Poor gas mileage and acceleration

Tiny
DGREGORY70
  • MEMBER
  • 2012 FORD F-150
  • 5.0L
  • V8
  • 4WD
  • AUTOMATIC
  • 85,000 MILES
Hi, I've been getting poor gas mileage and acceleration for awhile now. Based on the gas mileage displayed on the dash I used to get 17 city and 19 or 20 highway. My son used to race go karts and I would have the whole family and a small trailer and still get at least 17 on the highway and about 15 if I took the back way home on secondary stop and go roads. Now I can't get past 15-15.5 anywhere really. I used to play around and reset the trip computer on the highway and see how high the instantaneous mileage would go, usually I could get about 23 until I exited. I tried it to day and it never went higher than 18.5 and ended up at 16.5 when I exited. This was cruising around 70 MPH at 1,800 RPMs. It also doesn't accelerate very well. It doesn't stumble, it just feels like I have a big truck and a small engine. IT wasn't this bad pulling the trailer. It doesn't really accelerate well until about 2,500 RPMs and then it feels better, but not great.

I've replaced the MAF sensor, spark plugs and just installed a new K&N filter. No improvement. I've also cleaned the throttle body, checked the temperature readings on the Cat's sprayed brake cleaner and carb cleaner around the vacuum lines I could see while it was running to check for vacuum leaks and nothing. I've also checked the temps on the rotors after driving and they seem okay as well. I have a older scan tool and have checked what I could based on web research and nothing seems to stand out. It's also never thrown any codes. I have mechanics check it but not in the last 2 years and they just tell me everything fine, but I don't know if they've test drove it or just hooked it up to the computer.

Anything you can think of or recommend would be appreciated. It's driving me crazy, but the trucks paid for and really clean still, but I'm going to be pulling a boat later this summer and I'm afraid of how it's going to react now with a trailer.

Thank you
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Thursday, June 10th, 2021 AT 1:56 PM

6 Replies

Tiny
KASEKENNY1
  • EXPERT
There could be a couple other reasons for this and we need to start with revving the engine and look for black smoke. You may not have much but we need to start with this as I suspect you have some injectors starting to leak too much fuel.

https://www.2carpros.com/articles/engine-black-smoke

Next we need to use that scan tool and monitor the o2 sensor data when you are driving down the highway.

There is a chance you have a faulty o2 sensor and that is not telling the PCM that it is over fueling so it continues to dump too much fuel.

We need to see what the voltages are or your short term fuel trims.

Next we need to see what the old spark plugs looked like. Did they show signs of fouling?

Below are a couple charts that will help with this. Please let us know what questions you have, then what you find. Thanks
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Friday, June 11th, 2021 AT 6:20 PM
Tiny
DGREGORY70
  • MEMBER
Okay, I ll take a look at those things tomorrow and let you know what I find. I changed the plugs awhile ago and I think they re gone, but I did take some pictures which I ve included for you to look at. For some reason I only have 7, but they were all very similar. Tell me if you see anything, I always have a hard time reading them. It always seems like it could fall into several categories.
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Friday, June 11th, 2021 AT 7:16 PM
Tiny
DGREGORY70
  • MEMBER
I've got 3 videos for you here, I'm not sure if I've got the readings you want to look at or not. If not let me know and I'll go back out. I also apologize in advance for the quality of the 2nd video, my 12 year old was holding the camera because I thought that would be better than me doing it while driving and it didn't quite work out that way.

The 1st video was me accelerating onto the highway this morning after about a 5 minute drive.

The second video was me entering the highway again but with different measurements on the screen.

The last video was me cruising on the highway after the second vide, with a different set of readings. Hopefully one of these helps.
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Saturday, June 12th, 2021 AT 12:21 PM
Tiny
KASEKENNY1
  • EXPERT
The numbers are not out of line. However, the last three spark plugs that you had pictures of appear to be carbon fouled. Do you know which cylinders these came out of?

I would think about changing the injectors out of these cylinders. Clearly anytime we are dealing with fuel mileage it is very subjective to the amount of hard accelerating you do but if you have been driving the same the whole time then I am thinking you have an injector or two leaking.

The only other possible cause would be o2 sensor issues. However, they appear to be operating properly.
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Sunday, June 13th, 2021 AT 8:32 PM
Tiny
DGREGORY70
  • MEMBER
Yeah, the driving is pretty much the same, especially just cruis8ng down the freeway. I ll have to look and see if I still have the info for the spark plugs. Would the injectors account for sluggish acceleration?
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Monday, June 14th, 2021 AT 5:15 AM
Tiny
KASEKENNY1
  • EXPERT
Yes. The injectors over fueling will throw off the fuel mixture but I was hoping to see this in the o2 sensor data. However the plugs are showing it so I would check the fuel pressure and then let it sit with the gauge on it and see how long it takes to bleed off.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Monday, June 14th, 2021 AT 6:54 AM

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