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Have a general question relating to vehicles with EFI. Specifically the fuel supply system. Maybe some newer GWs operate this way, but regardless...

My vehicle has the fuel pump in the tank, which delivers pressurized fuel to the fuel rail for the injectors. A fuel pressure regulator controls the fuel pressure across the injectors, and there is a return fuel line to the tank.

My question is: Is the return fuel line generally under pressure (WRT atmospheric) or under vacuum? In other words, would a leak in the return line spew fuel, or suck air? Does the answer depend on engine loading?

Thanks in advance to this very knowledgable group of techies!

hossners
 

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Most cars have the pump & fuel sender combined in the tank. The fuel pump pumps fuel at the same rate regardless of engine speed and usually only stops with the engine off and the ignition live or dead. The return pipe is usually thinner than the feed pipe because obviously less fule is going to be returned to the tank. If you get a leak in the return line it will leak fuel rather than suck air. Same goes for the supply line.
 

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hossners wrote:
Have a general question relating to vehicles with EFI. Specifically the fuel supply system. Maybe some newer GWs operate this way, but regardless...

My vehicle has the fuel pump in the tank, which delivers pressurized fuel to the fuel rail for the injectors. A fuel pressure regulator controls the fuel pressure across the injectors, and there is a return fuel line to the tank.

My question is: Is the return fuel line generally under pressure (WRT atmospheric) or under vacuum? In other words, would a leak in the return line spew fuel, or suck air? Does the answer depend on engine loading?

Thanks in advance to this very knowledgable group of techies!

hossners
hossners, that depends on the vehicle, vehicle's age, & emission class. In most EFI vehicles (standard type) up until about ayear or so ago, fuel was pumped from the fuel tank by the (in tank)fuel pump, that pressurized fuelflowed to the fuel rail under pressure (that's where the injectors receive theirfuel from), the pressure in the injector rail (or rails) is controlled by the fuel pressure regulator (in most cases that fuel pressure regulator is vacuum compensated from manifold vacuum so the pressure can increase at low vacuum high engine load, orchange as the elevationchanges).. There is usually a fuelreturn line running from the pressure regulator back to the fuel tank. That return line isn't actually pressurized by fuel pressure as it is not restricted at either end but in most cases is a fair amountbelow normal surrounding atmospheric pressure due to the emission requirement that the fuel tank be kept at negative pressure (vacuum)for hydrocarbon emission vapor control. That means that any small leak in either the fuel tank or fuel return line will allow it to suck in air. This applies to the OBD 2 emissioned vehicles. On the later OBD 2 automobiles a service check light should light on the dash if an air leak into the fuel tank (or return line) allows the fuel tank pressure to raise to atmosphere or above.

Some of the very late model EFI vehicles do not use a fuel return line but instead use a variable output pump in the tank& on-demand type fuel supply system.

Very early EFI vehicles (prior to OBD 2) could have a slightly pressurized fuel tank if the vapor recovery system was in the non-recovery mode, or parked with hot fuel in the tank. On those, the return line would also be slightly pressurized under those conditions.

Twisty
 

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. .....and all we really know about Twisty is....... he is a veritable fountain of information, on almost EVERYTHING !!!!... a true GURU/forums/images/emoticons/cool.gif... and he lives in the USA ...  we could pass him at the Safeway, and not know just how close we were to greatness.... "Deep Bow !!!   and Salam !!" .. I am impressed !! Silver/forums/images/emoticons/emoticonsxtra/cooldj.gif
 

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you can think of it as the overflow of the radiator system. The extra fuel returns back to the gas tank. The fuel will be under some pressure. The fuel gets heated from the heat transfer from the surroundings in the engine compartment.


You can remove the gas cap and that will help relieve the pressure on the line.
 

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Hmmm. 2 votes for pressure. 1 vote for vacuum. If I had an extra fuel line, I'd go prick mine and tell you for sure.

Here's more about my particular problem. I have a '94 Ford Taurus (no snickers, please) 3.8l that a few years ago saw a very sudden drop in mpg. From about 24 mpg to 18mpg. Since then have had it to several different shops with no avail. Never find any problems. Has passes numerous emmissions tests, the CC runs in the normal heat range (the lostfuel is not existing via exhaust). Fuel rail holds pressure all night. Runs as good as it ever has. BUT - at times you can smell gas fumes while the engine is running, both in the engine compartment and in the cabin! I'm losing gas somewhere. I've been told that this might just be normal - but know it's not because the vehicle did much betting in the past, plus I smell gas.

The mileage these days is best (22-25) when it's all highway and worst (14-18) when it's stop and go city driving. Like there's a hole in the tank, but the hole is only there when the engine is on. There is no noticable leak or drip.

I thought perhaps a blind spot in the diagnostic chain could be the return line. It is not tested by the pressure testing of the fuel rail. It is only carrying fuel while the engine is running. If this line is pressurized and has a pin hole, fuel could be atomized and lost, affecting my mileage.

Any other thoughts?
 

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Your lines holding pressure for the entire night is a good thing. You can have a leak in your evap system.

There is a special tool that puts a pressure on the system and sees how much it is leaking.


The gas tank has a vent which is drawn to a charcoal canister in front of the car. The vapor is held in suspense in the charcoal. During certain conditions, a solenoid or a vac switch, will purge the canister and cause the vapors to get pushed into the throttle body.

You can also have fuel escaping the seal on the fuel pump or sending unit.


A bad gas cap with a weak spring can release gas into the atmosphere .


If your return line had a leak, you would see it being moist while the car is running.


Dragging brakes can also contribute to lower fuel mileage if your engine is running properly.
 

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hossners wrote:
Hmmm. 2 votes for pressure. 1 vote for vacuum. If I had an extra fuel line, I'd go prick mine and tell you for sure.

Here's more about my particular problem. I have a '94 Ford Taurus (no snickers, please) 3.8l that a few years ago saw a very sudden drop in mpg. From about 24 mpg to 18mpg. Since then have had it to several different shops with no avail. Never find any problems. Has passes numerous emmissions tests, the CC runs in the normal heat range (the lostfuel is not existing via exhaust). Fuel rail holds pressure all night. Runs as good as it ever has. BUT - at times you can smell gas fumes while the engine is running, both in the engine compartment and in the cabin! I'm losing gas somewhere. I've been told that this might just be normal - but know it's not because the vehicle did much betting in the past, plus I smell gas.

The mileage these days is best (22-25) when it's all highway and worst (14-18) when it's stop and go city driving. Like there's a hole in the tank, but the hole is only there when the engine is on. There is no noticable leak or drip.

I thought perhaps a blind spot in the diagnostic chain could be the return line. It is not tested by the pressure testing of the fuel rail. It is only carrying fuel while the engine is running. If this line is pressurized and has a pin hole, fuel could be atomized and lost, affecting my mileage.

Any other thoughts?

Hoosners, you didn't read my post completely you have 2 votes for pressure & one vote for anything from vacuum (OBD2 emission era up, it's the law to have vacuum in those),, to possible pressure due to non purge of the tank vapors under certain conditions to a very slight vacuum during a proper vapor purge mode in the older cars & trucks.

Because the return line is open at the tank end it pretty well sees the same pressure or vacuum that the fuel tank inside does. Probably a slightly higher return line pressure at the engine end than the tank end due to internal friction in the return fuel for the length of the line. (pull thefueltank cap & see if pressure comes out)

IF, your return line is leaking you will see a wet spot at the area it is leaking at.

Many things can be causing your lower MPG reading.

Driving faster than you did when the MPG was better..

Rusted brakes dragging a little

Different tires now than you had before (some can make a big difference)

More weight & junk in the vehicle than before (I find this on a lot of MPG complaint vehicles)

Dirty injectors (it can still pass emission testing as that is pretty easy to pass)

Wear on the camshaft lobes

Wear on the piston rings

Using gasoline with alcohol in it

Poorly functioning fuel vapor recovery system

Leaking spark plug wires or worn plugs

Front or rear end out of alignment (another thing I commonly find of older low MPG vehicles)

Air cleaner or plugging air intake problems.

Heavier engine oil than was in it before.

Dirty outside of the car (not much difference but measurable)

Replacement exhaust system that flows differently than the production exhaust.

02 sensor that is lazy or running out of cal (usually will still pass emission test)

A little of all the above added up..

Twisty
 

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hossners wrote:
Have a general question relating to vehicles with EFI. Specifically the fuel supply system. Maybe some newer GWs operate this way, but regardless...

My vehicle has the fuel pump in the tank, which delivers pressurized fuel to the fuel rail for the injectors. A fuel pressure regulator controls the fuel pressure across the injectors, and there is a return fuel line to the tank.

My question is: Is the return fuel line generally under pressure (WRT atmospheric) or under vacuum? In other words, would a leak in the return line spew fuel, or suck air? Does the answer depend on engine loading?

Thanks in advance to this very knowledgable group of techies!

hossners
Hossners, the return line always has a volume of fuel in it when the engine is running. At higher speeds more volume, lower speeds less volume of gasoline depending on how much the injectors bypass or consume. There will be some pressure in the return line due to the friction factor of the return line tubing, but, you won't see much real pressure in the return line unless you (theoretically)plug the outlet then it would equalize with the high pressure feed line when the injectors were closed and pulsate at varying pressuresas the injectors opened and closed. Otherwise the return line simply routes unneeded fuel back to the fuel tank at a much reduced pressure than the fuel supply line. Some systems don't even return the fuel to the tank, the excess fuel is simply routed back into the fuel supply line via a T ahead of the fuel pump which may be mounted near the engine.

Vic
 

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I've seen this complaint before on ford products. Most of the time it's a thermostst opening too early. Gauge in the car is not accurate enough to tell, try replacing the 'stat with a new 195f one and see if that helps.



oh, and you can't go by the smog test, thats what a catalytic convertor is for, cleans up the exhaust.
 

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Gentlemen,

Some excellent information and great suggestions. I have tried most of the ideas at some point or another in the last 60k miles. A couple tune-ups, rebuilt tranny, new head gasket, new radiator, hoses and thermo. New brakes and tires all inthat time period. The problem has persisted. Again, the drop in mpg was within one or twotanks of gas, so IMHO something broke that should be fixable. Also the data point that the fewer engine hours it takes to burn through a tank of gas the better my mileage seems to point to a possible constant rate leak.

Oldschoolgoldwing - I notice the fuel smell each time I drive the vehicle. Does the canister purge that frequently? Also, does this system test you speak of include the return line?

Twisty - sorry to miscaracterize your response. I reduced your answer to the 'normal operating mode' value for convenience, as this is where I've been for years with this car.

Gambler - the shop has measured the temp of the Cat and found it to be in normal range, therefor my earlies statement that the fuel is not exiting via exhaust (side of engine).

Solving this mystery always returns to the front of my mind as gas prices continue to rise. Maybe time to get my wife a new ride - there is a nice '03 Valkyrie for sale at the local shop that I could 'buy for her' ;)

hossners
 

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hossners wrote:
Gentlemen,

Some excellent information and great suggestions. I have tried most of the ideas at some point or another in the last 60k miles. A couple tune-ups, rebuilt tranny, new head gasket, new radiator, hoses and thermo. New brakes and tires all inthat time period. The problem has persisted. Again, the drop in mpg was within one or twotanks of gas, so IMHO something broke that should be fixable.
Oldschoolgoldwing - I notice the fuel smell each time I drive the vehicle. Does the canister purge that frequently? Also, does this system test you speak of include the return line?

Twisty - sorry to miscaracterize your response. I reduced your answer to the 'normal operating mode' value for convenience, as this is where I've been for years with this car.

Gambler - the shop has measured the temp of the Cat and found it to be in normal range, therefor my earlies statement that the fuel is not exiting via exhaust (side of engine).

Solving this mystery always returns to the front of my mind as gas prices continue to rise. Maybe time to get my wife a new ride - there is a nice '03 Valkyrie for sale at the local shop that I could 'buy for her' ;)

hossners
Hossners,
Also the data point that the fewer engine hours it takes to burn through a tank of gas the better my mileage seems to point to a possible constant rate leak.
That might not be as it seems.. The fewer running hours it takes to empty a tank could also mean it spends less time at idle or much less time in the cold fuel enrichment mode, or fewer cold starts with less stopping & starting. Fewer running hours per tank is consistent with higher mileage as long as the miles traveled are consistent.
I notice the fuel smell each time I drive the vehicle. Does the canister purge that frequently? Also, does this system test you speak of include the return line?
That vapor can should purge any time you have a warm engine that is above idle & the throttle is above closed. Not sure on the older Ford system but some are pulse width modulated to purge at different rates depending on o2 readings or throttle position/engine load.

the shop has measured the temp of the Cat and found it to be in normal range, therefor my earlies statement that the fuel is not exiting via exhaust (side of engine).
You really can't go by converter temperature alone. You could be running a little rich in some modes & still have the converter handle that without generating excess heat. If at one time someone used an incorrect type silicone sealant on your engine at re-assembly you could have a slightly poisoned oxygen sensor & it could be lazy or poisoned now & not keep tight control of the fuel mixture. Those o2 sensors have a large overall effect on your fuel air ratio.

A couple of more thoughts..

If you have a auto trans make sure the torque converter is locking at the correct times. If the trans is different or overhauled the converter lock-up points could be incorrect or not locking at all.. Verify the shift points are correct as just a little difference here can make a big difference in fuel economy.

Check your odo calibration. A slight odo inaccuracy can really show a drop in fuel economy calculations.

Maybe get your car on a chassis dyno & see if the rear wheel power is still there or you have rich place in you fuel cal (monitor the o2 sensor for cross counts as that will tell if youare staying in your mid cal zone).

Can you trace your drop in MPG back to any engine work or other vehicle work?

Sorry I can't be more specific on your problem as I'm not that up to speed on the old Ford emission/fuel control systems. I am presently working on future 2007 & 2008 vehicles for a different auto company.


If you keep smelling raw fuel & can't find the leak look at the top of the fuel tank. Over the years I have seen sender "O" rings seeping,or rust perforate the top of the fuel tank in the air dome area & they don't leak liquid fuel but gas off a lot of vapors & smell like the whole fuel tank is letting it's cargo out.


Twisty
 

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Very informative….

I can smell fuel around my Toyota Echo and the engine light is on. Has been for 2 years and I could not seem to find the problem. Recently found that my gas tank has a bit of rust on it that has seemed to gone through.

Excellent ideas guys!!!

Tim.
 
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