1979 mercruiser 260 steam coming from exhaust

[kyle50w]

Hello all,

This is my first post. I have a Mercruiser 351 260hp on a 24' Airslot. When I run it with the earmuffs hooked up while on the trailer it initially does not have any white steam/smoke? coming from exhaust. About five minutes in white steam/smoke starts to emit from the exhaust. The risers get hot, but I can still place my hand on them for about five seconds before I have to remove it. During this time my temp. gauge still reads cool with the needle still in the yellow before the green. I notice that the area where the water exits the lower unit is strong on the port side, but only spits out on the starboard side, but at times will become strong. While under load in the water the temp gauge raises to just before redline after about thirty minutes of 3/4 throttle but does not go into redline. Just trying to figure out if the steam/smoke is normal or not and if running just below the redline is also normal. I should also add that while under load and the gauge just below redline if I throttle down the gauge also goes down almost immediately. Thanks for any replies.

[Bond-o]

I have a Mercruiser 351 260hp

Ayuh,... Welcome Aboard,....
You're wrong there somewhere,.. a 350 Chevy is 260hp, a Ford 351 is 235hp or something like that...
It sounds kinda normal, but I have a Question for ya,...
When was the Impeller last changed,..??
If you don't know,.... It needs it....

[kyle50w]

Bond-o,

Thanks for the reply. You're right it's a 350 hit the one by mistake. The impeller was changed last year and have used the boat approximately five times since then. Could the temp gauge be off?

[Don S]

A faulty gauge does not cause steam.
Might try checking with one of the IR temp guns. Check the manifolds, risers, and the engine. Check right at the base of the temp sender on the engine to verify what the gauge is reading.
One other thing, hot exhaust and cold water will make some steam.

[kyle50w]

Thanks for the help and suggestions. The IR temp gun sounds like a good place to start. Does anybody know the median running temperature of this size Mercruiser?

[Don S]

You need to take it out and run it under load and take the temps. Should never be over 180°. Take the temps on the risers and the engine thermostat housing and temp sender. The temps will vary, depending on where you take them, and the oil pan will be around 230°, and be fine. So it depends on where you take the temps, get some readings and tell us where you took the readings.

[havasuboatman]

If you have steam, can't keep your hand on the riser at idle, and have poor water flow from the exhaust on one side, my friend, your impeller is on it's way out. Your water volume is low.

[jtybt]

This thread begs to ask FWC or raw water cooled and when was the last time the manifolds and risers were changed or even checked.

Sounds like water blockage from rust build-up in the riser.

[kyle50w]

Took the boat out today and got it to redline after about twenty minutes of 3/4 throttle. Took readings on the risers, thermostate housing and temp sender via IR temp gun. All readings were between 177F and 180F but the risers were extremely hot. Once again when I go to neutral, the engine begins to cool almost immediately. Not sure if this is normal or if I have a blockage somewhere.

[deves3000gt]

My boat 350 through hull exhaust will steam a bit and water flow is about the same one side being more then the other. I was told on here that it is the way the boat is sitting on the trailer and that most of the dule exhaust will tend to prefer on outlet over the other (at idel) so i would saya the steam is ok but, if the risers are that hot i would have to agree with the above of low water flow. IMO Good luck