Rainbow Exhaust 5.7 Mercruiser

[Boatbuyingloser]

Hey guys, I put a new prop on as well as had my carb rebuilt and when I first launched it, a rainbow appeared coming out of the exhaust. What can cause this and does it have anything to do with the prop or just a bad tune? It stopped after about 2 minutes of running but its a little worrisome and I don't want to be "that guy" leaking oil all over the lake.
P.S. after the rainbow stopped, small globs of oil would still float up behind the boat at idle for about 30 mins. You can see these on either side of the "rainbow"

1992 Maxum 2300 SCR

 

[Scott Danforth]

these problems dont care what boat you have. however can best be solved remotely on a forum if you let us know what you have for a motor and drive

Assuming you have a 5.7 and an alpha drive. You have a possibility of the following things:

• you are running way rich and that is unburnt fuel
• you have a dead cylinder and that is unburnt fuel
• if you have a fuel injected motor, you could have a bad fuel module and that is leaking fuel into your cooling water.
• you have a bad power steering cooler and that is power steering fluid
• you have a bad gear seal and that is gear oil
• if you have an oil cooler, that could be engine oil leaking from the oil cooler
• you could have warped heads or a bad head gasket from an overheat and that is engine oil
• your tilt/trim rams could be leaking oil

so you are "that guy."

pull the boat, start inspecting

look at all your fluids for presence of water, pressure test the PS cooler, pressure test the drive, pressure test the cooling water jacket

 

[JASinIL2006]

"that guy" would just go boating and ignore the sheen on the water until the problem kept him from being on the water.

Good for you, @Boatbuyingloser for taking care of this and not just ignoring it!

 

[Scott06]

that looks more like gear oil than gas to me. I would check your drive for water intrusion and look at remote oil reservoir lines up in transom housing

Are you loosing gear oil volume - doesnt take much.

Had a visually identical seen on my jetski a few years back was thinking it was too much fuel - was gear oil from jet pump plug being cross threaded

 

[QBhoy]

Hi. My thoughts on possibilities

If the boat is new to you, it may have been fogged with 2 stroke or oil.
It could be the choke sticking and it’s fuel
You may be burning oil
You have too much oil in it.
It could be the power trim
It could be gear oil from fittings on the lower unit or from a header tank pipe fitting that’s not connected or damaged. Easy to get them damaged during installation or removal of the drive occasionally