Reporting MPG

[Spurlee]

When I evaluate my MPG I just look at whatever my '96 ETC calculates. I noticed today, though, that it's measure of fuel consumption is about 10% less than what I actually put in the tank. If the computer used the gallons consumed figure to calculate the MPG it's seriously over estimating the actual figure.

I think for a while I'll keep track of MPG the old way with a piece of paper and see what the difference is.

[Ranger]

No I believe it calculates it based on fuel injector pulse counts and pulse width. PCM counts the pulses and does the math. I may be over simplifying it but I believe that is the basics of it. Pretty acurate from what I understand.

[KHE]

The fuel economy as calculated by the PCM is VERY accurate. As Ranger said, it keeps track of the injector pulse and using the flow rate of the injectors, calculates the fuel used and the miles per gallon. Assuming you reset the average fuel economy when you fill up, calculate the fuel economy from the gas pump gallons dispensed and it is significantly lower than the ave mpg on the fuel data center, it could indicate a leaking fuel injector or bad fuel pressure regulator.

[JasonA]

Yeah, the fuel mileage calculated by the car will be much more accurate than what you can calculate by hand, assuming there are no problems with your fuel system (like a physical leak or leaking injectors).

[Spurlee]

The fuel economy as calculated by the PCM is VERY accurate. As Ranger said, it keeps track of the injector pulse and using the flow rate of the injectors, calculates the fuel used and the miles per gallon. Assuming you reset the average fuel economy when you fill up, calculate the fuel economy from the gas pump gallons dispensed and it is significantly lower than the ave mpg on the fuel data center, it could indicate a leaking fuel injector or bad fuel pressure regulator.

I see...what's the diagnostic drill for detecting leaking injectors and checking the pressure regulator?

Thanks!

[WarrenJ]

I noticed today, though, that it's measure of fuel consumption is about 10% less than what I actually put in the tank  . . . .  it's seriously over estimating the actual figure.

Well now, that's a puzzlement!

I think we're all in agreement that the indicated MPG is VERY accurate in our cars. What then would explain this descrepancy?

Any failure mode I can think of would report a DECREASED MPG figure.

I guess we'll simply have to wait for your response. Please let us know what you find.

Regards,

Warren

[adallak]

I noticed today, though, that it's measure of fuel consumption is about 10% less than what I actually put in the tank  . . . .   it's seriously over estimating the actual figure.

Well now, that's a puzzlement!

I think we're all in agreement that the indicated MPG is VERY accurate in our cars. What then would explain this descrepancy?

Any failure mode I can think of would report a DECREASED MPG figure.

I guess we'll simply have to wait for your response. Please let us know what you find.

Regards,

Warren

Sounds like "measure of fuel consumption" is the "Fuel Used" data, not the MPG. Kind of confusing inputs.

[KHE]

The fuel economy as calculated by the PCM is VERY accurate.  As Ranger said, it keeps track of the injector pulse and using the flow rate of the injectors, calculates the fuel used and the miles per gallon.  Assuming you reset the average fuel economy when you fill up, calculate the fuel economy from the gas pump gallons dispensed and it is significantly lower than the ave mpg on the fuel data center, it could indicate a leaking fuel injector or bad fuel pressure regulator.

I see...what's the diagnostic drill for detecting leaking injectors and checking the pressure regulator?

Thanks!

FPR- disconnect the vacuum line and turn the key to on - if fuel drips out of the FPR nipple, it is bad.

For the injectors, remove the fuel rail from the manifold - leave the injectors attached to the fuel rail and turn the key to the ON position to pressurize the fuel rail. There should not be any fuel at the manifold side of the injectors - if there is, the injector is leaking.

[Spurlee]

The fuel economy as calculated by the PCM is VERY accurate.  As Ranger said, it keeps track of the injector pulse and using the flow rate of the injectors, calculates the fuel used and the miles per gallon.  Assuming you reset the average fuel economy when you fill up, calculate the fuel economy from the gas pump gallons dispensed and it is significantly lower than the ave mpg on the fuel data center, it could indicate a leaking fuel injector or bad fuel pressure regulator.

I see...what's the diagnostic drill for detecting leaking injectors and checking the pressure regulator?

Thanks!

FPR- disconnect the vacuum line and turn the key to on - if fuel drips out of the FPR nipple, it is bad.

For the injectors, remove the fuel rail from the manifold - leave the injectors attached to the fuel rail and turn the key to the ON position to pressurize the fuel rail. There should not be any fuel at the manifold side of the injectors - if there is, the injector is leaking.

Thanks. Can you point me to the FPR Vacuum line? I read a lot about FPR and should check it...

[KHE]

The fuel economy as calculated by the PCM is VERY accurate.  As Ranger said, it keeps track of the injector pulse and using the flow rate of the injectors, calculates the fuel used and the miles per gallon.  Assuming you reset the average fuel economy when you fill up, calculate the fuel economy from the gas pump gallons dispensed and it is significantly lower than the ave mpg on the fuel data center, it could indicate a leaking fuel injector or bad fuel pressure regulator.

I see...what's the diagnostic drill for detecting leaking injectors and checking the pressure regulator?

Thanks!

FPR- disconnect the vacuum line and turn the key to on - if fuel drips out of the FPR nipple, it is bad.

For the injectors, remove the fuel rail from the manifold - leave the injectors attached to the fuel rail and turn the key to the ON position to pressurize the fuel rail. There should not be any fuel at the manifold side of the injectors - if there is, the injector is leaking.

Thanks. Can you point me to the FPR Vacuum line? I read a lot about FPR and should check it...

The FPR is on the fuel rail - it is a cannister about 1" in diameter and 3/4" high. You will see the vacuum line when you locater the FPR. Watch for fuel at the FPR nipple, not the vacuum line.

[cadillacjeff7777]

I had a 93 Seville for a while. I once took it to this dim witted mechanic and he advised me to go to the store and put some injector cleaner in. I did so and then took off for a 400 mile trip. I was on the returning 400 miles and noticed at the pump that the gallons put in was about 2 gallons over what the fuel used was on the DIC. I swore that I had a leak. Took it to another mechanic's shop and he went over it with a fine tooth comb finding no leak. I then took it to the dealership and was told that the injector cleaner had eaten away something in the injector so that on every injection pulse it was injecting more fuel(so the computer was computing everything based on a correct pulse width)

[Spurlee]

I had a 93 Seville for a while. I once took it to this dim witted mechanic and he advised me to go to the store and put some injector cleaner in. I did so and then took off for a 400 mile trip. I was on the returning 400 miles and noticed at the pump that the gallons put in was about 2 gallons over what the fuel used was on the DIC. I swore that I had a leak. Took it to another mechanic's shop and he went over it with a fine tooth comb finding no leak. I then took it to the dealership and was told that the injector cleaner had eaten away something in the injector so that on every injection pulse it was injecting more fuel(so the computer was computing everything based on a correct pulse width)

I've used injector cleaner every other oil change for years with no ill effects. Seems like if the injectors were damaged in some way enough to seriously affect mileage I'd see other signals from running a too-rich fuel mixture...thrown codes, fouled plugs, etc.

[TDK]

Regarding the mileage calculation,

The BRD (Big Red Dog) has never had any fuel injector cleaner, injector work or FPR replaced.

It consistently runs 23.5 average mpg at about 82mph with little cross/head or tail winds on level ground for about 2-hours. This has been verified several times by re-fueling after the trip.

The only error I have ever noticed is when running above 90 or into the triples for some time, the mileage indication actually goes UP! Perhaps the mileage data register has an upper limit on injector pulse accumulations per time frame - compared to the miles covered input over the same time frame. Maybe the computer engineer figured that mileage accuracy at the upper end is not that important?

[adallak]

Have you ever tried to switch to neutral while coasting down with 70 MPG instant fuel economy? The MPG goes down! So, according to the computer, you are not saving fuel by switching to neutral.

[JasonA]

It's been said here before that during long coasting situations, the PCM can actually turn OFF any or all of the fuel injectors to save fuel, as long as the momentum of the car can keep the engine running. By putting it into neutral, you are forcing the engine to run on its own, albeit at a slower speed. The best way to save fuel going down a long hill is to let the PCM do the job it was given.

[adallak]

It's been said here before that during long coasting situations, the PCM can actually turn OFF any or all of the fuel injectors to save fuel, as long as the momentum of the car can keep the engine running. By putting it into neutral, you are forcing the engine to run on its own, albeit at a slower speed. The best way to save fuel going down a long hill is to let the PCM do the job it was given.

Won't turning the injectors OFF make the engine stall?

[JasonA]

That's the whole gist of keeping the car in drive when you're coasting down a hill. Turning the injectors off (one or more of them) of course will reduce the power of the engine. Turn them all off and, if the engine were just idling, it would stall. But when you're in drive coasting down a hill, the momentum of the car (and gravity) is keeping that engine (as well as your accessories) turning.

That's the very reason that putting the transmission in neutral on these cars is ineffective at reducing fuel use. If the transmission is in neutral, nothing is keeping the engine running but the air and fuel that the PCM is controlling. And in fact, the engine doesn't just "idle" as you roll down the road. You'll notice that the idle speed is somewhere between 1000-1200 rpm. This is to keep the engine running as you load it with A/C and power steering because the car would no longer keep the engine running (with the transmission in neutral).

That's exactly why it's best to leave the transmission in drive to achieve the best fuel economy.

A coworker swears that putting his Camry in neutral coasting down a hill extends his range by 100 miles a tank. Who knows. Maybe he's full of carp. Or maybe Toyota's idea of engine and fuel control is different from Cadillac's. Either way, neutral doesn't work for us (for fuel economy).

[adallak]

That's the whole gist of keeping the car in drive when you're coasting down a hill. Turning the injectors off (one or more of them) of course will reduce the power of the engine. Turn them all off and, if the engine were just idling, it would stall. But when you're in drive coasting down a hill, the momentum of the car (and gravity) is keeping that engine (as well as your accessories) turning.

That's the very reason that putting the transmission in neutral on these cars is ineffective at reducing fuel use. If the transmission is in neutral, nothing is keeping the engine running but the air and fuel that the PCM is controlling. And in fact, the engine doesn't just "idle" as you roll down the road. You'll notice that the idle speed is somewhere between 1000-1200 rpm. This is to keep the engine running as you load it with A/C and power steering because the car would no longer keep the engine running (with the transmission in neutral).

That's exactly why it's best to leave the transmission in drive to achieve the best fuel economy.

A coworker swears that putting his Camry in neutral coasting down a hill extends his range by 100 miles a tank. Who knows. Maybe he's full of carp. Or maybe Toyota's idea of engine and fuel control is different from Cadillac's. Either way, neutral doesn't work for us (for fuel economy).

To accurately calculate the average fuel economy one should take into consideration the speed of the car as well. While running down a hill car will go faster in neutral, which can affect the average fuel economy noticeably.

[JasonA]

The speed of the car is not a part of the average fuel economy calculation. Average fuel economy is very simple:

miles driven / gallons used

Whether you drive 10 mph or 100 mph, in drive or in neutral, the calculation will still be the same, miles driven / gallons used. Of course, if you drive 100 mph, the wind resistance is a huge factor and will drop your economy, etc. And there are many other factors which can affect the final result, but the calculation itself is the same regardless. For instance, you could turn your engine on and let it idle, and coast down a 50 mile slope, and evaluate your avg. economy at the bottom. 50 miles driven / .015 gallons used or whatever the incredibly small amount would be. It's like 3333 mpg. Accurate? Yes. Useful or realistic? No.

As long as you have an accurate count of the distance driven and the amount of fuel used, your avg. mpg calculation will be accurate, regardless of the vehicle speed during that duration.

[adallak]

The speed of the car is not a part of the average fuel economy calculation. Average fuel economy is very simple:

miles driven / gallons used

Whether you drive 10 mph or 100 mph, in drive or in neutral, the calculation will still be the same, miles driven / gallons used. Of course, if you drive 100 mph, the wind resistance is a huge factor and will drop your economy, etc. And there are many other factors which can affect the final result, but the calculation itself is the same regardless. For instance, you could turn your engine on and let it idle, and coast down a 50 mile slope, and evaluate your avg. economy at the bottom. 50 miles driven / .015 gallons used or whatever the incredibly small amount would be. It's like 3333 mpg. Accurate? Yes. Useful or realistic? No.

As long as you have an accurate count of the distance driven and the amount of fuel used, your avg. mpg calculation will be accurate, regardless of the vehicle speed during that duration.

if you coast down in neutral faster then in gear you will cover the same distance (miles driven) in shorter period of time, which means your engine will be burning gas for shorter period of time abd consiquently will use less fuel (galons used). Same [miles driven]/less[gallon used]=moreMPG=better average fuel economy.

50 miles driven is something like 50 minutes. Let your engine idle for 50 minute and believe me it will burn much more than 0.015 gallon. I am afraid more than a gallon.