I have a 88 Camaro LB9 EFI IROC F here in Australia. Car was Calif specified export to Japan. I had a O-ring fuel rail failure with a fire resulting in an engine rewire. AIR, EGR, intake heat removed. Cat is hollow with Cat-Back exhaust, K&N intake filters and Crane 6HI ignition. ECU Prom is labelled ACXS (assume $32) appears stock and have not yet read the bin.
Problem: Engine is very COLD, at 75F daytime engine temp is typically 135-140F, at WOT I can reach 160F (160F thermostat fitted). On hot 105F days I read 160F engine temp after 5 mins city run with some fan action (fan on at 175F). Coolant is water with inhibitors, Timing is stock 6deg BTC, O2 sensor reads .2-.3vdc at (idle/low rev) at 160F. MPG is 10-11 and I assume ECU runs CL often. No red light when running, shop fault code scaning did not indicate sensor failures or so I was told?
Laptop logging with Tunerpro is underway, however any creative suggestions are appreciated.

 

Didn't see thermostat degree info. I've seen two "bad" thermostats over the years. One time my blazer had a piece of the wire that is in the bottom radiator hose break off and get stuck in the thermostat causing it to stay open. It would get up to operating temps (190 F) when running down the road but would begin to cool off almost immediately at a stop light or when slowing down to the 35 to 45 mph range. The other time a thermostat seemd to be "bent" and it would never fully seat which allowed coolant to bypass it all the time.

Have you confirmed your ga. is correct? If the fan is coming on at 175 F but the ga. never gets that high something doesn't agree there.

I've been told that when a computer reads below normal operating temps. it will supply more fuel (stays in the warm up phase).

 

I'd agree that the fan should not be turning on at 175°F. If the PROM is stock, the fan temps should be more like 223°/218°F.

My initial guess is that the dash guage is wrong. Either the guage itself is faulty, or the themperature sender in the LH head is failing. If you can scan the ECM data you can compare the CTS reading versus the temp guage. You can also measure teh temp send resistance at a known temperature:

 

I have now jumped at many possible fixes and will now look at the basics. The gauge is fine, checked againist a DVM temp probe and in very hot weather I can see the 160F stat hold at 160F on the car gauge. The ECU has no errors and running at below 140F the ECU is at closed loop for sure, gas around ratio 12.7:1. The fans are currently controlled by a external thermo-switch until I learn more about writing bins to eprom. I noticed what appeared to be a washer loose inside the AC/heater temp control valve.....could be a flow resrictor out of place, who knows?
I,m also using clean water/additive for cooling not glycol, my experience with Hotrods indicated that water is better for heat transfer.
Anyway today I'm putting in a 180F thermostat and a 1/8 dia. flow restrictor plate at the radiator return inlet from the heater bypass loop. Heck I'm retired and this is fun right. 1:10 PM

3:40PM Ok. done it using 180F stat and a 3/16" restricter, reset fans to start at 195F. At idle with the fans set they were on at 195 and off at 120F which is a big bandwidth. A short road test and I was still reading a high of 185 at ilde and when underway it dropped to 120 or so and held till the next idle situation. The fan works too good and looks like the restricter is a bit large?