My FE is falling and I don't know why

[GasSavers_RoadWarrior]

Your two 42mpg tanks had smaller fills, the 6 gallons or so still in the tank was probably warm and took up more space, then you couldn't get as much in as you'd actually used, leading you to think you'd used less.

[optix]

Dirty air filter
Worn spark plugs

EDIT: Wait. This graph is MPG, not l/km. Heh. Looks to me like you're doing better. I have 'blips' all the time, I don't worry about them.

[theholycow]

With the exception of varying traffic conditons, the tanks I'm talking about have approximately the same EOC habits.

I seriously can't believe the car's condition could have changed at all. It's only got 8,000 miles on it, and the two 42mpg tanks were within the last 1000 miles or so; one of them was immediately before the first lower 38mpg EOC tank. I should check my tires, but there's no reason for anything to be worn or malfunctioning.

Ford Man, that's a great point about the large effect even a little bit of EOC has on average. I did the math after my first EOC tank and wrote about it in my gaslog.

RW, the latest short fill tank should have been as good as the others, but maybe filling error is where my inconsistency is.

Ok, I don't feel so bad about it now. It is a little demoralizing to make so much effort and have the payoff disappear, but I guess it's just normal inconsistency. I know my current tank is going to suck, traffic at my difficult left turns has been going fast, traffic where I'd normally have room to EOC has been going slow, and I've had to exercise the car's limits three times just this morning (two hard accelerations after left-turning and finding another car coming fast, and one semi-difficult pass).

[JanGeo]

8k on the clock is still new and breaking in and the ECU is probably programmed to change A/F mixtures a little as the breaking in progresses. You also are getting a little more comfortable with the car and not babying it as much.
Another BIG thing is your oil . . . you changed it when?

Lastly if you have been lugging the engine then you may not have broken in the engine bores as well as you could have with light throttle use and gentle revs. Even a moderate throttle opening at really low rpms results in high combustion pressures and low intake vacuum which equals heavy loads.

Try a 5 mile warmup and then a first gear rev to near redline with very light throttle use and see how much more gas you have to keep giving it as the RPM climbs. This loosens up the rings a little and gets the pistons traveling up the bore a little further getting some much needed oil to the top rings and also indicates how loose your engine is getting as it breaks in.

[theholycow]

Changed the synthetic oil at 5000. Next scheduled change is 10,000, then every 10,000.

I definitely did not baby it when I first got it...I kinda beat on it, enjoying my new speed and manual transmission too much (still beat the EPA estimates, though). RPM varied a lot during the first thousand miles or so, and I didn't shift really low until after that.

I don't lug but I do WOT+low RPM. It's possible that it hasn't broken in properly.

I don't understand the high combustion pressure thing; it's an idea I've seen a few times before but it never made sense to me. How does that work? It's not getting much more air or fuel and not making as much torque as it would at higher RPM.

I'll have to try that procedure to get a feel for the engine and maybe loosen it up. How could the pistons travel further up the bore while still attached to the same crankshaft?

There's always so much more to learn here.

[GasSavers_RoadWarrior]

Originally Posted by theholycow

I'll have to try that procedure to get a feel for the engine and maybe loosen it up. How could the pistons travel further up the bore while still attached to the same crankshaft?

Well he's either meaning the ~20 thou extra from hitting the top of the bearing clearnace and thermal expansion, or he's using one of these new fangled elastoviscous siloxane variable displacement crankshafts.

[Ford Man]

Another thing to think about if the car is still that new and still in the break in period fuel mileage will probably fluctuate a little until everything gets loosened up good. It has always taken my new cars several thousand miles before I started getting top FE. Once you know the engine is completely broken in you could also add a bottle of Slick 50 or equivalent to cut down on some of the friction. That's what I did in my '97 Escort Wagon and I've been using it in the '88 Escort since about 150K miles. A bottle about every 50,000 miles or so.