Mercuriser 260 overheat at low speed

[rlander06]

My 1985 Sea Ray 230 Weekender with a 260 Mercruiser overheats at low idle speed. I replaced the lower unit impeller even thou it did not need replacing. I removed the thermostat and it would not even cool at high speed. I installed a new thermostat (160 degree same as old one). I checked all hoses for flow and all works well. I took both exhaust Risers off and they look good. The starboard side looks like it has more water in the exhaust elbows (90s) then the port side. None of the water ports are plugged that I can see on the exhaust manifold or risers.

 

[Scott Danforth]

Removing a thermostat actually makes the motor run hotter as there is no control over the water flow and your velocity thru the block is too fast for tge water to pick up heat

Go back to the impeller, follow the hoses from the drive to the T-stat and on to the manifolds and look for blockage.

The raw water pump should fill a 5 gallon bucket in between 16 and 20 seconds at a 1000rpm speed

 

[zellerj]

Does the temp change from turning hard to port or starboard. If that makes a difference, then there may be a kink in the hose coming from the impeller. At low speeds the pressure from the impeller is such that a kink will limit the supply of cooling water. At higher speeds the restriction is overcome with water pressure and normal temps resume.

 

[alldodge]

Look in the exhaust Y pipe to see if the exhaust shutters are there, if not see if they are in the bottom of the Y pipe

Are you sure its over heating, check with an IR temp gun? Might be the gauge/sender

 

[QBhoy]

Check the exhaust shutters

 

[wahlejim]

Did you replace the impeller housing while you had it apart? There are wearable plates in there that will mess with water flow if there is too much clearance.

Other than that, isolate the problem. Do Scott's test. Takes 20 seconds to pull the hose and stick it in a bucket. It will tell you if you are dealing with a flow issue before water hits the engine or after.

 

[porscheguy]

Are you sure its over heating, check with an IR temp gun? Might be the gauge/sender

+1 on this. My temp gauge had only been reading 120 the other week. Bought a new t-stat and sender. Both Sierra. Installed them. Temp gauge read 190-200. Checked with IR thermometer and temp was properly fluctuating between 150-160 on the intake manifold. Old sender was bad. New Sierra sender was bad. Went up the street to mercury dealer and got a Quicksilver sender. Temp reads correct now.

 

[rlander06]

Did you replace the impeller housing while you had it apart? There are wearable plates in there that will mess with water flow if there is too much clearance.

Other than that, isolate the problem. Do Scott's test. Takes 20 seconds to pull the hose and stick it in a bucket. It will tell you if you are dealing with a flow issue before water hits the engine or after.

Will this test work with the boat out of the water and by using the muffs and a garden hose with high pressure?

 

[rlander06]

Did you replace the impeller housing while you had it apart? There are wearable plates in there that will mess with water flow if there is too much clearance.

Other than that, isolate the problem. Do Scott's test. Takes 20 seconds to pull the hose and stick it in a bucket. It will tell you if you are dealing with a flow issue before water hits the engine or after.

I did replace the housing and all.

 

[achris]

.... Old sender was bad. New Sierra sender was bad. Went up the street to mercury dealer and got a Quicksilver sender. Temp reads correct now.

And people wonder why I prefer to spend a bit more and get OEM parts.

Old Chinese proverb. Cheap no good, good no cheap.

Chris....

 

[rlander06]

+1 on this. My temp gauge had only been reading 120 the other week. Bought a new t-stat and sender. Both Sierra. Installed them. Temp gauge read 190-200. Checked with IR thermometer and temp was properly fluctuating between 150-160 on the intake manifold. Old sender was bad. New Sierra sender was bad. Went up the street to mercury dealer and got a Quicksilver sender. Temp reads correct now.

I was trolling one day and the engine started to get real hot and started to run rough, I got the engine up past idle and was able to get the engine to cool down. It still would go to very hot and run bad at idle.

 

[achris]

I was trolling one day and the engine started to get real hot and started to run rough, I got the engine up past idle and was able to get the engine to cool down. It still would go to very hot and run bad at idle.

I had that on a V6 recently. Turned out to be a leaking exhaust elbow gasket....

 

[Scott Danforth]

Will this test work with the boat out of the water and by using the muffs and a garden hose with high pressure?

No, needs to be in water

 

[rlander06]

My 1985 Sea Ray 230 Weekender with a 260 Mercruiser overheats at low idle speed. I replaced the lower unit impeller even thou it did not need replacing. I removed the thermostat and it would not even cool at high speed. I installed a new thermostat (160 degree same as old one). I checked all hoses for flow and all works well. I took both exhaust Risers off and they look good. The starboard side looks like it has more water in the exhaust elbows (90s) then the port side. None of the water ports are plugged that I can see on the exhaust manifold or risers.

I just took off the exhaust elbows to check the shutters, there is none on either side. The only thing I have not checked is the water pump on the motor,

 

[alldodge]

I just took off the exhaust elbows to check the shutters, there is none on either side.

Being none are there, they might just be in the bottom of the Y pipe

 

[rlander06]

Being none are there, they might just be in the bottom of the Y pipe

I will dig down deeper to the bottom of the Y pipe and have a look.

 

[Scott Danforth]

I would pull the drive and look for the pieces. Then formulate a plan to fix the ship that is broken

 

[rlander06]

Look in the exhaust Y pipe to see if the exhaust shutters are there, if not see if they are in the bottom of the Y pipe

Are you sure its over heating, check with an IR temp gun? Might be the gauge/sender

I Checked the Y pipe today and found no shutters in the bottom

 

[alldodge]

That's good, how about the IR temp?

 

[rlander06]

That's good, how about the IR temp?

As soon as I can find a IR reader I will check this out. Thanks