I have a 40 hp mercury boat motor runs great but running a little too hot Ive changed the impeller already?

Ive changed the water impeller and housing, compression on all cylinders are 90psi . the motor doesnt seem to come out of the hole and plain out like it should, but it is running great. the water coming out of the outlet is pretty hot though and I dont want to harm the motor. How can I cool this heat problem and get the boat to come out on plain like it used tooI have a 40 hp mercury boat motor runs great but running a little too hot Ive changed the impeller already?Is the tel-tale stream steady? Check the temp of the engine block and head. The temp of these componets is what matters. Does the engine run better before it warms up? If it doesnt then the temp is not effecting the running of the engine. The down on power problem could be related to something else. How are the plugs, is the fuel fresh? Is the prop in good shape, has the engine been sync and linked lately? Run the engine at 1000 rpms on a hose and pull the spark plug leads off at a time. Make sure all the cylinders make the engine drop in rpm. If not the engine is not running on all the cylinders.I have a 40 hp mercury boat motor runs great but running a little too hot Ive changed the impeller already?Did you check for blockage in water passages? The old impeller could have broken up and clogged something. Was the water-pump to engine transfer tube installed properly and was the grommets on both ends inspected or replaced ? Had some lizards climb into thru hub exhaust once and caused some problems. Timing also plays a part in engine temp. Have you flushed your engine lately ? How about the thermostat,if its got one, is it alright ? Same prop, have you hit something lately? More weight in the boat ? Lean fuel mixture. Is the bottom of your boat clean. Sinc and link ? I know I'm going to hear about this one, but use 89 octane instead of 87, because 89 has more detergents to keep carbs and tops of pistons clean and not so much to add to the problem. Water temp should be around 125degrees for that motor........Hey Boat tech, does this engine have a poppet valve and spring?...( Bingo Mercman !)....Octane is (derived )from (additives) now days. It's all (87). 89 has more detergent and it's already in there. You don't have to add anything. That's why they make aftermarket additives to begin with because people use 87. They think they are saving money. Figure the cost of additives compared to the extra cost of 89 and the inconvenience of not having to deal with it. And no, higher octane now days will not burn a hole in the piston or burn the valves, in a engineers point of veiw, meaning burn time. But excessive use of octane boosting additives over time will. And I know, that you know, that we know, that there wasn't enough information in the question to answer it properly, but sometimes thats what makes it fun .... Don't it give you a chuckle when you feel like a blubbering idiot answering some these incomplete questions? I do.I have a 40 hp mercury boat motor runs great but running a little too hot Ive changed the impeller already?Need to know what year or serial number. 2, 3 or 4 cyl. 2 stk or 4stk. Has the motor been sitting up for 6 months? What is the temp of the motor?



Hey Link, we need to know what 40 it is before the thermostat, and popet question can be answered, some had them some did not. Also, quit wasting you money on 89, there are no more detergents in it than the 87 or 91. Mercury Quick clean or Yamaha Ring free will keep the cylinders and carbs clean, of course the Mercury product is better.I have a 40 hp mercury boat motor runs great but running a little too hot Ive changed the impeller already?If this is a 2 stroke motor it could be a seal problem. If it is an older engine and has been setting around and not run in a while the engine seals will dry out letting air leak into the fuel air flow leaning out the fuel mixture which will causing heating and performance problems. The exhaust should be hot or your cooling system would not be working.