First I checked all grounds. Those were sound. Then I measured output voltage on the temp gauge sender wire (single red wire connects to the sensor on the lower intake manifold). I got 10.14 volts, which is correct. I also had voltage at the gauge (10.14 volts). I tried doing the 19 ohm resistor and 250 ohm resistor tests as noted in the manual, but got absolutely no readings. If you get zero readings, the manual says to replace the gauge. I then tried two other gauges and got exact results. I was thinking that the sensor wasn't getting hot enough (it was a brand-new sensor) and since the engine cooling fan wasn't working either, I figured it had to be a clogged coolant passage.
I started to remove the lower intake manifold (LIM) to clean it out but chickened out when I couldn't feel two of the lower mounting bolts on the manifold. Also couldn't get one of the fuel lines disconnected (takes a special Ford tool, says the shop manual). Put it all back together and vowed to take it to a mechanic. But after thinking about that for a while (didn't want to face Dave Planakis and admit I couldn't fix a stupid inoperative temp gauge), I decided I could at least remove the coolant line going into the T-fitting on the LIM and see if it was clogged. A bit of a bother, but I did that, draining the coolant directly from that line into a bucket beneath the car (had the car on drive-on ramps). It was immediately apparent that no coolant was going into the LIM (and therefore not heating up the temp sensor or the fan sensor); the fitting was stopped up where the line connected. I then rotated the T-fitting about a half turn out (better to fit a socket onto the purple-ring sender; this is the sensor with a single, red wire connector) and removed the fan sensor first (otherwise, the gauge sensor interferes with the TFI connection. Doing this was easier than removing the TFI). Then simply unscrew the T-fitting (takes a deep socket; 19mm I think; same socket unscrews the purple-ring sender). I found the T-fitting chocked full of grayish-greenish gunk. Cleaned that out, flushed it with a water hose until sparkling clean. Then I used a small screwdriver to rake more of the grayish-greenish gunk out of the LIM. I also shoved a small plastic tube up in there and rotated it around; even more stuff came out. Then I used a squeeze bottle (like ketchup comes in for picnics) to squirt warm water up into the LIM. More gunk came out. Kept doing this until only clear water came out (about four-five bottles worth). If you did this outside, you could simply squirt a water hose up into the LIM.
I then removed the engine cooling fan sensor (on lower back of LIM; has a multi-wire connector). Difficult to get to but eventually it came out using a 25mm socket and socket wrench. Cleaned and flushed that hole also, but it goes into the LIM so not much gunk came out of this hole.
Replaced everything, hooked up everything, poured the coolant back in, cranked the engine. After the engine warmed up, the temp gauge worked like a charm, needle staying at about the 4 p.m. mark.