Mercruiser Thermostat Issues

tank1949

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I have a 1994 Pro line with twin 5.7 Mercruiser (not original) that are sea-water cooled. Neither possess closed cooling systems. I recently installed new Sierra 142 degree thermostats each and one new temperature sender to starboard motor. The gauges are original, as best that I can tell. With outdrives hooked up with muffs, my gauges read 180-185 after motors heat up. This seem high. I thought 160-165 was normal, after they open? So, I inserted thermometer in-between hose and thermostat housing and tighten hose clamp around hose. This insertion would be very close to temperature sending unit with solid connection. The other end of hose connects to riser. I tested temperatures for both motors using this method. Either motor reads between 142-146 via thermometer. AND, the motors feel cool, regardless after running for several minutes each. My concern is whether gauges are calibrated correctly. My guess is that they are not. Both becoming defective seem a bit remote? The difference between the temperature at the thermostat housing and gauge seems too much.
 
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Re: Mercruiser Thermostat Issues

One possible cause comes to mind. Have you noticed if your fuel gauge is also reading higher than it should? If all three gauges are high, then the problem may be with the Instrument Voltage Regulator - either it's faulty or there are some bad connections in the circuit.
 

tank1949

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Re: Mercruiser Thermostat Issues

I don't have a clue to what you are refering to. Where would it be???? Fuel gauge seems ok. It never shows 100% full since it is trailered on a slight incline. Rest of gauges seem reasonable.
 
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Re: Mercruiser Thermostat Issues

Ok. First off, I don't know if your boat has the older style auto instruments that require a voltage regulator. If there is one, it will be located on the back of the instrument panel. It limits the voltage going to the fuel gauge and to the temp gauge. Some instruments may have an internal regulator.
See attached thread which discusses a fuel gauge problem, and check the final post. This thread only refers to the fuel gauge, but it helps explain what the regulator does. There are many other search results for auto's also.

Inaccurate Gas Guage readings - Boat Design Forums
 

alldodge

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Re: Mercruiser Thermostat Issues

Many gauges work off of different voltage levels. If your gauges are original the question is where the senders replaced, if they were not original what type replaced them and were the senders replaced? Different manufactures change things and since both gauges read the same it can be puzzling. As another retired guy mentioned it could be a regulator issue but it could also be a gauge/sender issue.

I would suggest determining what type of gauge you have, make and model. Then try to go to that manufacture and find out if they have their own system for determining meter movement. Then let us know what you find.

Exp: I have a boat which uses VDO gauges and it took me quite a while to determine which gauge I needed to replace a malfunctioning one.
 

tank1949

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Re: Mercruiser Thermostat Issues

What would be the correct voltage 12, 13, 14 volts??????
 

alldodge

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Re: Mercruiser Thermostat Issues

I'm not sure your 94 Proline will be using a gauge voltage regulator. Been looking all over the net and haven't found anyone using one in a long time. The old cars like ford used one which was set at 10V and Chrysler used one set at 5V, and other variants. If you don't see a something on the back of your dash which has 2 wires on, one coming in 12V and one going to the gauges your using battery voltage. Should be standard gauges which will operate from 12 to 14.4 volts.
 

tank1949

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Re: Mercruiser Thermostat Issues

I'm not sure your 94 Proline will be using a gauge voltage regulator. Been looking all over the net and haven't found anyone using one in a long time. The old cars like ford used one which was set at 10V and Chrysler used one set at 5V, and other variants. If you don't see a something on the back of your dash which has 2 wires on, one coming in 12V and one going to the gauges your using battery voltage. Should be standard gauges which will operate from 12 to 14.4 volts.


Thx to all. It turned out that vendor sold me sender not correct for gauages. They will refund and sell me correct senders.
 
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