Logo
FULL SITE MENU

Mercruiser 260 Inboard overheats under load

C

Chefbry

New member
Hello, I have a boat with twin 260 Straight inboards with closed cooling system. This engine warms up considerably slower than the other. Starboard engine over heats under load over 2500 rpms. I can idle around all day and it runs and cools fine. Water flow is good ( but seems a bit less than the port) New impeller, thermostat, thermostat housing, belt, Fresh waterpump. Trans cooler is clear, as is the heat exchanger. Water flow from intake is almost identical to the port motor( I measured it) infrared temps show 160 on pump, thermostat, risers are just warm, the heat exchanger and hoses are not too hot. The only part of motor that seems like it could be the culprit are the left manifold is showing like 190 ish. could it be the manifolds? I believe they are coolant cooled and the raw water only goes to risers and the heat exchanger? Also I am experiencing a oil or coolant slick from exhaust into water and a bit of white smoke at start up from this engine. Engine seems to run good. Idles fine, starts right up.
 
how old are your engines?

when you 'measured', was it at idle or under load at elevated RPM?

any idea how old the risers are?

on the 'slick' - oil and coolant are usually easy to distinguish...are you sure it isn't fuel?

on the 'left manifold', where you got the 190-ish value, where were you measuring at?
 
makomark said:
how old are your engine 1986 , Fresh water cooled. They seem to be about 800 hours

when you 'measured', was it at idle or under load at elevated RP Measured at idle and immediately following the temp increase.

any idea how old the risers are No Idea on risers, but i can put my hand on them while running

on the 'slick' - oil and coolant are usually easy to distinguish...are you sure it isn't fuel? It may be fuel. The Quadrajet carb is older and may be running rich. The port carb I replaced this year and runs smoother so I plan on replacing that one soon

on the 'left manifold', where you got the 190-ish value, where were you measuring at?
Click to expand...
​Close to the fresh water pump end.
 
were the symptoms the same before and after all of the new parts went in?

have you measured the product from the raw water pumps to see if they are equal and adequate?

The exhaust manifolds are cooled downstream of the thermostat so they will be warmer than the thermostat's opening value.

are you certain the thermostat housing was assembled with the parts in the correct order (per the factory manual)?
 
makomark said:
were the symptoms the same before and after all of the new parts went in?

have you measured the product from the raw water pumps to see if they are equal and adequate?

The exhaust manifolds are cooled downstream of the thermostat so they will be warmer than the thermostat's opening value.

are you certain the thermostat housing was assembled with the parts in the correct order (per the factory manual)?
Click to expand...
Basically I have done each part exchange and then sea trialed the boat. Each time with the same response. I have not measured the product from the pumps.
 
I'd be inclined to check the pump output...especially if it is original. there are other service parts beyond the impeller.
 
Look for an air leak before the raw water pump. I had a bad hose clamp (some boob used a non-marine clamp that rusted) on mine that let the pump suck in enough air to do this same overheating nonsense.

Jeff

"Let's go Brandon!"
 
Remove the rad cap, with engine warmed up raise the rpm to about 2000 and look for compression bubbles in the coolant
 
Back
Top