Riser Cooling Problem

[BethnKevin]

Yesterday we took the boat out for a trial run and as we were getting up to speed, we noticed smoke bellowing out of the transom area. After backing off the throttle we noticed it was coming from the port side riser area. The starbird side was cool enough to touch and engine temp was normal and below 150 degrees. What might I do to try and figure this out? I did take the water hose off of the riser and did notice very little water in the hose. Again I am learning as I go and all information is greatly appreciated. Thanks to all and have a great day.

[VetteLT193]

engine? risers? run in salt?

if it is factory merc and older the riser is probably backed up with rust. My best guess based on the info so far. Pull the riser off and look at the inside. This is the #1 reason for engine failure so I'd do this ASAP

[zelatore]

Vette's probably right based on the little we have to go on. Luckily risers are easy to pull off, assuming of course the bolts aren't seized.

[BethnKevin]

I'm sorry.. I forgot to tell you it is a 1989 Donzi Ragazza 25 with 5.7L 350 OMC Cobra motor. If it is not the riser could it be the thermostat housing or exhaust manifold? Yes we go in saltwater.

[DonziJon]

I would pull the theromostat housing first and see what you can see in there. Regardless what you find, you will need TWO new thermostat gaskets..upper and lower. They are slightly different. Running temp of under 150* seems a tad low..even for salt water use. Maybe the thermostat has been removed altogether or is frozen open. Thermostat is the first place to look. After that has checked out as being functional, then pull the Hot riser and have a look for rust scale.

My '86 Minx has a 350 and I had to replace the risers about 10 years ago. One was bad, I replaced both. The exhaust manifolds are still original. John

[zelatore]

1989......salt water.....and I'm betting it's raw water cooled.

If it hasn't been done before, it's a good bet you'll need to replace at least the risers if not the manifolds.

Not sure on the cooling routing of the OMC, but it certainly wouldn't hurt to check out the thermostat housing. Simple enough and cost much. For that matter, both it and the riser might need cleaned out/replaced.

And if one side is bad......

[Kirbyvv]

I had a similar issue. check to see if you have equal volumes of water exiting from the exhaust. I had one side partially blocked and ended up replacing the exhaust.

[BethnKevin]

I am going to check the thermostat housing, the risers and manifolds... And will replace whatever needs to be replaced. I have the next couple of days off so I can get into this then. I will keep you posted and updated. Thanks again for the info.
Kevin

[ANGUS]

I can't speak with experience on a smallblock OMC...but on my bigblock the manifolds have access caps on the rear that if original are rubber and easily removed. My point is that I have been amazed at the accumulation of sediment and rust at the rear of my port side manifold at the end of each season. If yours is equipped this way I would remove those end caps and push a cleaning rod thru from the front side( remove hoses). I now do this each year using a shotgun cleaning kit...that rear cylinder or 2 really get restricted cooling with crud buildup. This is easy and may resolve your problem on that side.Again..I am familiar with the BBC but am guessing the design is the same. :wrench:

[MOP]

Another possibility is if it has the ball reliefs in the Tstat housing, they can get sticky and only pass water to one side especially at low to low cruise speeds.