Mercury 300 Pro Max Issues

[thooker]

Hey Guys,
I have a 1999 Mercury 300 Pro Max on my 21 Cyclone Ski Race Boat.
I put a water temp gauge in my dash and for many years it never moved off of the lowest temperature. Recently the temperature started creeping up to around 160° when at idle even though the water pressure gauge is reading 3-5 psi and a steady stream of water is coming out of the side cowling port. It has been about 1.5 years since my last impeller change so I will definitely do that and maybe look at changing out the thermostats at the top of the heads as those have not been changed to my knowledge. Any other ideas for this issue?

The second thing I noticed is it sometimes sneezes while idling in gear. The engine every once in a while slightly sneezes but keeps right on running/idling. Could this be a lean or rich issue or an oil injection issue. I checked the fuel PSI and it is within spec and the direct injection seems to be working as I need to fill the oil tank every so often. Should I check my reeds as those have never been changed to my knowledge either. Service Injectors?

The engine runs great at WOT and has good compression in each cylinder. (Roughly 120 psi in each cylinder).

I eventually want to get a new Mercury 300 but am a little short on the funds. With that I need to keep this one going for a few more years.

Thanks and I appreciate any and all help.

[Merc 2.5]

When's last time injectors flowed, reeds checked , a misfire make her sneeze to I believe , but dont quote me on that. I'm no efi guy , just starting point, if comp good ,I'd put timin lite on ea plug wire at idle and make sure u ain't got a misfire 1st , does it sneeze on muffs to or just in water ?
Also a cross fire/double fire would prob raise temps also. But not 100% on that , but water pump great starting point

[thooker]

I agree and am probably due to flow injectors and check reeds.
I will definitely change the impeller, thermostats in top of heads as well as new plugs to be 100% sure on that part of it.
If that doesn't work I will toss a timing light on it and check that.

Thanks for the tips.