78 115hp overheats new pump/thermostats

[tcurtsinger]

I am trying to get my old motor running on a new boat, previous owner told me it would overheat. I replaced the water pump and both thermostats this week and got it started. The old thermostats were stuck and the groove in the impellar was ground down and not catching on the key. It runs fine but I got the overheat alarm within a minute or two at idle. The stream from the telltale is strong so I removed the two hoses leading from the heads into the thermostat housing and started it. There is good water flow coming from both heads and the alarm did not sound after idling several minutes this way. Put the hoses back on and got the alarm again. I'm guessing one or both of the thermostats, which are both new, are not opening or there is a blockage somewhere past them. I thought about removing the thermostats and testing it, but its such a pain to remove them. Is there anything that I am missing that could be causing it to overheat?

 

[emdsapmgr]

Re: 78 115hp overheats new pump/thermostats

When you installed the new thermostats, did you visually check the valve body? The valve body is the plastic pc sandwiched between the block and the outer housing. The valve body on yours has two pinholes toward the centerline of that pc. They frequently clog up with dirt/debris. If these holes clog up, the engine can oveheat at idle. Those pinholes let hot water get into the back of the stat housing to heat the stats up so that they will open. There are two ways to handle this. 1. Clean those two holes out when you have the housing apart. 2. drill a tiny hole in the top of each stat itself to let the water through.

 

[Haffiman]

Re: 78 115hp overheats new pump/thermostats

If those engines overheats, change of the water deflectors are close to a must.
This means off with heads.

 

[tcurtsinger]

Re: 78 115hp overheats new pump/thermostats

Everything is new, valve body, valves, springs, housing, gaskets. Broke all the old stuff getting it apart. Not sure where the water deflectors are, but if they are in the head and were bad wouldnt it overheat when I unhooked the hoses. I had good flow coming from the heads? I'm going to try the method I found here about pouring hot water in the hose and see if the thermostats open and drain.

 

[Haffiman]

Re: 78 115hp overheats new pump/thermostats

The deflectors are in the cylinder cooling channels and directs the water flow.
If 'defect', water may not circulate properly.
May even take a 'short cut' leaving parts without circulation.