One exhaust manifold hot

I have a 2000, 350 mag Mercruiser and one of the exhaust manifolds (port side)is getting hot. The other one stays cool. Any ideas?

It sounds like that side or one of the cylinders on that side is running lean.You might have an intake leak.Start the motor and take carb cleaner and spray around the intake manifold.If you here the motor start to increas in rpms(rev up) then thats where its leaking.Also make sure your exhuast is clear and there nothing hung up in that side of it.

Often times there is a T junction in your cooling system which is meant to distribute the water pressure evenly to both exhaust manifolds. Sometimes the ball/valve/flap gets stuck and sends more water to one side than the other, thus one side gets hotter than the other. Check the exhaust pipes when running the engine on land. See if more water is comN out of one side, this is your tell tell sign that you have a blockage. That’s what comes immediately to my mind...Good luck

I actually fired it up when I got home from work today and it looks consistant from both sides. After running for bit the manifold gets too hot to hold your hand on it for any more than 5 seconds.

I did notice the temp guage to be fluctuating. It would be 175, drop to 120, etc. The needle was not bouncing around crazy, just moving back and forth.

I have the same exact problem with my boat. I have 1997 Alpina and one exhaust manifold is hotter than the other and the temp guage does the exact same thing too. I first thought it was a bad temp sending unit but now I doubt it. Does your exhaust also smell rich? I can't idle for too long or I start getting naesous from the fumes. Please tell me if you find out what the problem is.

I took the boat in to the dealer for the manifold issue and some other things last week. Dealer said they called and Mercruiser said it is normal, nothing to be concerned about, for that side to be hotter than the other...the newer fuel injected engines do. I pulled the plugs to replace them yesterday and did notice that the port side plugs smelled of gas more than the starboard side, so yeah, it would be running rich. Had the boat out today and it ran like a champ.

Odd, I would expect a better explanation form the dealer but...is it still under warranty?

I don't know for sure, but it seems that the thermostat is opening and closing (therefor the fluctuating in the temp gauge). It seems that your engine doesn't reach operating temperature when in idle.
It could be that when your thermostat is closed only 1 side of the exhaust get enough water (the cold side) and the other side get less. When the thermostat is opened both sides gets equal water.

Hope that this helped.
Tommy

Chris, the boat is not under warranty. I trust my dealer completly and know they would not give me any mis-information, as long as Mercruiser is being right, and I have no reason to believe they are not.

Tommy, there is good water flow in each side of the exhaust, no problem there.

When is the last time you replaced the thermostat, perhaps it is sticking closed for some time and not opening at the proper point? How many hours on this thermostat?

I'm simply gonna have to call Merc out on the "it OK for one side to be hotter than the other". Engines are all about balance, if 4 cylinders are hotter than the other four (and I'd say 20 degrees is getting to be a BIG difference) then you are pushing one side harder than the other. Eventually rings/lifters/valves/your cam/crank will all wear due to the imbalance. Something is a miss. Could be a bad ring, bad valves, clogged manifold but something is bad. I would check the exhaust. Look for any smoke on starting it up, see if one side smells richer than the other. But, bottom line is, if it runs with no problem but the heat then is it a Big deal? I'd say run it until you see the ill effects then the problem will be easier to identify but no more expensive to fix, hopefully Good luck, and happy waken

It is normal. I have been working on these mercs awhile now and it used to scare me, until I talked to a merc technican. NO WORRIES. (LAKE TAPPS RULES)

I don't understand how merc can call this "normal". Maybe it doesn't have any immediate effects on the engine but in the long run it can't be good. If the temperature is fluctuating so much the heads/valves/lifters are expanding and contracting with each time the temp changes. I would think this would cause excessive and premature wear on the engine.