The elusive 10 mpg.

Thomas/NH wrote:
I've been waiting for this post to show up... I just got back from a 2 1/2 week -4500 mile trip down to Fl and back and my mileage just downright sucked. It was so bad I tried everything I could think of to improve it, I switch grades & brands of fuel, I hand figured the mileage BTW: the truck computer was dead on accurate (1/2 MPG), I varied the speeds, my methods of driving, nothing helped much. Best I can tell is; when I left NH it was in the single digits to low teens, the engine must have richen the fuel mixture as the exhaust pipe was black with soot and the tires never got warm at all (7.5-8.5 MPG). As I got further south and it started to warm up (in the 30 & 40s) mileage got better now up to 9-9.5 MPG. Once in the warm weather of FL (70-80*), I finely got back to 10-11 MPG and the exhaust pipe turned white again.

If you guys, recall, I'm the one who boosted that by putting my Kayaks on the roof of my truck I got 2 MPG better mileage... Well, this trip they didn't help me at all, I was ready to throw them off. Maybe I'll try them again next summer.

I have a 2013 F150 Ecoboost HD pulling a 9K Cougar 321RES. The truck did a fantastic job towing the trailer, an excellent combination. The trailer sway control is impressive, it was activated 6 time on the trip (30+ knot wide winds) as well as the integrated ABS sure save the day with a hard panic stop in DC.

Thomas,

I'm not sure that your cool weather mileage is unusual if the morning warm-up period is included in the days mileage check. Even our Prius loses significant fuel efficiency if the fuel check includes warm up on a cold day. If I reset the meter after it is warm and underway, it improves quite a bit for the rest of the trip. All cold gas engines always receive brief huge amounts of fuel for reliable starting and warm-up because the cold mix will not ignite otherwise. High energy ignitions were primarily designed to mitigate this for faster warm-up with a side benefit of less misfire at speed with worn plugs.

Some (or all) of these EcoBoost engines apparently have a mild cold starting problem with over-richness. In your case I suspect that the soot comes from the cold start of the day and the tail-pipe tip never gets warm enough to burn the rear-most residue off in the cool weather. One puff of soot leaves a lot of carbon-black. You might try to see if it stays off the rest of the day by wiping it off after warm-up on a cool day and driving normally. I predict it will no longer accumulate at highway towing speeds, even on cool days.

One of the reasons this solution of the quirky soot problem makes sense to me is that the catalytic converter normally burns excess fuel once it is warmed up. By the time the truck is warm, it has gone into closed-loop operation. Closed-loop merely means the oxygen sensors (and others) determine where the correct stoichiometric mix should be and the computer re-adjusts the amount of fuel delivered in a round-about "loop-back" (feed-back). If the fuel would not be adjusted this way, a rich mix might continue. This would, in turn, burn up the catalytic converters since their job is to burn off what is left, to prevent unburned hydrocarbon emissions out the tailpipe. Cat converters don't work until they are warm, however, so excess fuel can pass without arrest in cold conditions. Thus the soot.

Because the cats don't work until warm, all Fuel Injected (FI) engines, including Direct Injection (DI), run in what is known as open-loop during cold start conditions. By open-loop, I mean there is no feed-back and all fuel is determined by a pre-set "map" (chart) with minimal sensors to basically determine rpm, temperature, barometric pressure and throttle opening and make a good guess. A faulty guess might make soot under some conditions.

From observing reported above average EB mpg under load, and the certain knowledge the engine must meet strict EPA standards, it appears to me as though the turbo, and DI on the EcoBoost engine, may help deliver slightly higher thermal efficiency, therefore improved mpg under load (better BSFC), than competitive non-turbo'd gas engines. If so, this flies in the face of all past turbo gas engines which historically required more fuel (for cooling) under load than their naturally aspirated counter-parts. Such older turbo engines also always got poorer fuel economy and polluted heavily.

There also occasionally appears that there may be a brief deceleration error puff of soot, as some owners have reported tailpipe soot right after stopping a hard run. If this is true, whatever the cause, it seems to me it cannot be a chronic sooty, rich condition unless there is serious over-heating/plugging damage to the catalytic converter from excess hydrocarbons. Such a chronic rich engine should not pass emissions tests and should reveal defective emission control(s).

Wes
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