Temp gauge on 1973 XR7 351C 2 Barrel engine shows extreme overheating but the car runs fine and has not had any coolant overflow. I Just bought the car, have no history on it and I am new to the forum. Any thought overflow

Does anybody have any thoughts? I have owned other classic cars and when they have indicated a temp this high they usually overheated and overflowed. but this did not happen with my XR7. Has any of you had temp sending unit problems or temp gauge problems on a 73 351C?

Welcome to the forum! You can get an infrared temp sensor fairly cheap and use it make sure temps are OK. The 351C block on my 70 runs about 200 deg F at the sending unit, and has a 30 deg drop from top radiator hose to bottom radiator hose.

The temp sending units you get at the auto parts store are not the correct range. So that could be your problem. You can usually find a NOS temp sender on eBay. Once I found the right one, it measured about 30 ohms to ground at normal operating temperature causing the gauge to read just above 1/2.

You don’t need an overflow recovery rank if it doesn’t boil over - at least that’s my way of looking at it. A lot of others on here use and like them.

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It is very common for the generic auto parts store temperature sensors to not read the same as the factory sensors ~ so you get guages showing erroneous indications.

After you verify that the car is not actually overheating, you might try finding a correct Autolite / Motorcraft unit on eBay to swap with the parts store unit.

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If the wire from the sender is shorted it will peg the gauge. The gauge tester is designed to troubleshoot this.

Ford gauges operate on resistance. The more resistance, the lower they read, the less resistance, the higher they’ll read. First step is to unplug the wire from the sender, it’s located on the front of the block, just below the thermostat housing. The wire should be Red with a White tracer stripe. When you unplug the wire, it should drop the gauge to below “L”. Now ground the wire to the intake or block and it should peg to above “H”.

If all of that checks out, then it’s very likely that your gauge is fine and the sender is at fault.

If you have a multimeter, set to to Ohms and check resistance between the sender terminal post and the block. You should have a value between in Ohms, usually below 100. If you have no resistance, then the sender is internally shorted and needs to be replaced.

The correct gauge sender is Standard TS-58

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Don’t leave the gauge grounded for more than a second or two or it will overheat and damage the gauge. The Desert Classics Gauge Tester is a great way to check out all the gauges at min, max, and 1/2.

The value in ohms you get from the sensor terminal to ground is highly temp dependent. Here is a chart I generated for resistance vs. temp for a 351C NOS temp sender:
75 deg 300 ohms
90 deg 210 ohms
100 deg 180 ohms
110 deg 150 ohms
120 deg 130 ohms
130 deg 107 ohms
140 deg 91 ohms
150 deg 77 ohms
160 deg 65 ohms
170 deg 54 ohms
180 deg 46 ohms
190 deg 39 ohms
200 deg 33 ohms
210 deg 28 ohms
212 deg 27 ohms

*These measured resistances are likely a few ohms too high since the water temp was changing faster than the thermal inertia of the sensor. I didn’t take the time to let the temp stabilize at each reading.

Those numbers are way off, don’t know why it measures that way. The reading at 192 degrees should be 23 ohms to 25 ohms

My data is for a 1970 NOS D0WY-10884-A (Autolite SW-924). WCCC site says that sensor was used for 351W and C through 1973 and should measure 23-26 ohms at 220 deg which fits my curve.

For 67-69, WCCC shows a common sensor for all engines that measures 15-19 ohms at 220 deg. This must be the sensor you are thinking of.

With the correct 192 deg Robert Shaw thermostat, coolant temp of my 351C is 205 deg out of engine into top of radiator on a summer day. At that temp, my temp sensor measures 30 ohms which puts my temp gauge right at 1/2.

Your gauge tester gives me the following on my temp gauge:
L - bottom white line
M - 5/8
H - top white line
So IVR doesn’t seem off by too much?

Like tkapron reports, my car never boils over even idling on a 100 deg day. But my temp gauge will sometimes go to 5/8 which is 23 ohms on the sensor and 220 deg coolant temp. At 12 psi, a 50/50 mix of antifreeze shouldn’t boil until above 250 deg.

My 351C did used to boil over before the block was rodded out, and with original 20” 2-row radiator, water pump, and 5 blade fan.

Yes a ground issue will definitely peg your temp and fuel gauge.

One thing I didnt see mentioned, your 351C has a SPECIFIC thermostat, nothing your local parts store knows of or carries. Worth inspecting

Yes, the 351C needs the Robertshaw (formerly Dole Valve Co who also made the ported vacuum switch on the water pump) high-flow thermostat. It has a solid disc that extends downward when it opens, and plugs the large bypass hole in the engine block casting. Without it, your water pump is recirculating a lot of coolant through the engine rather than pumping it through the radiator.

Like CatsRock said, your local auto parts store will happily sell you the wrong thermostat like every other engine uses. And it will fit just fine and open like it’s supposed to, but very little coolant will go through it to the radiator.