Car is slowly losing MPG...

[Nograts]

And I don't know why. I shipped it maybe 7 months back from Hawaii to here, everything has been fine for the most part.

About two months ago the little LED screen that tells me MPG on average has been dropping about .1 MPG periodically. It has now gone from ~20.9 MPG to 17.7 MPG. What would be the likely cause of this?

It's just my work car. I drive it....literally about 6 miles round trip each day. Max speed on that route is 35.

 

[ThatsABigOne]

There is not enough detail needed to give you any advice. What car, what type of commute, what fuel, weather, etc, etc.

 

[darfstellar]

Temperature? Engines are less efficient in cold weather and you never had to contend with that in Hawaii. I lose 2-3 mpg in the winter months over what I get in warmer weather.

 

[Xtrem]

Different commute, stop and go vs highway speeds?

 

[Ferzerp]

Winter blend, or you had an arbitrarily high indicated MPG due to previous long driving and it has taken this long to equalize.

Manually calculate it the next few tanks and see if it truly is changing.

 

[Nograts]

Allright thanks.

It's a 02' Lexus RX300. Commute is 3 miles out, 3 miles back. All paved road, max speed is 35MPH. Temp right now is ~30-50 daily. Fuel is standard 87. Weather clear with some rain...

It is stop and go. It might be possible what Ferz mentioned about being high MPG from my 2 hour trip when I picked it up...that was like 5 months ago though.

 

[master_shake_]

how many miles?

 

[Nograts]

145k

 

[master_shake_]

ever done a tune up?

plugs, wires, connectors, oxygen sensors, filters?

 

[Nograts]

ever done a tune up?

plugs, wires, connectors, oxygen sensors, filters?

I did the oil/filter/air filter about a month ago...was saving all that stuff for a dealership 150k mile package thing. Think they'll take it in early?

 

[master_shake_]

I did the oil/filter/air filter about a month ago...was saving all that stuff for a dealership 150k mile package thing. Think they'll take it in early?

no idea.

 

[Zenmervolt]

Commute is 3 miles out, 3 miles back. All paved road, max speed is 35MPH. Temp right now is ~30-50 daily.

That's pretty much the worst possible operating environment for the car. It's going to be running in warm-up mode the entire trip, there won't be enough time for the oil to come up to temperature and burn off contaminants, and everything will be cold, which increases the resistance in bearings, etc.

Mileage in that scenario should be pretty poor, and that's likely all you're seeing.

ZV

 

[Nograts]

That's pretty much the worst possible operating environment for the car. It's going to be running in warm-up mode the entire trip, there won't be enough time for the oil to come up to temperature and burn off contaminants, and everything will be cold, which increases the resistance in bearings, etc.

Mileage in that scenario should be pretty poor, and that's likely all you're seeing.

ZV

Allright thanks that is somewhat reassuring. I guess I'll keep my eye on it, spring is almost here so I'll see if that makes a difference.

 

[tortillasoup]

And I don't know why. I shipped it maybe 7 months back from Hawaii to here, everything has been fine for the most part.

About two months ago the little LED screen that tells me MPG on average has been dropping about .1 MPG periodically. It has now gone from ~20.9 MPG to 17.7 MPG. What would be the likely cause of this?

It's just my work car. I drive it....literally about 6 miles round trip each day. Max speed on that route is 35.

battery..... If you care, you can get a battery charger such as a Batteryminder 1500 and charge it up, measure your fuel economy again and report back. I can almost guarantee you this is the issue. You just don't drive the vehicle enough to keep the battery topped up all the time, so when you do drive it, the alternator is loaded down, wasting gasoline.

Change the motor oil if you haven't already, really dirty motor oil can affect fuel economy but hopefully that's not the case because that would mean it's getting dirty really quick.

 

[Nograts]

Not sure it's worth the cost of a battery charger just to test the theory with gas being insanely cheap right now. I'll keep this in mind but I think i'll just make sure it doesn't drop too much drastically lower... take it in for 150k mile servers to the dealership here in a few months.

So lame they don't tell you prices they just tell you what is included, but no price

 

[tortillasoup]

Not sure it's worth the cost of a battery charger just to test the theory with gas being insanely cheap right now. I'll keep this in mind but I think i'll just make sure it doesn't drop too much drastically lower... take it in for 150k mile servers to the dealership here in a few months.

So lame they don't tell you prices they just tell you what is included, but no price

Well the cheapest thing you can do is get a "Free" multimeter from harbor freight with one of those coupons. If your battery voltage is like less than >12.5v, then you know that this is the issue. If it's around less than >12.3V, then you definitely have a future problem on your hands. You might need to put a battery tender on this shit occasionally unless you want to start replacing batteries and alternators... For point of reference, on a new flooded lead acid battery, fully discharged is 11.9V while full is 12.7V. You can interpolate the relative state of charge by knowing that 11.9V is 0% and 12.7V = 100%.

 

[brainhulk]

3 miles. use a bicycle

 

[leper84]

Your car probably isn't warming up enough to switch from open to closed loop. You're going to use more gas that way. There probably isn't anything horribly wrong aside from being way too late on your maintenance.

 

[BUTCH1]

That's pretty much the worst possible operating environment for the car. It's going to be running in warm-up mode the entire trip, there won't be enough time for the oil to come up to temperature and burn off contaminants, and everything will be cold, which increases the resistance in bearings, etc.

Mileage in that scenario should be pretty poor, and that's likely all you're seeing.

ZV

Yup, when cold the O2 sensors cannot function properly so the car is in "open loop" mode for the first couple of miles, then the ECU knows the outside air temp and will feed the injectors a longer pulse (sort of the same function a choke used to do) so the car will run until it warms up a bit, then it will go into "closed loop" mode where the O2 sensors, MAF sensor, MAP sensor are all used to compute how much fuel is needed. In OP's case his car probably gets to closed loop about 1/2 mile before he's done driving it. I'm in a similar situation, my job is 2.3 miles from home and my gas mileage has dropped quite a bit and the OLM is (correctly) dropping the % of life my oil has much faster then when I drove 13 miles to work. I'm actually thinking of buying a scooter to avoid the wear and tear on my car LOL.

 

[Zenmervolt]

when you do drive it, the alternator is loaded down, wasting gasoline.

Even if the alternator is running at full output the entire time, that's not even remotely close enough to account for the 15% drop in economy that the OP is seeing. Your theory that this is the chief reason for the reduced mileage is absurd.

The difference from having the alternator always running at 100% cycle is maybe going to be 2% to 3% and that's being very, very generous with the estimate of the effect.

The OP's issue is that he's basically operating the car in open loop all the time due to the short trip and the colder weather (he shipped the car from Hawai'i) is further reducing mileage.

ZV

 

[tortillasoup]

Even if the alternator is running at full output the entire time, that's not even remotely close enough to account for the 15% drop in economy that the OP is seeing. Your theory that this is the chief reason for the reduced mileage is absurd.

Based upon what research have you concluded that?

Here is a hint: An Alternator is approximately 50% efficient. When charging a lead acid battery, approximately 50% of the energy going into the battery is dissipated as heat... That means when you charge a lead acid battery with an alternator, only 25% of the energy from the crank actually ends up in the battery.

Alternators are well known for sapping about 10% of a vehicle's fuel economy and that's assuming a fully charged battery. Don't believe me? Go on any hypermiling forum and one of the things discussed was removal of the alternator belt and just charging the lead acid battery when you reach your destination.
Here is one link: http://www.metrompg.com/posts/alternator-optional.htm


This removal of the alternator belt to save fuel has been tested ad-nauseam which is why I know about it.

The difference from having the alternator always running at 100% cycle is maybe going to be 2% to 3% and that's being very, very generous with the estimate of the effect.

The OP's issue is that he's basically operating the car in open loop all the time due to the short trip and the colder weather (he shipped the car from Hawai'i) is further reducing mileage.

ZV

As for OP "operating in open loop", unless his car was manufactured over 20 years ago, most cars enter into closed loop within 10 seconds of start up... that's what oxygen heaters are for... (cars that don't have heated oxygen sensors take significantly longer to enter into closed loop operation.)

 

[Zenmervolt]

Alternators are well known for sapping about 10% of a vehicle's fuel economy and that's assuming a fully charged battery. Don't believe me? Go on any hypermiling forum and one of the things discussed was removal of the alternator belt and just charging the lead acid battery when you reach your destination.

The primary resistance from the alternator is resistance from the belt itself and from the inertia or the pulleys and resistance from the bearings. Removing the alternator entirely is going to make a crap-ton more difference than just changing the alternator duty cycle. The fact that you think removing the alternator entirely is comparable to changing the duty cycle is a perfect illustration of why you're mistaken here.

Let's look at a typical alternator. You're dealing with a maximum output of somewhere around 130 amps. At 12 volts, that's 1,560 watts. For simplicity's sake we'll call it 1,600 watts.

If we assume a 50% efficient alternator, that's 3,200 watts of power sapped from the engine (not counting frictional losses from the belt and pulley system). This is 3,200 Joules/second or 11.52 MJ/hour. Gasoline has 120 MJ/US Gal.

With a 20% efficient engine, a maxed-out alternator will use 11.52/(120*.2) gallons of fuel/hour, or 0.48 gallons/hour.

That's the difference between zero output and max output, so it's easily double the difference between normal load and full load, so we're dealing with maybe 0.24 gallons/hour of difference in fuel consumption.

OP's vehicle was rated, under the old, optimistic EPA standards, at 23 mpg highway. At 60 mph and 23 mpg, that's approximately 2.6 gallons/hour. Adding another 0.24 gallons/hour brings us to 2.84 gallons/hour and brings mileage down to 21.12 mpg. For a difference of 8% (1.88/23).

I was off on my initial estimates of the effect of the alternator being run at 100%, but that doesn't change the fact that you're just plain wrong if you think that the main culprit here is the battery charging. Even if he got that 8% improvement over his current 17.7 mpg back, he'd still only be up to 19.1 mpg (17.7*1.08)

The OP's vehicle is EPA rated (using the newer, more accurate numbers) at 17 mpg city. The OP is driving only 3 miles, still in warm-up enrichment, in cold weather. That's going to be the biggest issue for him. He could remove the alternator entirely he still wouldn't get back to the 20.9 mpg he was getting before.

The major player here is simply a change in driving habits.

As for OP "operating in open loop", unless his car was manufactured over 20 years ago, most cars enter into closed loop within 10 seconds of start up... that's what oxygen heaters are for... (cars that don't have heated oxygen sensors take significantly longer to enter into closed loop operation.)

OK, you caught me, I played fast and loose with the terminology. It will still be in cold-start enrichment. Your terminological nit-pick has no practical effect. The fact remains that the vehicle, with a cold engine, will still be operating with a richer mixture even if it's technically in closed loop.

ZV

 

[tortillasoup]

The primary resistance from the alternator is resistance from the belt itself and from the inertia or the pulleys and resistance from the bearings. Removing the alternator entirely is going to make a crap-ton more difference than just changing the alternator duty cycle. The fact that you think removing the alternator entirely is comparable to changing the duty cycle is a perfect illustration of why you're mistaken here.

You just said a whole lot of nothing. Are you feeling alright? You remove the belt from the alternator so there isn't a load on the engine. I.e. The alternator doesn't charge the battery.

Let's look at a typical alternator. You're dealing with a maximum output of somewhere around 130 amps. At 12 volts, that's 1,560 watts. For simplicity's sake we'll call it 1,600 watts.

If we assume a 50% efficient alternator, that's 3,200 watts of power sapped from the engine (not counting frictional losses from the belt and pulley system). This is 3,200 Joules/second or 11.52 MJ/hour. Gasoline has 120 MJ/US Gal.

With a 20% efficient engine, a maxed-out alternator will use 11.52/(120*.2) gallons of fuel/hour, or 0.48 gallons/hour.

That's the difference between zero output and max output, so it's easily double the difference between normal load and full load, so we're dealing with maybe 0.24 gallons/hour of difference in fuel consumption.

You're mostly correct, your math is copacetic and I'm ok with your assumptions.

OP's vehicle was rated, under the old, optimistic EPA standards, at 23 mpg highway. At 60 mph and 23 mpg, that's approximately 2.6 gallons/hour. Adding another 0.24 gallons/hour brings us to 2.84 gallons/hour and brings mileage down to 21.12 mpg. For a difference of 8% (1.88/23).

I was off on my initial estimates of the effect of the alternator being run at 100%, but that doesn't change the fact that you're just plain wrong if you think that the main culprit here is the battery charging. Even if he got that 8% improvement over his current 17.7 mpg back, he'd still only be up to 19.1 mpg (17.7*1.08)

Here is where your math and assumptions fall flat. The guy isn't cruising at 60mph, he's cruising at a maximum of 35mph. In city driving, the load from the alternator becomes a significant factor as there is fuel consumption when idling. Also the load curve on the engine is not linear. Finally, this isn't a question of zero output and 100% output, it's a question of more like 20% output and 100% output. On a discharged battery, that alternator really will be pegging 100% because you've not only got the car's own electrical load but also charging the battery as well. Also that 20% efficiency could be peak efficiency, as the efficiency will vary depending on the driving load and whatnot. The engine's efficiency is different throughout the entire load and RPM curve and in city driving, something such as an alternator becomes a major fuel consumer...especially when you have to charge a battery.

The OP's vehicle is EPA rated (using the newer, more accurate numbers) at 17 mpg city. The OP is driving only 3 miles, still in warm-up enrichment, in cold weather. That's going to be the biggest issue for him. He could remove the alternator entirely he still wouldn't get back to the 20.9 mpg he was getting before.

The major player here is simply a change in driving habits.

I disagree. Remember, why this thread started. OP is complaining about steadily decreasing fuel economy. Since we're assuming OP's car is running properly, I say it's safe to say the battery being drained is the culprit. True being in warm up enrichment is no good but he's in hawaii, the weather is approx the same all year round. going from 20mpg to 17mpg is clearly something related to a drained battery. Short trips like OP is doing would clearly explain the battery being drained as it just doesn't have enough time to charge the battery to make up for the losses of cranking the engine + parasitic losses from when the car isn't being driven.

OK, you caught me, I played fast and loose with the terminology. It will still be in cold-start enrichment. Your terminological nit-pick has no practical effect. The fact remains that the vehicle, with a cold engine, will still be operating with a richer mixture even if it's technically in closed loop.

ZV

Well a car in open loop and cold engine uses significantly more fuel than one in closed loop and cold engine. If it was running in open loop for a long time, the oil would get much more polluted than it is now and that will drag down fuel economy. I'm not saying OP's oil isn't heavily polluted anyway but it shouldn't be nearly as bad as it would have been 20+ years ago.

 

[nerp]

You need to take the car on the highway and drive for 30-45 minutes at least once a week. Get her up to speed and nice and hot. It's not going to help fuel consumption but it will keep your car healthy. Your decreasing mileage is because your commute is so short.

 

[Meghan54]

You need to take the car on the highway and drive for 30-45 minutes at least once a week. Get her up to speed and nice and hot. It's not going to help fuel consumption but it will keep your car healthy. Your decreasing mileage is because your commute is so short.

Exactly. You and ZV hit it on the head.

As an aside, the OP is the perfect candidate for a pure elec. car, or at least a good hybrid.