Mercruiser 233 Overheating--Out of Ideas

[johnkom]

New engine two seasons ago and it runs great. But on plane, 2500 rpm plus, it starts to overheat. Temperature creeps up slowly into the red until I throttle back after about 3 to 5 minutes. Then engine cools down to normal and you can run all day off plane never heating up. I have replaced almost everything in the cooling system. New manifolds and elbows. New outdrive pump and water pocket. New outdrive water pump hose to stern tube. New marine engine water pump. New thermostat. Removed thermostat to trouble shoot and still same symptom. All temperatures verified with two different IR temp guns. Bore scoped outdrive passages and engine hoses and saw no restrictions. Engine timing checked. Compressions all good. Don’t know what else to check. Ideas??

JK

 

[alldodge]

More info about the motor
Your postings show and 5.7 and a 233 (not counting 302). Unable to find a 233 but can find a 230. What year and serial number is the motor?

 

[johnkom]

1976 Ford 351W, serial #4465387.

 

[Scott Danforth]

Removing the thermostat actually makes things worse.

Check your raw water flow rate

Check the manifolds for rust

 

[alldodge]

Thanks, inboard 351
Are the hoses routed as in the pic?
Note the ports on top, front of motor go to elbows

Next is there 2 gaskets and 1 plate separating the elbow from the manifold

 

[johnkom]

"Thanks, inboard 351
Are the hoses routed as in the pic?
Note the ports on top, front of motor go to elbows

Next is there 2 gaskets and 1 plate separating the elbow from the manifold"

Yes, all checked.

 

[johnkom]

"Check the manifolds for rust"

They're new.

 

[alldodge]

Guess your not moving enough water
Have you tried clear hose from drive to see if its drawing air (bubbles)

 

[Scott Danforth]

Are you running the boat on muffs or is it in the water?

Hope you are not trying to suck up from a bucket

 

[johnkom]

Boat is in the water in a marina. I can't put clear hose on supply from stern as it is below water line. I can check flow. Will do that Monday.

 

[alldodge]

Yes you can, get a short piece of clear hose, a double male barbd fitting and couple hose clamps. Remove the hose going into the thermostat housing from the drive and install short hose.

 

[johnkom]

OK. Question. When checking flow, obviously it is ample for low rpm. How do I check for high rpm?

 

[johnkom]

"Yes you can, get a short piece of clear hose, a double male barbd fitting and couple hose clamps. Remove the hose going into the thermostat housing from the drive and install short hose."

Ah. Check upper end. Cool. Will do.

 

[kenny nunez]

What about the hose from the bell housing to the gimbal housing?
Can you hold back the incoming water to the thermostat housing from the drive at idle with the hose removed with your hands?

 

[johnkom]

"What about the hose from the bell housing to the gimbal housing?"

If you are referring to the hose in the outdrive, it was replaced this spring with a new one from Mercruiser along with new bellows.

The fresh water hose going into the thermostat housing is pretty soft and I'm sure I can squeeze it but not sure I can stop the water flow. I'll try that tomorrow when I have the clear section in.

 

[Scott06]

The new impeller was this an oem merc impeller or aftermarket?

 

[johnkom]

Mecruiser

 

[dubs283]

Mecruiser

Did you replace the separately purchased water pump base/seal housing and gasket?

 

[kenny nunez]

If you cannot stop the water by hand then that rules out the drive pump delivery to the engine.
You really need to add a short section of clear hose to the in coming water hose at the thermostat housing. If there is a combustion leak the pressure will overcome the incoming drive pump supply. You will see the water disappear in the clear hose if there is a combustion leak.

 

[Lou C]

That’s exactly what I was about to say Kenny! Note that you can have close to normal compression test results and still have leaky head gaskets. I had that with mine then noticed a bit of water in the cyls.
Also I’d make up a pair of clear hoses from the stat housing to the manifolds. Bubbles there def suggest leaky head gaskets.