I have an 89 iroc with a 350 tpi. My temp gauge reads hotter than what the temp really is. I have read alot of posts of other people having the same problem even though they know that they are not overheating and all systems are working fine. I can compare my gauge engine temp with my scanner which shows engine temp and see the difference. The difference is about 30 degrees hotter on the gauge than the scanner. My primary fan kicks on around 230 on the scanner, but my gauge is reading almost 260. What could be causing the problem for this much difference? I have tried 2 different temp sending units(the one in the driver side head) 1 of them being acdelco with no change. Tried a different temp gauge and still the same result. The gauge reads correct to around 160 or so but after that, the gauge starts climbing ahead of what the scanner is reading. I've heard that these cars got crappy gauges, but they had to be good enough and close to accurate for GM to put them in the cars. Read about and seen alot of other irocs with there temp gauge reading correctly. I'm just trying to track down this problem. Is it a wiring issue, gauge problem, sending unit problem? Since the gauge works by resistance from the sending unit, could it be a resistance problem with the wire that comes off the sending unit going to the gauge? I've did alot of searching about this and read other posts about the same problem, but a fix for the problem seems to get lost as more people reply to the post.

I also have a temp gauge inaccuracy. I swapped a V6 for a 1972 350 and cannot get the temp gauge to work. All it reads is zero. It worked with the V6 in it and I have tried two different sending units. Do I have the correct wire? What color would the wire be? If anyone has any suggestions it would be greatly appreciated.

Try grounding the wire on the sending unit. When grounded, the gauge should peg. That will atleast tell you if your gauge is working and if its a sending unit or wiring problem. The wire should be a single brown wire, but since your car is an earlier year, it could be a different color, but mine is brown. I'm not forsure, but have you tried and gauge from a v-8 car. v-6 and v-8 gauges might be different, but I am not forsure. Also, are you getting a sending unit for a v-8 camaro for your year and not for a v-6.

The temp gauge wire should be dark green, and actually gets grounded in the bulb test & start position of the key.
Any resistance in the wire would only make the gauge read less, in particular at high temps. So your problem is the opposite.
Try measuring the voltage between engine ground and chassis while the engine is running. Anything substantial there will affect the gauge accuracy, esp. at high temps.