Engine Temp vs MPG

[JanGeo]

I emailed Synlube this morning and found out that I am really doing well in the MPG department. My question was if my engine temperature was in the proper range for best mileage as it seems to like operating a little higher than 182F - seems the idle comes down a little at 186-188F but Miro says that the EFI computer uses a little extra gas injection to cool the engine when the temp gets a little high. Also the AC lowers FE by about 10mpg in Cali at least. Anyone ever checked engine temp vs MPG while monitoring Air Fuel mixture ratio?

[Matt Timion]

Originally Posted by JanGeo

Also the AC lowers FE by about 10mpg in Cali at least.

10MPG???

HOLY CRAP!

I'd be ordering a small electric fan right away :P

[95metro]

No kidding, 10mpg? That seems way too high. I'm certain you'd see your rpms drop and feel the engine start lugging at that kind of load.

[SVOboy]

According to my ecu, it leans itself out as the temperature get higher. I posted about this earlier with a picture of the graph.

[JanGeo]

According to my ecu, it leans itself out as the temperature get higher.

That is programmed into your ECU - is that the intake air temp or the engine temp? He was referring to the VVT-i engine in my xB and other vehicles with the same engine like the Echo Yaris xA - makes some sense as a richer mixture burns cooler - also said that it could be programmed out if someone hacked into the code - some on the Scion World want it done to prevent power robbing corrections when they hop up their motors.

[JanGeo]

No kidding, 10mpg? That seems way too high.

Well it would depend upon the AC compressor and the MPG that you are operating at - if you are only getting 17mpg then it would not be a 10mpg drop but if you are getting 50 then 10 is realistic - as the joke goes for the past 12 years of Geo ownership - that little engine couldn't run the AC. I already see a drop in MPG while coasting when I turn on the heater fan but that is when the ScanGauge shows 125mpg and it drops to 120 mpg so that is realistic. Funny but I didn't see much of a drop when I turned on the AC going 30-40mph however my compressor will not run without the blower motor running which I would expect would not be needed if driving fast enough to force the air without the fan on but it does not turn on the AC light if the fan is off so I guess the fan is required.

[SVOboy]

That is programmed into your ECU - is that the intake air temp or the engine temp?

Engine temp, air temp is similar but different. The correction factor is more severe except on the cold end of things.

[philmcneal]

.8 liters per second when idling at full warm temps

1.3 liters per second when idling at full warm temps with AC On

difference of .5 so almost a 40% decrease in FE...