cold weather causing bad fuel economy?

[jaidedeye]

its been getting down to low 20's here in NC lately with a daytime high 0f maybe 40. i have noticed that my fuel economy has sucked all of a sudden. doesnt seem like my car is warming up completely, i have no A/C condensor, and a 18 inch flex fan, and a 180 thermostat. i know the stock guages are junk but my car only reads at the 130 mark and my heat isnt really hot. i jave no bubbles in my cooling system, i already checked that. i have new 0-2 sensors, coolant temp sensor and air temp sensor that i still ned to install. would those make a big difference or should i change the thermostat or block off part of the radiator to get it ti operating temp?

 

[jaidedeye]

ttt

 

[Michael Yount]

I'd definitely install the new sensors if you have them - no reason not to. I'd pull the t'stat and see if it's closing all the way; put it in a pan of coolant on the stovetop with a thermometer and see when it's opening. Perhaps it's not quite closing all the way letting coolant through which is driving operating temps down.

Fuel mileage usually drops off just a hair in the winter and in the summer. Lower ambient temps are good for engine efficiency, but bad for aerodynamic drag (cold air is more dense). In the dead of summer, the air is less dense which helps aero, but the higher ambient/intake temps hurt efficiency. Peak mileage probably comes in the spring/fall with temps in the 50-60F range - best balance of lower intake air temps and aerodynamic drag.

Be sure to check tire pressures. They drop significantly with temps, and much lower tire pressure will hurt mileage.

 

[87'GTstang]

Not to mention the wonderful additive in "winter gas" - MTBE. But along with the denser air concept in the same amount of given space there are more oxygen molecules - more oxygen calls for more fuel.........

It's not a theory it's a fact, you are going to get worse gas mileage with cold versus warm weather.

 

[Michael Yount]

Good catch on the winter gas formulas; they don't help.

With regard to air density/efficiency - remember, at peak - colder air, more density = more air, more oxygen, more fuel, more power, uses more fuel. But for most street driving, much, much less than peak power is needed. The mass air meter takes the higher density into account (that's why it's a MASS meter and not a FLOW meter) and proportions fuel accordingly. One of the beauties of efi is that it takes care of things like that. Carb? Yes - colder = more fuel. But not with efi.

 

[87'GTstang]

One of the beauties of efi is that it takes care of things like that. Carb? Yes - colder = more fuel. But not with efi.

Not even with good ole' speed density?

 

[87'GTstang]

With regard to air density/efficiency - remember, at peak - colder air, more density = more air, more oxygen, more fuel, more power, uses more fuel. But for most street driving, much, much less than peak power is needed. The mass air meter takes the higher density into account (that's why it's a MASS meter and not a FLOW meter) and proportions fuel accordingly.

Wouldn't "proportioning fuel accordingly" in cold weather mean burning more fuel in response to the reading of more oxygen present in the same amount of given volume of air intake in the winter versus summer? In the end, more fuel is burnt isn't it?

 

[Michael Yount]

Give some more thought to it - let's use an around-town case to take highway aero out of the equation. You basically need the same amount of power to move the car around. If you really are using more air/fuel - you're making more power. Which means for a given throttle opening, you'd be going faster/accelerating quicker. I have to assume you'd drive the car the same basic way year round - i.e., you wouldn't be going faster/accelerating quicker. So, you'd have very slightly less throttle openings to create the same amount of power. If you assume the car's driven in the same manner, and you remove the fuel additive issue, and the mileage penalty due to having to push the car through more dense air - the engine's gonna run more efficiently (use less fuel) when ambient's are lower. The lower the intake air temp - the more efficiently it will run, and the more peak power it's capable of.

The problem is that all these differences are fairly minor - and the colder intake air efficiency gain does not outweigh the fuel additive loss, and the aero loss.

 

[Michael Yount]

SD systems do take temperature into consideration when they do their charge mass calculations. But they're not as precise or accurate as a well calibrated mass air system at dealing with changing external conditions.

 

[87'GTstang]

Yeah I suppose after the wind drag coefficiency and the winter fuel mix - it really doesn't matter what gas mileage you get, it's already taken care of.

 

[jaidedeye]

im willing to bet if i get the car to run at a warmer temp that my mfuel economy will increase, i say this because two weeks ago when i was 60-70 degrees i was getting good mileage.

 

[Michael Yount]

jaidedeye - don't confuse warmer operating temp with warmer intake air temp. I've read dyno tests where they changed the t'stat to see what impact that would have on intake charge air temp - and there was little difference. I think if you're running a 160F t'stat, and you switch to a 180F or 195F you may see improvement. If you have a 180F and you switch to a 195F, I don't think you'll see much difference. If you drive your car to FL, I think you might see one mpg improvement or so. There shouldn't be a huge difference in any event - maybe a mpg or so worse in the dead of winter or the dead of summer, with the best mileages coming during the fall and the spring when ambient temps are not too cool or too hot.