Vette has been hovering around 24MPG in mixed driving. Yeah, I know "drive the damn thing and don't worry about mileage", but it's one of my OCD triggers.

I recently had some work done on it (some day I'll calm down enough about that to pitch a bitch that won't melt your screen...) that included a new thermostat. I noticed that the water temp gauge now sits about mid-range where it used to run about 1/4 of the way up from cold.

Last two fill-ups have calculated in the high 25's. Wheee!

My thinking is that the engine running hotter allows the ECU to adjust the mixture (leaner?) resulting in better mileage.

Or maybe it's all a coinkydink.

Thoughts?

I would agree with you. Like mileage dropping in the winter months.

Agreed.
I had a '68 Camaro that showed the temperature at about a 1/4 on the gauge. When I put a new higher-temp thermostat in, it allowed the motor to get up to proper operating temperature and it ran much better after that.

I’m actually surprised that your gauge shows differently. Because of OCD people most gauges are no longer accurate and are typically designed to stay in the straight up or straight horizontal position as long as it’s in the normal operating range to avoid a lot of the unnecessary something’s wrong visits.

Of course the type of driving factors in heavily. Just a check or 2, 1 MPG difference, could be part wishful thinking.

Not trying to be a downer, with more checks it may be evident.

My truck averages from 15 to 21 on the ‘last 50 miles’. It all depends on the driving conditions.

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My thinking is that the engine running hotter allows the ECU to adjust the mixture (leaner?) resulting in better mileage.

You're most likely right. Every EFI system - or at least those back then - richens the mixture for cold starts and running. If the old thermostat was stuck open a little bit, not fully closing or opening early and the engine was running too cool before, that would explain it. On a post-1995 car with OBD-II, slow warmup would have triggered the engine light. You should also notice an increase in the heater output.

Corvettes with EFI get surprisingly good gas mileage if you take the brick out your right shoe. Because of tall, top-speed-oriented gearing, the engine is loafing at 2000 rpm or less on the highway and they are streamlined.

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A functional coolant temperature gauge isn’t shocking on a C4; the C7’s even has numbers.

I’m surprised that a Corvette doesn’t have a temp gauge that gives a readout in degrees fahrenheit.

Not sure what year vette you have. But yes, on some of them if the engine is running too cold, the ECM is in closed loop and runs off of a predetermined fuel/air ratio on the rich side. Once if reaches a certain coolant temperature it switches to open loop and reads the EGT sensors and other parameters and adjust fuel and timing to match what it's reading.

quote:

Originally posted by jimmy123x:
Not sure what year vette you have.

1990 with TPI.

You should have a 195 degree thermostat, your car was designed to run that temperature and your mileage will improve vs running a colder thermostat such as a 180 degree.

quote:

Originally posted by jimmy123x:
Not sure what year vette you have. But yes, on some of them if the engine is running too cold, the ECM is in closed loop and runs off of a predetermined fuel/air ratio on the rich side. Once if reaches a certain coolant temperature it switches to open loop and reads the EGT sensors and other parameters and adjust fuel and timing to match what it's reading.

I'm sure jimmy123x inadvertently reversed the words OPEN and CLOSED above.

Vehicles run for a very short time in a predetermined rich mixture, and that operation is called OPEN loop. In the mid 80s this was based on coolant temperature, but today the ECM is far more sophisticated. Decisions are made on temperatures of O2 sensors, catalyst, etc. OPEN loop is bad for emissions, bad for fuel economy, etc. So, modern vehicles go from OPEN to CLOSED as quickly as possible. Closed loop operation ensures "stoichiometric" air-to-fuel ratio, and is adjusted many many times per second through a "short term fuel trim" calculation, then over time, a more global "long term fuel trim" adjustment is made.

Now, if your car is old enough, has an ECM and your thermostat were defective, and it was stuck OPEN, your engine would run cooler than desired. The only way a thermostat sticks open is for the wax actuator seal to be compromised, and for coolant to be sucked into that wax reservoir. The coolant, being liquid and incompressible, essentially jacks open the thermostat and believe it or not, a thermostat is "open" when the disk moves 0.003 inches. At that distance away from the flange seat, the curtain area of the valve will flow sufficient coolant to keep the engine running cool.

But, your thermostat seemed to be controlling to a lower temperature, so as others have surmised, it might not have been the correct temperature thermostat. If that temperature makes the older ECM think you are not fully warmed up, then it will command the richer than necessary air-to-fuel ratio.

The correct "stoichiometric" air-to-fuel ratio is 14.7 to 1. In cold rich operation, it could go as low as 12 or 12.5 to 1.

If the new 'stat is getting your engine up to temperature, then the air-to-fuel ratio might be 14.7 to 1, and your fuel economy would increase.