Before throwing cardboard up front check a few things.

Have the engine temp relative to the ECM's sensor compared with the gauge. GM gauges are notorious for going goofy and reading off.

No scanner to connect get a temp gun and run the engine up to temp, when the gauge reads 150 shot the temp gun at the top of the T-stat housing. If it's hotter then the gauge the gauge is off.

Back to the scanner use looking at the ECM. If the ECM shows normal (190-210) and the gauge is low, everything is fine except a misreading gauge.

My reasoning for thinking the gauge is off is you mentioned the fan kicks in at 175. No GM fan that I can recall kicks in at 175 unless the ECM/PCM has been chipped or reprogrammed.

Has it been chipped?

 

I'm up near smokey point. Many Many miles north of Oly...I wouldn't use cardboard anyway, too wet here. Maybe like perforated plexiglass so as to allow some air, but not as much. You are absolutly right, it should not be that low cruising, I do need check my gauge for proper readings. LOT TO READ BELOW

Actually I have the USB ALDL cable. I may have typo'd the fan kicks on at around 185* on the gauge readout. If anything I'd lean towards a bad temp sender. Nordskog digital dash....Though when I do turn the car on my gauge reads about 51*F, the aldl says 65*F. I have much to learn about this car. Its not typical and I didn't build it...second time making that mistake...The main issue is while driving down I-5 the gauge reads around 145-150 and the heater is weak. City driving is like 165 w/ ok heat, stop and go gets up to 185 and heater works great.

Had a fun little vehicluar meltdown...On the 10th I did a basic tune up on the car, and drove home from my bro's shop. Ran ok, still needed tuning, check the timing, and the timing marks broke off. I have no idea what my timing is now! When doing the SP wires, I had him pull up the wiring diagram for the 5.7l Corvette and he gave me the diagram for a 4.1 vette w/ reverse dist rotation...that was fun. After figuring that part out there was a loud metallic crack/pop/tack sound. Not three sounds, just one combinded sound. After that mishap my starter now sounds like its trying out for the 40yd dash. Thing spins loud and fast. Got the plug wires fixed. Monday I go to run some errands after tweaking my AFPR back to 46psi, about 1 or 2 miles into my trip I have massive loss of power, engine RPMs go up but the car will not accel. Turn around head home. Back in the garage odd smells, sounds and light smoke. Pop the hood, get out look under the car..Cat is glowing bright orange. Wire on Cyl 6 was resting on the header and burned it through. Cyl 7 wire melted the boot completely off. Cyl 5 wire shot off the plug with burn marks too. That and the drive home was pretty damned loud as other cars breezed by me I could hear the exhaust just sputtering from my car as they drove by.

It sucks to be plagued with so many issues and the inability to rectify them all at once. Now my new set of wires are toast. And yes I used the stand offs, just cheap wires and hot headers.
That by far is my longest post. Lot a stuff happened over the weekend...

 

about 20% off then. So at 210F you'd be around 168F on your dash....if the gauge reads 20% off all the time.

So 190F which is a nice running temp would be 152F if it's 20% off.

 

I know it would be, but it only seems to be off when the motor is not running. According to WinALDL my gauge is reading an average of 12* less than the CTS. I'm going to log some more specific data and try to corrilate it with my gauge and with a laser temp gauge at each of the sensor locations and the top of the T-Stat housing. Hopefully I didn't damage anything more with my latest exhaust problem.

Would be a pain in the @$$ if I were actually near overheating everytime as opposed to underheating....That and I plan on checking my heater core. Just for good cause.

 

**Update** Checked my coolant temp via Winaldl. And low and behold...my gauge is wrong. At about 175 on the gauge, the vehicle is actually at about 210. Looks like before I was looking at the raw data and saw that the raw data and my gauge were not that far off. The converted side though is quite a bit off. Cruzin down the freeway at 65mph has my wacked out temp gauge at 145* and winaldl shows the temp sensor reading 189. As for the heater....with these numbers, I'm willing to bet there is a clog in the heater core. I'll give that a flush this weekend and see what that happens.

Thanks for all the help. Now to address some other issues in a new post.