Have a stock 8RT flathead in an F6 that is indicating HOT.

Baseline:

*Obtained vehicle with 5,000 original miles about 10 years ago.
*Had engine rebuilt about 8 years ago. Now has 15,000 original miles.

In the past two years I have:

*Radiator professionally re-cored and rebuilt.
*Rebuilt water pumps
*Replaced thermostats (180°F 160°F)
*Rebuilt carburetor and replaced leaking economizer valve (Ford 7RT or 8RT (cannot recall immediately) Holley 94)

Now for the issue: She's indicating hot. I just replaced the 8A-10884 temperature sending unit with a used (original) one I picked up along the way (don't know for sure it is good, but I've heard not good things about the aftermarket ones available today). Unfortunately, still indicating hot. From a cold start to 5 miles down the road, she almost maxed out:



How do I sort this out?

P.S. - I am very aware of the left and right side temp units and King-Sealy communication protocol differences from the standard resistance/ohm setup.

 

 

I ran into the same issue years ago with the aftermarket sender for my 223 I6 and I too question the reliability of the aftermarket senders. I went with a mechanical gauge under the dash. I also have a 46 Ford coupe with a Flathead and use a pair if mechanical gauges as well. In my opinion, it's good peace of mind when you can see the number readings. Steve

 

I have now clarified that the used replacement is of the original style and not a modern aftermarket.

As for JB's suggestion, I am planning on getting a HF unit in the morning. What should the expected temp be? 180 (Thermostat setting), 190? 220?

 

 

I think you are running right where I'd expect with 180 thermostats. Switch to Stant 160's from NAPA and you'll be center scale.

The thermostats ratings are the cracking temperature, not the running temp. With 160's my temps at the thermostat housings is 175 - 195 depending on outside temps. My gauge reads dead center.

 

Is the engine blowing coolant out the overflow? If not I would agree with using an infrared heat gun. They are not expensive and you can get one in a couple days with Amazon Prime. Here is but one of many that they list:



.

 

Some followup from today. Went to the store and got a thermometer and 160 thermostats. Drained the coolant and removed the existing thermostats to find that they were 160 (bad memory?). I made sure they were installed spring down into the block. Filled back up with coolant and let her fast idle in the driveway for 15 minutes. Pointing at the heads where the water outlets are got to 170-172. Was the same on the heads where the temp sending units go. Top of the radiator was about 160, bottom 135. Gauge was hovering around at the mark under H, not centered as I would expect. Thinking about replacing the wiring for the sending units...

 

You can adjust the gauge unit. It has a toothed arm accessed thru a hole in the back of the unit, that you can make adjustments with. If you remove the unit from the dash, and connect a 1.5v battery across the gauge, it should read half-scale. If it doesn't, make a small change on the toothed arm, and try again.