[Dan Banquer]

O.K. guys, I've got a few questions on cars that someone might be able to help me with.
1. Why does my car get poorer gas mileage in the winter?
2. My Mechanic says that now that we have 10% ethanol in the gas we no longer need to use dry gas.
Is this true?
    Thanks in advance for your input;
              d.b.

[Thebiker]

Well, the dry gas thing is accurate. Dry gas is just alcohol, which works as a drying agent and binds moisture.  So if your already have ethanol in the fuel, you don't need to add more.

As to the decrease in gas mileage .  I drive a diesel, so what do I know?  I average 45mpg year round.  But then idling will not warm up a diesel and most folks do warm up their gas engines.....more time idling means a decrease in mpg?  That's just speculation, but the logic seems to work.

[Dan Banquer]

Well I can see the logic of longer idling, but I have noted a decrease in highway mileage that does not appear to account for idling; Or so I think at the moment.
                 d.b.

[ctviggen]

In my car, which has a turbo, I get better mileage in the winter (as compared to other seasons where the AC isn't used -- the worst mileage is always with the AC on).  The more dense air helps the gas mileage, and certainly helps power.  Perhaps it's the opposite with non-turbo cars?

[Dan Banquer]

Yep; I certainly get less mileage with the AC on. I have observed about 10 miles to the gallon less with the AC on for long trips.
                             d.b.

[aerius]

Several things actually.

1. All the lubricating fluids in the car are thicker and less slippery which leads to more friction until everything warms up.

2.  When the engine is cold, the fuel/air mix and spark timing isn't optimum so it's less efficient.

3.  Cold air is denser, which means the engine needs a smaller volume of it to provide a given power.  Therefore the throttle valve will be open less than usual at any given cruising speed, this means more pumping losses for the engine since it now works harder to suck in air.

4.  The locking torque convertor in an automatic transmission may not lock up until it's warms up sufficiently.  This leads to the engine reving higher and suffering more frictional and pumping losses which kills efficiency.

[Daygloworange]

2.  When the engine is cold, the fuel/air mix and spark timing isn't optimum so it's less efficient.

3.  Cold air is denser, which means the engine needs a smaller volume of it to provide a given power.  Therefore the throttle valve will be open less than usual at any given cruising speed, this means more pumping losses for the engine since it now works harder to suck in air.

This would be more applicable to carburated engines versus fuel injected. Yes, till the engine is warm the car will run in a slighlty "lean" condition so that the engine gets up to operating temperature sooner(for cleaner combustion, and therefore cleaner emissions). But after that the ECU will monitor O2 and adjust spark and fuel based on RPM and load to optimise the air/fuel ratio.

Technically, your car's engine will almost always make more power (therefore work less harder) when it's colder, due to air being denser. Therefore, more efficiant, and more fuel efficiant. The load (or drag) from ancillaries (heater, defroster, wipers) will place a load via the alternator on the engine. Basically the same as a human cycling on level pavement vs on an incline. You will dispense more energy to maintain your speed when cycling on an incline vs on a level terrain. Same goes for your engine.

Transmission slippage, tire slippage due to icy conditions, higher viscosity of cold fluids (engine oil, transmission oil) account for some increase, but it shouldn't equal or surpass the load from ancillaries.

You may have a faulty sensor that is malfunctioning, and not properly accounting for temperature, therefore making your car run at a poor air/fuel mixture during the cold.

Cheers

[Dan Banquer]

Well this is getting interesting. I did have a main engine sensor replaced this summer that was connected to the manifold. The difference in overall performance was rather noticeable. However, it did not appear to make much difference in the mileage.
Thanks to all for their input,
             d.b.

[mfsoa]

Tire pressure?

[Bob in St. Louis]

Dan what make/model/year vehicle do you have?
Have you seen any malfunction indicator warning lights/messages?
Good thinking about the air pressure. Suprised it took this long to see that.
I've got more questions but need to know what kind of beast we're talking about first.

Bob

p.s. By the way, I smell a Saab 9.3 owner here.

[Dan Banquer]

Dan what make/model/year vehicle do you have?
Have you seen any malfunction indicator warning lights/messages?
Good thinking about the air pressure. Suprised it took this long to see that.
I've got more questions but need to know what kind of beast we're talking about first.

Bob

p.s. By the way, I smell a Saab 9.3 owner here.

I think your smelling something else Bob; I own a 1996 SW1 Saturn with a single overhead cam engine and a 5 speed manual transmission. It's got about 139,000 miles on it. No warning or idiot lights are coming on.

I will certainly check my tire pressure; and good call, check the obvious things first!
             d.b.

[lazydays]

In my car, which has a turbo, I get better mileage in the winter (as compared to other seasons where the AC isn't used -- the worst mileage is always with the AC on).  The more dense air helps the gas mileage, and certainly helps power.  Perhaps it's the opposite with non-turbo cars?

my last two cars as well as my last two trucks got the same gas milage with the air conditioning on in the summer. Small engines are much more affected by loads than a modern V6 or V8. But, I've noticed that when driving on the highway in very hilly country the multi displacment feature on my new car will go back to the full displacment more than I'd like.
gary

[JLM]

Petroleum fueled engines use heat as the primary motive force (explosions of fuel in the cylinders causing expansion) so it seems reasonable that until the engine warms up, some of the heat and therefore expansion is lost.  The cold weather of the last few days has reminded me of how stiff the automatic transmission fluid gets (validating what was mentioned above).

[lazydays]

Several things actually.

1. All the lubricating fluids in the car are thicker and less slippery which leads to more friction until everything warms up.

2.  When the engine is cold, the fuel/air mix and spark timing isn't optimum so it's less efficient.

3.  Cold air is denser, which means the engine needs a smaller volume of it to provide a given power.  Therefore the throttle valve will be open less than usual at any given cruising speed, this means more pumping losses for the engine since it now works harder to suck in air.

4.  The locking torque convertor in an automatic transmission may not lock up until it's warms up sufficiently.  This leads to the engine reving higher and suffering more frictional and pumping losses which kills efficiency.

1** once the engine has circulated the oil for three or four minutes (most of the manufacturers are specing 5w-30) the oil is right where it should be. If you happen to be using synthetics; there is no real difference between cold oil and hot oil.

2**that would be true in an older car built in the seventies or sixties. But all the newer ones sense the ambiant temp, as well as engine temp, and adjust from there. You can't change it as it's in the computor that runs the engine and fuel delivery system.

3**virtually all fuel injection systems have a gas return line back to the tank or fuel delivery line. You will never use anymore gas than needed because the CPU will not allow it. Still if you happen to have a bad sensor the fuel injection system will automaticly go into a full fuel delivery. Might want to have a scan run on the sensors.

4**torque convertors are set to lock up at speed above forty mph, unless the speed sensor is out of wack. It's pretty much a mechanical thing after that.

gary

[lazydays]

Petroleum fueled engines use heat as the primary motive force (explosions of fuel in the cylinders causing expansion) so it seems reasonable that until the engine warms up, some of the heat and therefore expansion is lost.  The cold weather of the last few days has reminded me of how stiff the automatic transmission fluid gets (validating what was mentioned above).

automatic transmission fluid is a synthetic , and is not bothered all that much with the ambient temp outside. What you maybe encountering is the temp of the valve body causing the spools inside tobe a little sluggish. There are only three manufacturers that have that part figured out. All are domestic manufacturers. Rest are still using technology from the sixties. With the advent of solenoid controlled valve bodies this is even more critical. To get around this some of the foreign folks are making the spools kinda loose in the basic body thus creating what is known as slippage in the oil supply.
gary

[aerius]

1** once the engine has circulated the oil for three or four minutes (most of the manufacturers are specing 5w-30) the oil is right where it should be. If you happen to be using synthetics; there is no real difference between cold oil and hot oil.

Yes, the engine oil, which still leaves the transmission fluid, the differential oil, and the grease in the various bearings.

2**that would be true in an older car built in the seventies or sixties. But all the newer ones sense the ambiant temp, as well as engine temp, and adjust from there. You can't change it as it's in the computor that runs the engine and fuel delivery system.

Still true today.  Try flooring the gas before the engine warms up to see how sluggish it is.  When the engine is in warmup mode, power and efficiency go down the crapper, this is what it's supposed to do, as directed by the ECU for emmissions control reasons.

3**virtually all fuel injection systems have a gas return line back to the tank or fuel delivery line. You will never use anymore gas than needed because the CPU will not allow it. Still if you happen to have a bad sensor the fuel injection system will automaticly go into a full fuel delivery. Might want to have a scan run on the sensors.

I think we have a mis-understanding here.  The engine is essentially an air pump, it takes X units of air to make Y units of power.  When it's cold the air is denser, thus X units of air will fit into a smaller volume, so to make the same amount of power, for say cruising on the highway, a smaller volume of air is needed.  To do that, the throttle valve is choked back, which results in more engine vacuum and pumping losses since you're now sucking air in through a smaller tube.

4**torque convertors are set to lock up at speed above forty mph, unless the speed sensor is out of wack. It's pretty much a mechanical thing after that.

In theory, and it also depends on the design of the darn thing.  Some, like the one in my car don't lock up until the fluid is warm enough.  Others have fuzzy logic which juggle speed, temperature, and a bunch of other parameters.  Still others are strictly speed based.

[Bob in St. Louis]

Dan, the Saab comment was directed at ctviggen (Viggen being a type of Saab).

Anything that would cause a 10mpg decrease in fuel economy that is monitored or controlled the PCM (Powertrain Control Module) {Also known by several other generic names: ECM ECU ect.."The brain" :roll:as some folks call it} SHOULD immediately turn on a MIL (Malfunction Indicator Light) in a U.S. vehicle built in 1996 or newer. All cars and light trucks built after that operate on a system called OBD II (On Board Diagnostics, second generation) All inputs and outputs of that module have a voltage range they operate in. If a sensor went 'bad' (full open or shorted circuit) would absolutely turn that light on. If one of these sensors (of which there are dozens of driveability/emission related sensors, solenoids, actuators, motors ect..) was 'bad' but stayed within it voltage 'window/spectrum', it may not turn the light on, because the PCM might be seeing that reading as plusible. (96 being an early OBD II vehicle, this system was in it's infancy and didn't "look at" as many things as a '07 vehicle does).

As a service department employee at a dealership, I'd be instantly sceptical of a vehicle that had that large of a drop in economy with no MIL.
That being said, if I were a technician I'd look at the following:

CTS/ETS (Coolant/Engine temperature sensors). A two wire sensor(s) located on the cylinder head. (Usually)
If the PCM was looking at a coolant temp of -225F (If I remember correctly, that was the default temp of most GM vehicles with an electrical 'open' in the CTS circuit) it would REALLY dump fuel in. Cold engines are less efficient than one at operating temperature, so the PCM is going to do everything in it's power to get it there. Cold air is more dense, meaning more fuel by nature, but within reason. NOT 10mpg. If the circuit was open, it WOULD turn the MIL on.

IAT (Intake Air Temperature) A Two wire sensor located in the air intake, between the air fiilter box and throttle body housing.
Once again, the PCM is looking at the temperature of the air coming into the engine.

MAF/MAP (Mass Air Flow/Manifold Absolute Pressure) 5-6 wire sensors (MAF is just after the IAT, MAF is usually remotely located {on the firewall?} with a vacuum hose running to the intake manifold.)
Can't remember which one or both a 96 Saturn has, but a MAF tells the PCM all about the density of air coming in the engine. MAF tell the PCM about the intake manifold level of vacuum. Both very important to fuel delivery/fuel management.

O2 (Oxygen Sensor) is a 4 wire sensor located in the exhaust manifold or slightly below, and a second one past the catalytic converter. VERY VERY important sensor that will cause a multitude of MIL's, driveability concerns as bad as needing a tow truck. This is the #1 sensor the PCM looks at for fuel delivery. It is the "God of all engine management sensors", no shit. They tell the PCM how much oxygen is in the exhaust stream (too much oxygen = too little fuel and visa versa). These operated on a rapidly floating/bouncing voltage (from .1 to 1.0 volt) and should cross the .5volt threshold many times a second back and forth (rich - lean - rich - lean) get it? If the sensor is slow/groggy, or is lingering on the end of the spectrum that tells the PCM, "HEY, I've got too much oxygen in here,--turn up the fuel!!", then you'd have crappy mpg. got it?

Those are some of the most common sensors I saw with GM vehicle (when I was working on them). There is many others that could be the culprit, but those are the one's I'd check first.
I've seen many bizzare things over the years that could create your symptoms that would NOT turn on the MIL.
Mice building a nest in your air filter box (choking the air).
Brakes dragging. Happens a lot in the winter time. Folks use the parking brake in sub freezing temps, the cable is frozen in the engaged position even though the brake handle/pedal is released. Causes the rear brakes to drag. Given enough time and speed, it would get the brakes hot enough to start causing damage. Do this: Park on a hill, nose down. Put transmission in neutral, slowly let off brake. If car wants to roll immediately, the brake isn't the problem. Got it? This condition is worsened by heat. So driving the car for awhile THEN do the test. (locking brakes = friction = heat = expansion of brake components = brakes apply harder = more friction = heat = more expansion...........)

My fingers are getting tired of typing, damn!

Have the car checked if your uncomfortable. Most libraries have service manuals if you want. But remember your digging in the EPA's back yard here, not real easy stuff... You could take it to a shop, but expect to pay about an hour diagnosis for the guy/shops time. They may not find anything after all that. Cheap solution, if your state/location has a manditory emission testing facility, have it checked. It's cheaper than a shop. If you fail the test some testing facilities will give the OBD DTC (Diagnostic Trouble Code) that was in your car. THEN you can follow down the path of that particular flow chart for that sensor.
Or, now that I think about it, "Auto Zone" (I assume you have those?) will check your codes for free.

Does that help at all? My fingers need a rest.

Bob

[ctviggen]

Dan what make/model/year vehicle do you have?
Have you seen any malfunction indicator warning lights/messages?
Good thinking about the air pressure. Suprised it took this long to see that.
I've got more questions but need to know what kind of beast we're talking about first.

Bob

p.s. By the way, I smell a Saab 9.3 owner here.

I do have a 9-3 Viggen.  It's a fun car.  And useful to boot.  (I chose my moniker not because I'm a car guy -- I'm not -- but because no one has this name anywhere.)

[Dan Banquer]

Hi Bob;
    Thank you very much for your rather extensive reply. I will study your response further and talk to my mechanic. BTW: The 10 miles per gallon reduction appears to be due to airconditioning only. Without the AC I will get close to 40 miles per gallon on the highway for long trips. I sincerely hope you were not confused by that.
Thanks again for taking the time and wear and tear on your fingers;
        I owe you;
                d.b.

[Bob in St. Louis]

Hey guys, though I'd comment on a few things I've seen:

Dan does not have an automatic, so he does't have a valve body.
I don't believe Saturns EVER had a throttle valve.
He doesn't have a torque converter.
Transmission and differential fluid are the same fluid (same case housing).
This talk about fluid vicosity, jeez man, He'd have to have 90W in every fluid to make that much of a difference.
Dan, DON'T floor the gas of a cold engine. EVER. NEVER!!!   No reason to. Where you live, that oil gets very thick and won't be lubricating the valvetrain components immediately. DON'T DO THAT!
Of course it's going to be sluggish on a cold start up.

Bob