Temperature Related Anomalies

[hawk2100n]

Hi everyone, I am beginning to suspect that I am having problems with either my IAT or Water Temp sensor or a combination of the two. I make a 65 mile one way commute about 3 times per week and depending on the temperature I see wild fluctuations in observed MPG. Prime example, yesterday the ambient was roughly 70 degrees and I averaged 42 MPG at 70. This morning it was 43 and I was returning 33-34 at 70-75 on the same road. Winds were light and variable and traffic wasn't an issue. There is no significant elevation change and I have been able to record 40+ MPG in this direction with warmer weather. I know cold has an effect but 20% indicates a problem to me.

The evidence is attached...
fIA 60
fWT 175

I have never seem my fWT go above 180 and I know this is low. Before I dive deeper and invest in a new thermostat and temp sensors can anyone tell me if this is normal for D16s? Or do I need to get coolant temps back to 210-220 like most cars? If the car always believes its cold then it's logical that it would always try to warm up with extra fuel. The computer hasn't thrown any codes in 20k miles.

[hawk2100n]

[user removed]

Thermostats will fail in the open position. Find out what the OE thermostat temp range was and use a good quality or OE thermostat (no chinese crap). Probably a 195 degree stat. At highway speeds you should see operating temps above the thermostat opening temp. I would avoid a higher than original thermostat opening temp, which could contribute to a head gasket failure.

If you replace the thermostat, test for mileage before you do any other repairs, unless you know there are other maintenance issues, especially with no CEL.

Also consider a warm air intake to bring up the incoming air temperature for better atomization, also a grille. or radiator block if not already done. Cold coolant leaving the radiator and entering the engine will kill fuel mileage.

regards
Mech

[mcrews]

my understanding is that engines are designed to open and close the thermostat. SO if it's not getting above 180-185 then the thermostat is already open as Mech suggested.

[hawk2100n]

I suspect it might have a cold 170 deg thermostat installed. 180 is the OE and 190 and 195 are available. I'll think about it but probably a 180 will be a wise choice. Thanks for the input!