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Discussion Starter · #1 ·
My vintage 1978 raw water cooled 3qm30 engine runs fine. However i have a growing concern about operating temp. Maybe I am too anal about this but my otherwise reliable vdo guage has begun showing temps in the 190 range when after couple hours at 1800rpm. It has always been my understanding that RWC runs closer to 140 as this the temp salt drops out. So with guage reading 190 I have checked engine by hand and also heat gun. Max readings anywhere on head or temp guage housing is about 138...call it 140 or pretty much perfect. As added confirmation I pulled both thermostats and tested them. They open at 140 as well. But staring at a guage telling me 190 just bugs me. I have two senders the second one being the on/off switch type from yanmar that has tested working and should go off at 165. It feeds the alarm. The alarm works but has never gone off. So the engine seems to be operating properly but it just bugs me that the main guage consistently reads 50-60 over. Should I live with it or dig in to solve? If dig in where should I begin? Both the guage and sender are at least 15 yrs old and have always been off for the 4 years of my ownership. I suppose having the proven temp gun as backup and maybe marking the guage somewhere to indicate red zone might calm concerns but it would be even better to have an accurate guage. Thanks in advance for opinions.
 

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One of None
Hunter 34
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Replace the gauge and sender they're cheap enough,
 

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Discussion Starter · #3 ·
Yes I suppose that would be the easiest way. But part of me likes to understand why something is happening before I attempt a fix. To go all new would likely fix it but I would not have gained the knowledge of how gauges and senders work together. I am actually fairly clear on gauge function but I cannot say I understand how the sender works. I understand the sender is about resistance and that can be measured in ohms but how it communicates with the gauge escapes me. Maybe I need some more youtube time :)
 

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One of None
Hunter 34
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Temperature gauges work on electrical Resistance to temperature, the needle on the gauge is showing you resistance, they're just putting a temperature number next to it instead of ohms, to know the specifics of the resistance in a sender, most manufacturers have a chart showing the resistance at different temperatures,

many of the sensors on automotive applications are simply the same thing except the computer is reading the resistance not a guage.

So you need, at the very minimum a multimeter, then you can learn how to check circuits open, closed & how read resistance.

mechanical gauges work on pressure.
This should help:
https://www.autozone.com/repairguid...mperature-Sensor/Testing/_/P-0996b43f8037e980
 

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Get yourself a direct read infrared thermometer and simply follow your hosing and cooling components to develop a profile of the 'heat soak' of your engine after running for a goodly amount of time.

Start at the exhaust outlet and simply measure each individual component of the cooling system ... and record. Any blockage, failing thermostat will show up as a significant change in temperature.

The 3QM has its temperature sensor located near the front of the 'valve cover' ... and that place is where you do a quick check of the engine temp. with an infrared direct read thermometer on a 3QM.
These temp sensors can go bad quite often ... and all the wiring back all the way to any gage readout & sometimes needs to be also replaced. For the temp 'sender', there are several thread configurations (some are tapered threads some are straight thread connections, so if you suspect the temp. sender is at fault, you should remove it, check thread profile, etc. and not blindly order from a Yanmar dealer.

On a 3QM engine the chief suspect for gross overheating is the development of 'slab rust' inside the exhaust manifold wherein the loose slabs will constrict the water passages. If this is the case, be VERY careful in removing the internal rust slabs from the inside of the manifold ... as these old manifolds are very RARE to impossible to find from a dealership and usually need a specialized cast-iron metal-magician to repair/'rebuild' ... especially when there are 'pin hole' internal leaks of the manifold ... and should be periodically checked by gentle 'pressure testing' of the 'water side' of the manifold.
FWIW --- to preserve the old manifolds, never ever drain the manifold and leave it 'dry' for long term lay up. Always fill with ethylene glycol antifreeze (including rust inhibitors) and 'well boiled' distilled water when long term storing. "Running the hell out of such a cast iron engine" will keep a goodly layer of 'black rust' inside the water passages ... the black rust is 'protective'; but, will turn gradually over time into destructive red rust if you dont store with antifreeze and rust inhibiters when storing the engine long term.
Hope this helps.

https://www.amazon.com/Temperature-...d+thermometer&qid=1566587821&s=gateway&sr=8-3
 
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