imported post
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