Water Temp Reading Low
 

'93 5.7l 340k mi. I am noticing my water temp gauge is not going over 120 degrees. Motor appears to be coming up to normal temp as usual with nice hot heater, pressure on hoses and hot to touch, runs fine. All other gauges working as expected. I assume I need a new sensor? Where's it located and any tricks to changing this 20+ yr old part w/o causing more problems!

I'd replace the t-stat first.

I have a '96 GMC k2500 with the 5.7 Vortec engine. My gauge wouldn't come up past about a 120 degrees. I replaced the thermostat and it didn't help. The sensor you want to replace is on the front left side of the engine in the head near the first spark plug. It's brass so be careful.
I don't know if you got the Vortec or not. A regular small block has it in the manifold near the thermostat housing I believe.

I believe the thermostat is fine based on how hot the heater is and how hot the radiator hoses feel, and expansion tank goes to hot fill as usual. Was wondering if the gauge sender was the one down by the left spark plugs. Think the one up on front of the manifold is for the computer. When I unplug that one the motor instantly dies.

You really should do a more accurate temp reading, than by hand or judging the air output.

You can use a simple stick (A/C) thermometer.
Crank with the RCap off, let it come up to temp, then check for 185 deg.
Close system BEFORE you shut engine off.

Good idea SW - Put my trusty meat thermometer on the upper radiator hose and it reads 160 with the motor revved - while dash gauge reads 120. Sounds like it's time to try a new sender.

HA, a meat thermometer, cool :D

But given those may not be exactly correct, and, the space between the probe and hose, i'm thinking your actual coolant temp is probably right at 180 degrees. Which is ok.

Yes, i'd go for the sender too.

Does the new sender need anything on the threads, or does it need to be bare naked for proper function?

Depends, if it is brass, then nothing. If just metal, maybe some Teflon Tape. Sealing or not sealing the threads, shouldn't effect the accuracy. Even with tape, you can not avoid getting a ground.