Merc 250hp bravo drive temp fluctuating

"replaced temp sensor,manifold

"replaced temp sensor,manifolds and risers,thermostat,and new raw water pump.I grounded temp gauge and pinned it.When i changed the thermostat part of the impeller was in the therm housing.Still having problem when i run rpm's around 2000. temp gauge goes from 0 to high 200's and drops back to 0 and stays for a while can't figuer it out any help please."

jumping like that generally me

jumping like that generally means it's ingesting air or loosing water pressure. I just had to replace my circulating water pump after having the same symptoms as you describe.

thanks bill i will look into t

thanks bill i will look into that.Ihad sent a email to mercruiser and they suggested a defective temp gauge or possible blown head gasket.The problem is i thought you would see water in the oil.

"Hey Bill thanks for your inpu

"Hey Bill thanks for your input but I have a Question on the syptoms your engine had.My engine runs fine manifolds,risers,and therm housing cool to warm by touch so isn't the water coming from outdrive.The temp gauge reads 100 to above 210.What happens is if I let the engine idle for 15 minutes and then put rpms up to 2000 30 seconds later or more is when temp gauge will go to 210 and the drop to 100 or it will go to 140 and drop to 100.Is this because the water circulator does not have enough output say if the thermostat opens or closes?
thanks paul"

"Paul:

The engine circulati

"Paul:

The engine circulation pump keeps moving water around in the block via the pump until the thermostat senses it is time to open up to lower that water temp. When it opens the exiting hot water blends w/cool water and the T'stat hsg. sends some of it to the exhaust manifolds and circulates some cool water into the block.

If the circ. pump belt is tight and the pump pulley does not wobble meaning a worn shaft bearing, it should be OK. The impeller is stainless steel and isn't supposed to rust away.

At the base of the pump hsg. is a weep hole. Can you see water dripping out of it or a telltale stain of a former leak? That leak means the grease/water seal is comprimized and the circ. pump should be replaced.

When you found the impeller chunk in the T'stat hsg. did you take the hsg. apart to check for more? Did you back flush from the hsg. to the impeller?

Bill stated above that he thought the engine may be injesting air or loosing water pressure. I am leaning towards agreement w/him. The temp. sender tells the gauge what is going on in the block. When it jumps around as you stated air is getting in the cooling system. The big drops in temp. on the gauge are evidence of it.

Pull the water inlet hose where it connects at the transom and check for a narrowed opening. There have been several boaters report that corrosion built up around the passage and crushed the pipe liner. With the hose off remove the water inlet grills on the lower unit.

Now shove a garden hose into the water inlet that you removed the hose from and see if the water flows freely down and out the water pickup.
You may have to connect a short piece of hose to connect a garden hose to it.

Lastly, carefully check the entire length of the water inlet hose for kinks, flat spots and areas where something may be squeezing it causing the water flow restriction."