YACT- Gas milage=> Is it my temp sensor that's the problem?

[JEDI]

i have a 2002 Chevy Prizm w/34k miles. (i drive ALOT.) Sticker says 41mpg highway. but i get 36mpg hwy usually.

i've noticed when i startup my car in a cold climate (~70 F) and drive to much warmer climate (~90 F), my gas milage is 40mpg. for example, i startup in the morning in NYC (70 F) and drive 300miles to DC. as i'm driving south, it gets warmer because i'm going south, and it's now the afternoon. (my car has an on board thermometer.)

but other times when i do that drive at those temps, and stop at a rest stop half way, my mpg=38. it's like when i start up my car again at 90F and continue the drive at 90F, i'm back to 36mpg for the remainder of that trip.

i've done this about a dozen times this summer, and the results are always the same.

So is it my temp sensor? How can I tell? and what do i tell the dealer to replace it?

if i do it myself, How much is a new temp sensor for my car, and how hard is it to install?

THX

edit: a dozen times from tne nyc to dc trip. going from dc to nyc i get the normal 36mpg. thats because i leave in the afternoon after work.

 

[NogginBoink]

I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

 

[Heisenberg]

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

 

[Ornery]

It may be you're generally going more up hill in one direction.

 

[JEDI]

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

 

[Heisenberg]

Originally posted by: JEDI

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

Warmer air = less dense = less oxygen per unit volume = less power/gas mileage. Also, warmer air means the cooling system can't operate as effeciently.

 

[Zenmervolt]

Originally posted by: Heisenberg

Originally posted by: JEDI

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

Warmer air = less dense = less oxygen per unit volume = less power/gas mileage. Also, warmer air means the cooling system can't operate as effeciently.

WRONG! Colder air = WORSE mileage. You're right that colder air is more dense, which means more oxygen. However, modern fuel-injection systems compensate for this by injecting MORE fuel into the cylinders for a given throttle opening. So for the same throttle opening, the car will get fewer miles per gallon in cold weather than in warm weather. You get more power with cold air, yes, but more power for a given engine design ALWAYS means more fuel is being used.

As for the cooling system, doesn't matter unless you're in very, very hot or very, very cold weather. You generally need to get over 100 degrees Fahrenheit or less than 0 degrees Fahrenheit to have an impact on the engine's operating temperature.

This is not a temperature sensor problem. If it were, you wouldn't have to re-start the car for the mileage to decline with temperature. For what you are describing to be a temperature sensor problem, it would have to be functional during startup and then stop functioning once the engine was running. That's an extremely unlikely fault.

ZV

 

[cavemanmoron]

Originally posted by: Ornery
It may be you're generally going more up hill in one direction.

correct.
the road may also be smmother concret/blacktop,in one direction,and also you may be going just a little faster one way,etc.


The thing to do is average your fuel mileage.

I get about 12 mpg in my Camaro,and could car less/notice if it varies by 2 mpg.

 

[Eli]

Originally posted by: Zenmervolt

Originally posted by: Heisenberg

Originally posted by: JEDI

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

Warmer air = less dense = less oxygen per unit volume = less power/gas mileage. Also, warmer air means the cooling system can't operate as effeciently.

WRONG! Colder air = WORSE mileage. You're right that colder air is more dense, which means more oxygen. However, modern fuel-injection systems compensate for this by injecting MORE fuel into the cylinders for a given throttle opening. So for the same throttle opening, the car will get fewer miles per gallon in cold weather than in warm weather. You get more power with cold air, yes, but more power for a given engine design ALWAYS means more fuel is being used.

As for the cooling system, doesn't matter unless you're in very, very hot or very, very cold weather. You generally need to get over 100 degrees Fahrenheit or less than 0 degrees Fahrenheit to have an impact on the engine's operating temperature.

This is not a temperature sensor problem. If it were, you wouldn't have to re-start the car for the mileage to decline with temperature. For what you are describing to be a temperature sensor problem, it would have to be functional during startup and then stop functioning once the engine was running. That's an extremely unlikely fault.

ZV

Hmm......... Are you sure ECUs monitor intake air temperature?

I would be interested to know just how much variance there is in fuel injection volume vs. air temperature..

How does the ECU determine what the air temperature is? Is there a temperature sensor in the intake? Are you sure it doesen't just go by engine temp(cold/rich hot/normal)?

I mean, what you say would totally make sense.. as long as the engine really varies injection volume by air temperature.

I have never heard of this though.

 

[Eli]

As for your original problem..

There is probably a variable you aren't taking into consideration.. I think the hill suggestion was a good one.

If you're going slightly up hill one way, it could reduce your milage.. while on the return trip, you'll get all that back and then some due to the downhill nature.

 

[JEDI]

Originally posted by: Eli
As for your original problem..

There is probably a variable you aren't taking into consideration.. I think the hill suggestion was a good one.

If you're going slightly up hill one way, it could reduce your milage.. while on the return trip, you'll get all that back and then some due to the downhill nature.

no, i mean a dozen times from nyc to dc. going from dc to nyc is always the same since i leave in the afternoon after work.

 

[JEDI]

so if it's not the temp sensor, then what could it be? my O2 sensor?

oh, as for not stopping during my trip. it's HARD..5 hr trip = gota pee sometime. i did that couple of times by no drinking anyting when i woke up and drove like that. i was very thirsty when i reached dc.

 

[Zenmervolt]

Originally posted by: Eli

Originally posted by: Zenmervolt

Originally posted by: Heisenberg

Originally posted by: JEDI

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

Warmer air = less dense = less oxygen per unit volume = less power/gas mileage. Also, warmer air means the cooling system can't operate as effeciently.

WRONG! Colder air = WORSE mileage. You're right that colder air is more dense, which means more oxygen. However, modern fuel-injection systems compensate for this by injecting MORE fuel into the cylinders for a given throttle opening. So for the same throttle opening, the car will get fewer miles per gallon in cold weather than in warm weather. You get more power with cold air, yes, but more power for a given engine design ALWAYS means more fuel is being used.

As for the cooling system, doesn't matter unless you're in very, very hot or very, very cold weather. You generally need to get over 100 degrees Fahrenheit or less than 0 degrees Fahrenheit to have an impact on the engine's operating temperature.

This is not a temperature sensor problem. If it were, you wouldn't have to re-start the car for the mileage to decline with temperature. For what you are describing to be a temperature sensor problem, it would have to be functional during startup and then stop functioning once the engine was running. That's an extremely unlikely fault.

ZV

Hmm......... Are you sure ECUs monitor intake air temperature?

I would be interested to know just how much variance there is in fuel injection volume vs. air temperature..

How does the ECU determine what the air temperature is? Is there a temperature sensor in the intake? Are you sure it doesen't just go by engine temp(cold/rich hot/normal)?

I mean, what you say would totally make sense.. as long as the engine really varies injection volume by air temperature.

I have never heard of this though.

It doesn't measure intake air temperature. It measures oxygen content. That's what the oxygen sensor does. It's keeps the mixture stoichiometric regardless of air density. At least, that's why the fuel mileage changes.

The temp sensors typically measure coolant temperature to determine which operating mode the engine should be in. But if they mis-read the coolant temp then they could order the wrong operation mode. Still the problem he describes is extremely unlikely to be a temp sensor problem.

ZV

 

[Zenmervolt]

Originally posted by: JEDI
so if it's not the temp sensor, then what could it be? my O2 sensor?

oh, as for not stopping during my trip. it's HARD..5 hr trip = gota pee sometime. i did that couple of times by no drinking anyting when i woke up and drove like that. i was very thirsty when i reached dc.

No, any failure should result in a _constant_ dicrease in mileage. Not this odd thing you're describing.

ZV

 

[JEDI]

Originally posted by: Zenmervolt It doesn't measure intake air temperature. It measures oxygen content. That's what the oxygen sensor does. It's keeps the mixture stoichiometric regardless of air density. At least, that's why the fuel mileage changes.

The temp sensors typically measure coolant temperature to determine which operating mode the engine should be in. But if they mis-read the coolant temp then they could order the wrong operation mode. Still the problem he describes is extremely unlikely to be a temp sensor problem.

ZV

so less oxygen = more gas milage?

but even if i tape off half of my air intake, the o2 sensor will adjust fuel/air mixture back to normal?!?!?!

 

[PG]

You guys thinking too much about this. Either way 36 or 40 mpg is great and I wouldn't worry at all. Lots of cars don't get the full mileage stated on the sticker for various reasons but you are pretty close. If you only got around 25 then it would be time to worry.

 

[Eli]

Originally posted by: Zenmervolt

Originally posted by: Eli

Originally posted by: Zenmervolt

Originally posted by: Heisenberg

Originally posted by: JEDI

Originally posted by: Heisenberg

Originally posted by: NogginBoink
I can't quite make out the details... but a car will normally get lower mileage as temperature goes up.

Yeah, I'm guessing it's just a result of the air temp. When it gets hot out, you normally get about a 5-10% drop in power.

huh? colder air = better gas milage?! gota store 20 gallons of summer gas and try it in the winter. cause in winter they switch to ethanol.

so you're saying my mpg should actually drop as i'm going into a warmer climate?! WHY?

Warmer air = less dense = less oxygen per unit volume = less power/gas mileage. Also, warmer air means the cooling system can't operate as effeciently.

WRONG! Colder air = WORSE mileage. You're right that colder air is more dense, which means more oxygen. However, modern fuel-injection systems compensate for this by injecting MORE fuel into the cylinders for a given throttle opening. So for the same throttle opening, the car will get fewer miles per gallon in cold weather than in warm weather. You get more power with cold air, yes, but more power for a given engine design ALWAYS means more fuel is being used.

As for the cooling system, doesn't matter unless you're in very, very hot or very, very cold weather. You generally need to get over 100 degrees Fahrenheit or less than 0 degrees Fahrenheit to have an impact on the engine's operating temperature.

This is not a temperature sensor problem. If it were, you wouldn't have to re-start the car for the mileage to decline with temperature. For what you are describing to be a temperature sensor problem, it would have to be functional during startup and then stop functioning once the engine was running. That's an extremely unlikely fault.

ZV

Hmm......... Are you sure ECUs monitor intake air temperature?

I would be interested to know just how much variance there is in fuel injection volume vs. air temperature..

How does the ECU determine what the air temperature is? Is there a temperature sensor in the intake? Are you sure it doesen't just go by engine temp(cold/rich hot/normal)?

I mean, what you say would totally make sense.. as long as the engine really varies injection volume by air temperature.

I have never heard of this though.

It doesn't measure intake air temperature. It measures oxygen content. That's what the oxygen sensor does. It's keeps the mixture stoichiometric regardless of air density. At least, that's why the fuel mileage changes.

The temp sensors typically measure coolant temperature to determine which operating mode the engine should be in. But if they mis-read the coolant temp then they could order the wrong operation mode. Still the problem he describes is extremely unlikely to be a temp sensor problem.

ZV

Dur.

My brain must've been sleeping when I posted that. I was totally thinking along the lines of air temp, not oxygen content.. lol

 

[TechnoKid]

I have seen hondas (96+) and other cars have air intake temperature sensors, so I'm sure the ECU uses this senosr in one way or another in correlation with the O2 sensors in the exhaust system.

 

[notfred]

Oxygen sensors are in the exhaust, not the intake. There's a mass air sensor in the intake that measures the mass of air that is flowing into the intake manifold. It does NOT measure oxygen content.

 

ZEM is absolutely correct, the colder the temp, the lower the fuel mileage, why you ask ?

Takes longer to warm up (ECM stays in open loop longer)

Thicker oil, more internal friction and pumping losses

The ECM also richens up the mixture when the ambient temp are cooler to maximize the efficiancy of the catalytic convertor.

There is nothing wrong with your Prizm, this gas mileage descrepency is absolutely normal, there are many variables to consider including but not limited to the following ;

Brand of fuel

Terrain

Head or tail winds

Ambient air temp

Road surface quality

Speed

Load

etc, etc, etc.

 

[Thegonagle]

HOW on earth do you measure your fuel economy in the middle of a tank-full, at a rest stop? Do you fill up along the way? I'd think you would only fill up once, at the beginning of your 300 miles.

 

[JEDI]

Originally posted by: Thegonagle
HOW on earth do you measure your fuel economy in the middle of a tank-full, at a rest stop? Do you fill up along the way? I'd think you would only fill up once, at the beginning of your 300 miles.

yup, fill up right b4 i leave. fill up again when i arrive.

how do i force the ecm to be in closed loop? and what's the downside of doing this while the engine is cold?

 

[JEDI]

Originally posted by: Roger
ZEM is absolutely correct, the colder the temp, the lower the fuel mileage, why you ask ?

Takes longer to warm up (ECM stays in open loop longer)

Thicker oil, more internal friction and pumping losses

The ECM also richens up the mixture when the ambient temp are cooler to maximize the efficiancy of the catalytic convertor.

There is nothing wrong with your Prizm, this gas mileage descrepency is absolutely normal, there are many variables to consider including but not limited to the following ;

Brand of fuel

Terrain

Head or tail winds

Ambient air temp

Road surface quality

Speed

Load

etc, etc, etc.

So what would happen if i tape off 1/2 the air intake? (ie: tape off 1/2 the air filter)

less oxygen = less power, but more mpg?

 

So what would happen if i tape off 1/2 the air intake? (ie: tape off 1/2 the air filter)

less oxygen = less power, but more mpg?

No,no and no.
Blocking off the air intake will lower your mileage.

Do not screw around with the sensors or ecm, want more mileage ?

Drive at a steady speed

Avoid "jackrabbit" starts

Drive at reasonable speeds

Keep tires inflated to recomended pressures

Unload all that crap in your trunk

Keep the vehicle clean and waxed

during cold starts, let the engine warm up for no more than one minute

Avoid excessive idling

Change your oil and filter every three thousand miles

Use the correct viscosity oil (read your owners manual)

Avoid using the A/C as much as possible

 

[Lizardman]

if you are getting withint %5 of the gas mileage they advertise I would call that acceptable.