Temperature sender

I’m sick with a cold & don’t feel much like climbing under the car to finish the transmission mounts so this was what I made today.

temp sender9

The thing at the bottom is a 5/8-inch hose splitter from AutoMeter, designed to allow one to put a temperature sensor in-line. The problem? It was tapped to 1/8 NPT (national pipe thread)–the size that works perfectly for modern idiot lights and electronic gauges. But I intend to use an old-school “bulb” sender and that needs a 3/8 NPT hole.

The bulb thingie is also like 2 inches long–fine for installation on a 350 Chevy manifold, but way too much to jam inside a 5/8-inch pipe.

What to do?

Well, first, I guess I need to say why I’m doing it.

Turns out the Subaru EJ22 engine is made with a strange kind of thermostat system. Instead of installing the thermostat on top of the engine, where the heat would be hottest, and allowing said heat to trigger the evacuation of too-hot coolant from the engine, Suby engineers placed it on the bottom, and set it to allow the coolant to enter the engine.

Fair enough I suppose.

But maybe not, because then the Suby guys rigged up a system whereby the heater core water is piped back down to a spot just behind the thermostat. The idea is that heater core water, which comes out the top of the engine, would be used to trigger the thermo. And so in Subys the heater core always has plenty of coolant flowing through it.

That makes the temperature in that heater hose the closest thing to what’s going on in the engine…that is not in the engine. It’s also a key metric for anyone wanting to know what the thermostat is feeling.

Aaand since my engine is going to be placed five feet behind the dashboard instead of two feet in front of it, the heater hose–extended four feet and brought through the floor up under the dash so it can be piped to my new heater, also just happens to be the most convenient place to tap-in a secondary* temperature sender, whose un-moddable ether-filled springy line is in no way long enough to reach the engine from the dash anyway–save maybe with a straight shot past the occupants’ shoulders which would look fairly silly.

So that brings us back to the problem of the too-small tapped hole. Auto Meter does sell a reducer to get us to 3/8 inches (that’s the red thing). But that looks like more of a hole than this little brass barrel can comfortably take. I also don’t want the temp bulb reducing flow.

The answer: a 1/4 inch adaptor, and a tap to make my 1/8-NPT into a 1/4 NPT. Plus the inevitable 7/16 drill.

temp sender1temp sender2temp sender3temp sender5temp sender7temp sender10temp sender11

With this rig, my temp bulb will get to within a half inch of the line, with coolant flowing past it and (hopefully) passing the heat to the sensor about as well as it would anywhere else in the system. But it won’t be impeding the flow one little bit.

So now, hopefully, I can has a nice, accurate read on the coolant temp and the gauge will look and work like something you might expect to see on a proper old British car.

*Yes, I am leaving the Suby original in its original location and wiring it to a light on the dash.

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About stuntmidget

I'm a poor mechanic and general wisenheimer. I love old cars and the stories behind them, true or not.
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