Re: Strange overheat twin 1996 200hp SWII Yamaha posted corrected
Re: Strange overheat twin 1996 200hp SWII Yamaha posted corrected
I found this post by WilleBright:
"I'm betting that you first noticed it at full throttle only and as time goes by, at lower and lower throttle. It will continue to decline with use.
My experience is that such alarms are caused by scale build-up on the outer walls of the cylinders in the block and in the heads. That insulates the aluminum from the cooling water and the heat stays in rather than transferring to the water. I'm afraid that a thermosensor won't detect this condition because the outer cooling surfaces don't get as much build-up and the block will show normal temperatures.
To remove the scale, the heads have to come off. Then lay them flat on their backs and fill the cooling cavities with vinegar or milkstone remover (a farm supply product). The scale on the outside of the cylinder walls can be removed manually with an old toothbrush and milkstone remover. Protect electrical components when you do this, and wear protective goggles and rubber gloves. And definately do it outside where there's plenty of ventilation.
Once done and reassembled, run plenty of fresh water through the motor to clean out the residual acid. Do not use muriatic acid or Zing. Those eat aluminum. Milkstone remover doesn't. A very similar product to milkstone remover is called Acid Wash. They are both phosphoric acids. They come by the gallon and are rather inexpensive. "
