnt at auto parts and don't think it was even $80 and was simple replacement job.

Poorsche

Thanks for the input guys!