1997 Yamaha P150TLRV Overheateing at 4800 rpm!!

[BUDA13]

I can run all day long less than 4800 rpms without an overheat alarm, but once I exceed that RPM it takes less than a minute for the alarm to go off. The boat has been to two local dealerships and stumped their mechanics. We replaced the two overheat sensors and the problem went away for 6 months, now it is back. From the moment that the overheat alarm comes on I can shut down, pull the cowling and touch the heads. They are warm but not HOT by any means. This motor has only been in freshwater. The yamaha mechanics that I have been working with have replaced the impeller and waterpump , as well as a complete de carb to no avail. I was told by one mechanic that that was the first year Skeeter owned yamaha and their were some issues with some boats having an aftermarket wiring harness that caused a few problems like this. The weird thing is I know the previous owner and it never did this in the 8 years he owned the boat. Please help!!!

Matt

 

[bigbrownbuku]

Re: 1997 Yamaha P150TLRV Overheateing at 4800 rpm!!

first determine it isnt actually overheating with an infra red temp gun. i never leave home without mine. then determine why the alarm is being set by unhooking senders when the alarm is happening, could possibly be another sender causing the alarm, low oil etc.
did they service the pressure relief valve when they serviced the water pump?

 

[BUDA13]

Re: 1997 Yamaha P150TLRV Overheateing at 4800 rpm!!

They did service the pressure relief valve and I have a separate light and alarm for a low oil condition. ONe other thing comes to mind, when they replaces the sensors they also replaces the fuel line but I dont see a connection between the two. Thanks fir the response!

 

[WillyBWright]

Re: 1997 Yamaha P150TLRV Overheateing at 4800 rpm!!

They may have noticed that the lines were getting hard, or maybe suspected an air leak making it run lean.

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.