@GeorgeSanJose How/why does coolant temp have an effect on mph? (In layman’s terms, please.)
My understanding is that when the engine is cold, the computer keeps the RPM’s higher to compensate and to warm the engine up to run more efficiently.
Now, we always jump on tire pressure, but I was looking at the new car edition of consumer report and in their test even pressure of around 20 PSI caused only a small percent drop in gas mileage.
How/why does coolant temp have an effect on mph? (In layman's terms, please.)
When the coolant temp is lower than normal operating temp or the temp sensor is sending a lower temp signal the ECM/ECU will think the engine is not at operating temp. The ECM/ECU will add feed more fuel to the engine to make the mixture rich. And will also raise the idle to get the engine to operating temp. quicker. On carburated (sp?) engines this is done by the choke plate restricting the amount of air (choking) and dumping more fuel to get the rich mixture to run a cold engine. Also will have a whats called a fast idle cam or some such method to raise the idle. So the short answer is cold engine equals higher idle and richer fuel mixture equals more fuel burned equals less MPG. I hope this helps.
Also, when the coolant temp is low, so is the oil temp so its higher viscosity saps away a little energy.
A little more combustion heat (and thus pressure) is lost to the cold cylinder walls during the power stroke.
Smokey Yunick did some research running engines at high block temps to raise efficiency.
Running an engine at 300F would pose a lot of challenges.
A layer of insulation would be needed to keep under hood temps reasonable.
High grade synthetic oil. No rubber coolant hoses, gaskets or seals…
Well the guys here have provided you with AMPLE Ideas for what would cause this issue. They pretty much covered it all. My FIRST educated guess is that whomever changed your oil did NOT use the EXPENSIVE FULL SYNTHETIC Motor Oil in your engine. This is first and Foremost. Then you can factor in Cold Weather/Fuel Blend issues with “winter fuel”. But right off the bat…find out what oil is or was put into that engine. Your vehicle is NOT a farm tractor…and these days the mfg recommendation for oil…is more like a DEMAND…Its HARDLY Recommended. A specific oil weight of 0W-20 IS LIKE WATER…which provides lubrication as well as very low friction coefficient. This CAN and WILL mess with your mileage…esp if the engine is overfilled with the wrong weight.
Methinks your issue lies right there…
Blackbird
jtsanders Since mandated E10 and winter formulation 10% lower mpg is my average too. At least this winter the lower prices plus unusually mild tempuratures have offset that quite nicely.
There are winter temperatures, and then there are…winter temperatures.
When the average daytime temps were in the 30s, my gas mileage fell off by only 10%
However, now that the daytime temps have been in the 14-18 degree range, my gas mileage has fallen off by exactly 20%.
@PvtPublic Thank you!
The experts above answered @Marnet 's question about the effect of engine temp on mpg admirably. Thanks.
@sgtrock21, you must live out west. It’s been very cold this winter on the right coast.
Not sure where Marnet lives, but it is 70 degrees here in San Jose.
I live in the St. Louis area. Yes, several of you experts generous with your knowledge answered my question admirably. Thank you all very kindly. I am having no problem, unlike the poor OP; I was simply quite curious to understand. Years ago my car of the time had a broken thermostat and I recall it took forever to warm up and that the mpg was horrid while the problem lasted.
Your trip meter that shows cumulative overall average MPG has likely been reset(trip B?). I cannot recall if it only goes 10k miles as I have an 07 MDX. If nothing has changed trip A shows your current tank fillup MPG while trip B shows the cumulative MPG.
It is cold out and MPG drops and your getting your current report. Winter MPG is always lower and you likely don’t have your warm weather average moving it up.
Case in point my normal around town MPG with my MDX is 19MPG however in winter it drops as low as 15MPG due to real cold/short trips I make.
GeorgeSanJose Yesterday Pendleton, OR (NE part of state) hit +64 degrees. It has been known to hit -20 degrees there this time of year! The West side where I live has been extremely mild. Mother Nature could be making amends for last Winter when it hit single digits (+7) here for the first time in over 50 years. Normal Winter lows are mid 20s for 2 or 3 days.
If your cars computer gets a signal from the coolant temp sensor that the car is not warmed up, the computer changes the fuel injector pulse to provide more fuel per the amount of air coming in through the throttle body, because that is what a cold engine needs. The extra fuel burn up in the catalytic converter and does nothing to propel the car.