After chasing the reason for why my starboard riser is running about 30 degrees hotter then port, and blowing steam, for about a year, I found something interesting.

1999 502 MPI, Bravo 1
New within last 1.5 years:
Manifolds
Risers
Replaced riser/manifold gasket on problem side 3x, filed mating surfaces to make sure they were true
Raw water impeller and housing
Circulating pump
T-stat

Thinking it's a water flow problem, today I decided to start pulling hoses. Pulled both hoses that feed the bottom of the manifolds off at the tstat housing. Started engine (it was cold), had even flow coming out of each end of housing. Reinstalled the hoses and let engine come up to temp (+/- 165). Shut down and immediately removed the same hoses from the tstat housing and had friend restart motor while it was hot. While holding my hand a few inches from each side of the housing to feel the flow rate (as I had when the engine was cold) and see if it was still even to port and starboard after the tstat opened, the water coming to the starboard side just about burned me, while the port side was basically lake temperature. I'm talking a major difference here. So, that solves my problem of why my one riser and manifold are so much hotter. Pulled tstat housing and explored with a scope, but I honestly wasn't sure what I should be looking for. Just ordered a new housing.

Any clues why or how this would happen? It seems that all the hot water being discharged was sent to the starboard side rather then being distributed evenly. I'm really hoping someone says that the tstat housing is obviously corroded or there is a blockage of some sort....I've had enough of this...

when you changed the t-stat,did you use one for marine use?

Now im really going to upset you after all that work .

Its normal

I have seen MANY MANY boats like this. My buddies does it after a fresh BBC, my boat does it aswell, with fresh thunder risers, pipes. Is it normal, hard to say, but again I have seen a bunch like this. I would be curious to know why aswell, never bothered me, so I never really checked into it

John jr

It's a Merc t-stat

Guys, listen, if it's normal then I'm one happy bastard cause I can forget about it. It's just that it wasn't always like this and seems ludicrous to me that I'd be dumping all the hot water out of one side and the cool out of the other. Lots of friends with the same motor don't have this issue.

Anyway, I'm going to put the new tstat housing on for piece of mind and see what happens. By the way, I found it for $135 on ebay, it's a brand new take-off.

when you installed # 6 what position did you install it? also make sure you install a hp thermostat with the bypass built into it..

http://www.google.com/imgres?q=1999+...x3Ki3Cg&zoom=1

when installing #6 you can turn it to direct the water to favor one port or the other,in your case i would slightly turn the colder side towards the warmer side and see if that helps with the steam..

I installed exactly as in the diagram. You're saying to rotate it a bit to direct more cold water to starboard, right? So, looking down at it from the top, do you know if I would rotate it clockwise or counter clockwise?

..you could also leave it straight up and put a hose clamp on the colder hose for trial and error to see it it takes away the stram,,kinda like restricting the water going to the cooler side and forcing it towards the warmer side..

mercruiser phucked up when they got rid of the cooling balls rod and springs..

Question - What method are you using to measure the temp, are you using a "laser temperature gun", if so measure the temps at the front of the manifold and at the rear of the manifold. It is my understanding that some temp variation is going to be normal form left to right manifolds. I saw a thread on here quite some time ago where a gentleman had a similar problem. I believe the gentleman solved it by increasing water circulation in the back of the block by modifying his intake manifold, installing hoses from the rear of the manifold water passages to the either the risers or to the thermostat housing, can't remember which, but the idea is to have the water exit the rear of the block, where steam pockets can develop. I know that some people do this modification and then have the water dump overboard. Hope that this helps.
Cliff

Yea there's actually a groove on the housing and ridge on the sleeve so I can't adjust the position. It only fits one way.