Getting semi poor gas mileage, not sure why

[Fk8_FiST386]

Was a Ford tech for about 10 years, didn’t deal with many FiST’s though so this has me kind of stumped.
15 FiST with 54k miles. I’m getting 18-19mpg in town not really even getting into boost. Mountune turboback with catted downpipe and Mishimoto intake for bolt-ons is all it has so far. Gonna go hybrid turbo etc.
Bought the car about 3 weeks ago, they ran regular gas in it for half a tank(maybe more). I’ve been transitioning back to premium(92 octane here) and I feel like each fill up the mileage is going down. Haven’t really been beating on the car, and shifting when the “shift light” comes on to try and get better mileage. Does it need to regulate everything through a few tanks? Or do I need a tune to compliment everything? A lot of driving is in town but I know people are getting better in town. Thanks

 

[FiestaSTdude]

Was a Ford tech for about 10 years, didn’t deal with many FiST’s though so this has me kind of stumped.
15 FiST with 54k miles. I’m getting 18-19mpg in town not really even getting into boost. Mountune turboback with catted downpipe and Mishimoto intake for bolt-ons is all it has so far. Gonna go hybrid turbo etc.
Bought the car about 3 weeks ago, they ran regular gas in it for half a tank(maybe more). I’ve been transitioning back to premium(92 octane here) and I feel like each fill up the mileage is going down. Haven’t really been beating on the car, and shifting when the “shift light” comes on to try and get better mileage. Does it need to regulate everything through a few tanks? Or do I need a tune to compliment everything? A lot of driving is in town but I know people are getting better in town. Thanks

Weird, even getting on it I can get 24-22mpg in town. Have you tried going back to a tank of regular to see if mpg changes? Also, if you've been filling up at the same station, I would try a different one to see if anything changes.

 

[Fk8_FiST386]

Weird, even getting on it I can get 24-22mpg in town. Have you tried going back to a tank of regular to see if mpg changes? Also, if you've been filling up at the same station, I would try a different one to see if anything changes.

No I haven’t tried putting regular in, I don’t want to potentially ruin anything or start pinging. I fill up at Costco and have had WAY better luck with gas there than anywhere else. I didn’t have the issue with my SVTF and I tried a few places with it.

 

[Jabbit]

Clean the MAF, check your air filter, calculate mpg from the pump/odometer and not the onboard estimate.

 

[Capri to ST]

You're right, you should be getting better mileage in town, the lowest mileage I've ever gotten is probably around 24 MPG, and that was with a significant amount of town driving, and getting on it hard when I can.I completely ignore the shift light, I think it wants you to shift way too early, and makes me think that I would be lugging the engine.
I think Jabbit had some good suggestions in the post before this for places to start. I would also check and be sure your tires aren't significantly under inflated, although the TPMS light should have alerted you if that was the case.

 

[Fk8_FiST386]

You're right, you should be getting better mileage in town, the lowest mileage I've ever gotten is probably around 24 MPG, and that was with a significant amount of town driving, and getting on it hard when I can.I completely ignore the shift light, I think it wants you to shift way too early, and makes me think that I would be lugging the engine.
I think Jabbit had some good suggestions in the post before this for places to start. I would also check and be sure your tires aren't significantly under inflated, although the TPMS light should have alerted you if that was the case.

I checked pressure yesterday for a 3rd time. I don’t like how it wants higher pressure in the front so it’s 35psi all around.
I have a completely full tank right now, it’s only my second time filling up. When it gets down to a half tank, I’ll try gas at another place. I’ll also start ignoring the shift light. Any other suggestions? Nothing is wrong with the car, so I’m hoping it just needs a few tanks ran through it.

 

[SteveS]

Regular unleaded (87 octane) will not damage anything. That said, switching between 87 and 91 or 93 octane will not affect your mileage that much. Even using ethanol won't change it that much.
Something else is wrong. Being stuck in open loop might do it. A misfire might do it (but should also trigger a code). Faulty fuel gauge sender could screw things up.

 

[Fk8_FiST386]

Clean the MAF, check your air filter, calculate mpg from the pump/odometer and not the onboard estimate.

Everything is clean, checked it all. I was getting 24mpg. I’ll have to calculate my mileage. I’ve gone 60 miles and my needle hasn’t moved, so idk if my mpg meter is off or needs to recalibrate itself. I reset it when I filled up last and I didn’t do that the first time I filled up.

 

[Fk8_FiST386]

Regular unleaded (87 octane) will not damage anything. That said, switching between 87 and 91 or 93 octane will not affect your mileage that much. Even using ethanol won't change it that much.
Something else is wrong. Being stuck in open loop might do it. A misfire might do it (but should also trigger a code). Faulty fuel gauge sender could screw things up.

It’s throwing zero codes, warms up fine, never smells rich. Gonna go check codes again right now with my snap-on scanner.

 

[Fk8_FiST386]

Nothing..

 

[FiestaSTdude]

Everything is clean, checked it all. I was getting 24mpg. I’ll have to calculate my mileage. I’ve gone 60 miles and my needle hasn’t moved, so idk if my mpg meter is off or needs to recalibrate itself. I reset it when I filled up last and I didn’t do that the first time I filled up.

My needle never moves for the 50 miles or so.

 

[FiestaSTdude]

I checked pressure yesterday for a 3rd time. I don’t like how it wants higher pressure in the front so it’s 35psi all around.
I have a completely full tank right now, it’s only my second time filling up. When it gets down to a half tank, I’ll try gas at another place. I’ll also start ignoring the shift light. Any other suggestions? Nothing is wrong with the car, so I’m hoping it just needs a few tanks ran through it.

From what I've heard others say on here the pressure difference is something you definitely want to have, it helps with preventing oversteer or something like that.

 

[Fk8_FiST386]

From what I've heard others say on here the pressure difference is something you definitely want to have, it helps with preventing oversteer or something like that.

I’ll go back to 39psi in the front. I’m just not used to having so much air in a fwd car that spins.
We’ll see. Cleaned my air filter for shits and giggles.

 

[SteveS]

Even in 1978 we ran higher front tire pressure in the Fiesta. Very common in a FWD car. Lot more weight over the front tires. Virtually none in back.

 

[Alkoran]

Clean the MAF, check your air filter, calculate mpg from the pump/odometer and not the onboard estimate.

Why clean the MAF? It's not even used by the ECM to calculate mass air in the US cars, only the temp signal is used.

 

[Fk8_FiST386]

PO just got ahold of me and has a Cobb AP he forgot to give me.. Just picked it up, gonna check if it has a tune on it after I get off.

 

[Fk8_FiST386]

Fixed it. Mountune stage 1. Car feels quite a bit quicker now. Gonna do my intercooler and a hybrid turbo when my tax returns hit my account. And of course a Dizzy tune.

 

[FiestaSTdude]

Fixed it. Mountune stage 1. Car feels quite a bit quicker now. Gonna do my intercooler and a hybrid turbo when my tax returns hit my account. And of course a Dizzy tune.

So the tune was the issue?

 

[Fk8_FiST386]

So the tune was the issue?

Yep. Flashed the tune, drove it around, and gas mileage was jumping up so fast. Feels much quicker now and is getting 25’ish mpg driving fairly spirited.

 

[FiestaSTdude]

Yep. Flashed the tune, drove it around, and gas mileage was jumping up so fast. Feels much quicker now and is getting 25’ish mpg driving fairly spirited.

Glad you are back in action, enjoying your car!