Mercruiser 260 overheating

I have a 1978 Seay Ray SRV220 with Mercruiser 260HP.
I have serious mystery overheating!
My temp gauge slowly rises whilst underway until it gets close to the Red. Then I slow right down and the temp goes back down well into the green. It looked like impeller delivery problem so I disconected the delivery pipe from the thermostat manifold, connected a garden hose in it's place and extended the raw water delivery pipe to an extention. On running the engine I measured 5 gallons in 90 seconds at idle, 5 gallons in 75 seconds at 2000rpm and 5 gallons in 50 seconds at 3000 rpm.
This looked to me like the impeller is working great. I then removed the risers and these checked out clean and clear. I sought advice from the Mercruiser agent who suggested that I should have blanking plates between the risers and manifolds, these I fitted and it was still the same! So I removed the thermostat and it is still the same? The timing, dwell and mixture are all good. Where do I go next?

V2Climb

 

If the engine overheats without thermostat and with enough raw water pumped up, the problem must be in or near the circulation pump. Unscrew it from the block and see how little is left over from the impeller, it probably has no vanes at all.

 

Thanks for the input, unfortunately I have taken off the engine driven water pump and it looks like new!

V2climb

 

"Thanks for the input, unfortunately I have taken off the engine driven water pump and it looks like new!"
That cannot be. A water pump from a raw water cooled engine only looks like new when it is new. Mercruiser uses 70 degrees C. thermostats to prevent serious corrosion problems during the 12 months warranty. On a '78 engine the sea must have digested a lot of cast iron. Most of that has disappeared through the exhaust, but some will have accumulated in the lower part of the block and in the hoses. I've once seen a complete blockage in 2" rubber hoses on an engine that was not yet 10 years old.

 

Engine was recently overhauled! hence new pump and clean water ways!

 

Thank you all for the help and advice. Be advised that I purchased a digital infra-red thermometer.
I ran the boat in the dock and the gauge went up to the top of the green. The digital thermometer gave me readings of 165 all over the engine and 170 at the hottest point on the riser!
I took the boat out for a spin and at full throttle the temp gauge went up to the top of the red....I never let it get that high before. I checked the tempreture all over the engine and the hottest spot I could find anywhere on the engine was 175 and the hottest point of the riser was 185, and this on a 96 degree day!
So all along I was believing a duff gauge. Now I am going to get a new matched sender and gauge on Monday morning.

V2Climb