OMC 350 chevy runs hot above 2800rpm

[shulamite]

"Have replaced;headers, risors

"Have replaced;headers, risors, impeller, water pump, thermostat 160, gauge and sender, and have done the OMC prescribed test on raw water supply. We have app. 3-4 ins of water at the therm housing. Motor runs like a watch, however as the rpms rise above 2800 so does the temp, to as high a 220. As soon as I drop the rpms so goes the temp back to 160. Its like the throttle was hooked to the temp gauge. 350 chevy 260hp Donzi Regazza 25'1989. I think the water passages are partially blocked from 500 hrs in salt water, though the boat never set in salt water long. Any new ideas much appreciated. The motor runs so well I hate to tear it apart."

 

[charli]

Try pulling the plugs from the

Try pulling the plugs from the bottom of the block just above the oil pan. These are the block drains. If there is a lot of crud behind these plugs the bottom of the block could be full of crud cutting down the amount of coolant in the block.

Another way is to get a lazer temperature gun and check the temperature at different points on the block.
You may find the bottom stays cooler longer.

Sorry to say there is not much you can do if the water jacket is full of crud short of having the block boiled.

You can try flushing it. This may work if you can break up the stuff the best you can by poking a wire around in the drain holes while running water through the block.
You'll need a wire long enough to reach the back and front of the block. The problem is the area you can't get at on the inside / middle of the block.

Even if the boat was not left in salt water very long the block will rust if the engine wasn't flushed after each use. If the engine was left will salt water in the block a few times for very long you may have a problem.
Good luck.

 

[John Mason]

"Sounds simple, but have you c

"Sounds simple, but have you checked the tention on the belt?

I had a similar situation on a 2.5L this summer. Tried removing the thermostat, checked the water moving through the system... nothing seemed to help. Then tightened the belt and it stayed cool as a cucumber.

The next thing to go was the belt later that weekend! It was a very old belt anyhow. Put a fresh belt on (nice and snug)... works great and stays cool now!

Good luck!

John."