Had the boat out today and noticed one side of exhaust was running a little hot? Steam coming from exhaust. Other side is cool. 1999 502MPI. I"ve seen other posts about this, but can't remember?. Is this reversion? Stock Merc. manifolds. Pulled the plugs and # 5 is white. The rest are tan. What do I need to do to fix this problem?
Thanks,
Steve

How is the impeller in the sea water pump?

It was my experience that the port side exhaust runs hotter than the stbd. It is in the design of the merc cooling system in how the water is routed to the exhaust manifolds. The #5 and #7 cylinders are next to each other in the firing order and usually run hotter than the rest, but if it is white, you may need to check the injector and or fuel pressure. The factory setup is on the rich side and I almost always had plugs on the dark side from running so rich.

The impeller should be alright, as I checked it about a month ago and it looked good. The #4 cylinder is the one running hot (sorry typo) . Port side cool, Starboard side hot. Any suggestions ?

The water has two ways out of the engine...the right side or the left side. Water will take tha path of least resistance , sometimes leaving one side drier (and steamier) that the other . I've seen quite a few that have this situation , particularly at an idle when you one have a very small amount of water pressure. When running the pressure comes up and the flow increases out of both sides barring any major obstruction.
As others have stated , #5 could be fuel. Unfortunately , the one time I saw it it was a bad head gasket allowing a small amount of water in and steaming the plug and combustion chamber clean

How do I check to see if head gasket is leaking?

couldnt it just be cold lake water temps?

The water was cold but I don't think that is the problem. The starboard exhaust manifold was hot to the touch. Port side manifold was cool. The white spark plug on #4 cylinder is what has me concerned. I'll do a compression test on #4 in the next couple of days and see what that looks like.

Upon further thought I think I was wrong on which side runs hotter. If you can put your hand on it without burning it is far from running too hot. Especially with cooler lake temps, steam doesn't surprise me. #4 running lean is another issue. Hope you get to the bottom of it.

Bill

I have a 1994 502 mpi and I havn't noticed a temp difference but my #4 is always clean compared to the others. I havn't figured it out yet. I just changed the plugs last week, I have a lean condition across the board but I will adjust that with fuel pressure.