1990 Chevy g-20 w/5.7L -
Recently replaced the IAC because the idle was up and down. solved the idle issue then I noticed the temp gauge seemed to vary a lot more than i have been used to seeing it do.
Typically, even after driving awhile, the pin would sit on the second hash mark; hardly ever moving beyond it. kinda thought this was weird since the second mark signifies 140°, which seems like a pretty low operating temp. So I decided to replace the coolant temp sensor, the one near the Throttle body, and the electrical connector was totally disintegrated and wasn't even attached so swapped that out and replaced the thermostat at the same time because it had been awhile (spring facing down). and around this time I cleaned and repaired a bunch of chassis to engine ground for good measure. The radiator is new and coolant is new. did a flush not too long ago. have no reason to think it's not circulating properly. Heaters blow hot.

Yesterday I was in some standstill highway traffic which will makes cars run hotter anyway but the gauge was getting way up there. at times it would climb PAST the 3rd hash mark, 75% of the way to 260°... If i'm reading the gaige right that would be 220°. so I would blast the heater and this would keep it just below that (see pic); once i got moving again the gauge would chill out and lay mostly on the middle hash, pointing straight up. It was only in the mid/low 70's yesterday; texas hasn't even begun to get hot.I'm pretty worried about what this will do when it's 110 and i'm stuck in traffic...

The Question:
Am i just being paranoid from being so used to seeing faulty gauge readings for so long
-OR-
is the engine temperature being erratic and getting WAY TOO hot at times?
-OR-
Something else?

I had a dodge ram that had a bad thermostat and overheated once. everything after that moment was hell and I've come too far with this old gal to let anything like that happen again.
I still have PTSD from all the times that van exploded hoses while i was 1000's of miles from home.

the pic isn't the worst that it got...

 

The temp is a bit high.
The water pump could be worn out.

 

yeah that wouldnt surprise me... It's prob 10+ years at least.
Bought a Coolant temp Switch to install tomorrow. Hope that it does the trick; maybe the old one is sending bad signals.

 

I am not a mechanic, 30 year old vehicle I do not trust the 30 year old gauge and sender. I will check the sender temp using what ever I can, usually an IR temp gun or a multi meter can be used. I have never found a bad sender or gauge! Google test car temperature sensor.

 

Problem Solved! Drove around in slow traffic and the highest the needle got was to barley uncover the 25% mark.
Now here's the incredibly embarrassing part...
I decided to test the fan clutch with a few spins but it didn't want to spin because the fn upper water hose was jamming it!
When I replaced the thermostat I remember pulling the upper hose toward me and pushing it up to get it out of my way. Cleary it got stuffed under the fan shroud when i went to reattach.
I never bothered to open the hood for this repair, did it all from the inside.
Lucky the hose didn't get shredding. yes i know... total stupid noob move.
When I was young and there was any kind of problem my father used to say "It's probably something you did"
I STILL hate it when that d**k is right.

 

My fault auto repair 101.

 

HAHA, yup . One more question though: Went ahead and replaced water pump, fan clutch and coolant temp switch anyways; now the temp needle fluctuates somewhere between the 25% (140°) and 50% (180°) hash marks when at full operating temp... am i good to stop worrying about the temp of the engine and get on with my life?

 

Per the owner's manual, operating temp spec for the 5.7L is 195F.

140F is way too low. The thermostat shouldn't even open until 188F.

 

Since I've replaced everything having to do with the temp control (radiator, thermostat, both coolent sensors, fan clutch in the last few days what do you suggest?" Could it reading low be the symptom of an old gauge?

 

Of course it could.
But I suggest more diagnosing. Not more guessing and firing the parts cannon at the problem.
Did you try jjrbus's suggestion to actually measure the temperature, to verify that the gauge is reading correctly?

There are still many possibilities that you need to narrow down.
Go back and check your work. Is the coolant full? Did you burp all the air out? Does the thermostat work correctly? Is the sensor producing the correct resistance?