Merc 120 I/O overheats on plane

[ralf9788]

My Merc 120, in a 16' boat, has an overheating problem. A new therm and lower unit impeller has not helped. The timing & temp gauge has been checked. Two different props have been used.<br />The "normal" temp is 1/3 green (below 1/2). The problem is that up to 1600 rpm the gauge stays at 1/3 green. As rpm's are increased above 1600, the temp gauge starts up until at 2500 (plane speed) it moves into red in just over 1 minute. Going back to 1600, the gauge will return to 1/3 in just over 1 minute. At 2200 rpm, the gauge reads 3/4 green. Out-of-gear at 2500 the gauge remains at 1/3 green. It would seem that the load on the engine was greater at 1600 then at plane speed.

 

[a.d.mobile]

Re: Merc 120 I/O overheats on plane

what did the upper and lower water pump housings look like? check cooler for restriction it a long shot on a merc. splice in a clear plastic hose in the water line so when you run engine at the problem area you can look and see the water supply. you are loking for a steady supply not a lot of air bubbles.

 

[akriverrat]

Re: Merc 120 I/O overheats on plane

overheating at speed is normally a restriction in the system. the usual suspect is the exhaust riser/neck. overheat at idle and ok at speed is normally pump.

 

[ralf9788]

Re: Merc 120 I/O overheats on plane

The upper & lower pump hsg were in good shape. The lower was replaced for the new seal. One impeller blade was missing a piece about 3/16x1, its whereabouts unknown. I've considered that it may be stuck in the engine, have been told the water passages are probably too large to hold it. I assume the cooler refers to a closed system, it doesn't have one. Like the clear plastic idea. <br /><br />The riser/neck was replaced a couple of years ago after it forced an engine overhaul because of water leaking into the exhaust valve.

 

[akriverrat]

Re: Merc 120 I/O overheats on plane

if you have oil cooler or power steering cooler impeller piece may be there.

 

[ralf9788]

Re: Merc 120 I/O overheats on plane

The boat does not have power steering or an oil cooler.<br /><br />Does anyone have any thoughts concerning that if there was an exhaust leak into the water part of the manifold that could cause "bubbles" that restricted the hot water flow out of the engine. Possible?