Incredibly bad MPG's

[Tomm1468]

I swapped in a freshly built 5.0 about 3000 miles ago. everything is new, and mostly motorcraft parts, minus the spark plugs and 02 sensors. I was getting about 17 mpg in the city when i first started driving it. but its down to about 9 in the city now and 15 on a pure hwy drive. I'm trying to figure out why its so bad. I did have my cam position sensor go out on me, which i replaced, didnt time, and after about 2 weeks i went back in and timed it properly, and replaced my spark plugs. but still, terrible mpg. my only thoughts now are a vacuum leak, or bad 02 sensors. I went around and replaced the questionable vacuum lines, then went around with propane and tested all the places there could be a leak, it seems to be all tightened up. so now my only guess is the 02 sensors. my b2s2 jumps all over the place, while my b1s2 stays basically the same while idling. my short term fuel trim is pretty steady on bank 1 around -1.6 -> .8 -> 1.6. my bank2 short term is around -3 -> +3. so obviously these are still "within spec" but im thinking about getting some motorcraft upstream sensors to do the sensing. I've also heard that MAF could be causing the issue, that was hanging at .8lbs/ min at idle with the TP at 18%. I've been trying my best to make sense of these numbers since its my first time even looking at fuel trim. I already have a new MAF sensor that I was going to put in when i rebuilt everything, but ended just sticking with the OEM one (because i'm lazy and the OEM one seems to be working). I know i'm kind of just throwing parts and money at a problem trying to solve it, but its the best i could do, but i'm sick of doing that, and neeeeeeed to get back to the 17 mpg. I cant drive a half size truck on 9 mpg.

 

[Tomm1468]

so I took it out driving and watched the trims, the short term 1 and 2 both fluctuate a lot while city driving -8 to +8 while my long term 1 and 2 usually are between -3 and +4. It seems like a pretty wide range? Even though it’s all technically “within spec” so idk anymore. The pic is while idling.

 

[superj]

i have no idea but i would be in the same boat you are, if it was happening to me. i would have no idea what the numbers should be or if they are bad. i cannot believe no one has come and offered advice though as some of these dudes really know their engine stuff

 

[RonD]

Those fuel trims, STFT and LTFT are fine, yes STFT jumps around a lot, LTFT jumps around less but will also change
The "ranges" are not an issue, -10 to +10 is acceptable
Upstream O2s(Sensor 1s) can do False Leans so computer runs engine too rich, so STFT looks good but is wrong, pull a couple of easy to reach spark plugs and look at the tips, light brown is about as dark as you want to see, the darker the color the Richer the mix that cylinder is getting

S2(sensor2's after the Cats) O2's voltage should be 0.75v-0.85v, if its lower the computer will add extra fuel to try and get the Cats hotter, but wouldn't be that much extra, i.e. 17 down to 9mpg


You most likely have a fuel leak
In cooler temps its often not smelt, top of gas tank, maybe fuel filter, engine bay is warm enough so you would smell it there

 

[Tomm1468]

I did have a period of a few days that I was smelling gas when I started the truck, but I attributed that to the crank position sensor being misaligned, but that smell was coming from right around the fuel pressure regulator, but I was unable to find any gas around it. Could it be leaking into the vacuum?

 

[Bear in the woods]

Yes. When they go bad. They will have gas in the vacuum line where it’s hooked. They will cause bad fuel mileage. On rare occasions they will cause low fuel pressure and make truck weak. Think they cause flooding most of the time when bad

 

[Bear in the woods]

Yes. When they go bad. They will have gas in the vacuum line where it’s hooked. They will cause bad fuel mileage. On rare occasions they will cause low fuel pressure and make truck weak. Think they cause flooding most of the time when bad

@RonD see I do pay attention to things you have told me on my truck problems I agree.HE‘S THE MAN

 

[Tomm1468]

Soooo, how to fix this ? Do I have to get a new fuel rail? Or what ? lol, cuz I can’t find these parts for sale anywhere

 

[Bear in the woods]

Soooo, how to fix this ? Do I have to get a new fuel rail? Or what ? lol, cuz I can’t find these parts for sale anywhere
[

. Get a new fuel pressure regulator

 

[Tomm1468]

Alright, update. I pry’d off the upper intake and looking for a fuel leak. I didn’t see any visible signs of leak, the upper intake had a strong smell of gasoline coming from where it connects to the lower intake. I did smell a slight smell from the vacuum hose that attaches to the fuel rail, but when I turned the fuel pump on a few times in a row, nothing leaked out from it. So I do smell gas, but it’s unclear where it’s coming from at this point.

 

[bobbywalter]

Are you using 100 percent OEM induction?

I have seen 16 mpg go to 9mpg with filter change and goo on meter.

 

[Tomm1468]

Yes everything is oem, with 1 exception, my iac gave out a month back or so, I replaced it with whatever they sell at oreillys, but that was long after the mpg died, I do however still occasionally get a check engine on my new cam synchronizer as being p1309 not aligned, I know that it has some control over the way the fuel sprays, so maybe I just didn’t get it all the way lined up. (One or two teeth off) or some such. But I always attributed that to my ground being weird since I put in a new alternator, because the warning only pops up sometimes. But idk anymore it this point. Maybe one of the injectors is stuck open ?

 

[bobbywalter]

Hmm

 

[Tomm1468]

so what I’m thinking rn, I have to get another alternator, cuz this one has issues. I’ll get that installed (which hopefully will fix the voltage issues.) after which I’ll pull the fuel injectors, and check each one, to make sure it’s working, and I’ll double check the alignment on that cam synchronizer. Put everything back together. Unless I can find a new fuel rail at the pick n pull down the road from me, which then I’ll install that. Put it all back together, and see if that helps. If not I’ll put in the new MAF sensor I have on hand, and if that still doesn’t help, I’ll be back