Poor Fuel Mileage
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Junior User
Joined: Jun 2011
Posts: 52
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From: Ravenna,Ohio
Poor Fuel Mileage
Ok some of you may already know I done a whole mess of work on my ol truck the past month or so. New cap rotor distributor plugs wires Icm Iac Egr Tps valve cover gaskets and Upper manifold gasket. Thats off the top of my head. Its running but the problem is im only getting 6.5-7.5 mpg. I have no clue and the few mechanics I took it to tell me there is nothing wrong. It does not have any codes. Before all this I was getting around 12 city. Thats a big difference. I can almost watch the gauge move. Any ideas/info would be great. 86 f150 302 efi
Senior User
Joined: Mar 2013
Posts: 258
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From: Arkansas
Is all your emissions stuff hooked up correctly? Have you performed a compression test? Are you burning oil or getting blow by? Do you have cats? Does your truck feel sluggish and underpowered? A little bit more info would help try and figure it out.
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Junior User
Joined: Jun 2011
Posts: 52
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From: Ravenna,Ohio
Everything is hooked up. Compression is fine. No burning oil no blow by no cats. The truck seems to have all the power except when it downshifts it sounds kinda sluggish. As soon as you give it the gas it goes though but its been like that since I got it 2 1/2yrs ago
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
Several thoughts, but just guesses. Are you SURE you got the plug wires on in the right order? It is possible to swap a couple and still have it run fairly well.
And, when you say "no codes" that includes both tests?
And, when you say "no codes" that includes both tests?
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Junior User
Joined: Jun 2011
Posts: 52
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From: Ravenna,Ohio
There is no fuel leaks. Tank sending unit and lines are all fairly new no fuel leaking anywhere. I did the koeo test and got the 11. Mechanic said he checked codes and there was none. I just re checked the timing and it was around 5 or 6 btdc i got it on 10 now and yes I pulled the spout to set and re-installed it after. I dont think that that little bit off on the timing would make that big of a difference in fuel but I will see when I get some miles on her
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Junior User
Joined: Jun 2011
Posts: 52
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From: Ravenna,Ohio
Im starting to think that someone turned my throttle screw and I know that is a no-no. Either that or it is wore out because I looked at it today and it was backed almost all the way out. Is there any way to get it set back to where it should be or should I take it to a dealer and let them adjust it? I can also hear a small exhaust leak. Could it cause the mileage to drop like that?
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Cargo Master
Joined: Apr 2003
Posts: 2,408
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From: Parkesburg PA
Just for an comparison my 86 F150 EFI 302 gets 13 city, 19 highway.
My 87 F250 5.8 HO on the other hand... That's in the same boat but I'm slowly eliminating as many issues as I can. I'm hoping it won't be too long before I get to test mileage. It's my backup/plow truck and gets driven 50-60 miles a year...
If the TB screw has been messed with it's not the end of the world. Basically with the IAC out the screw should be set so the truck barely runs - think like 400rpm or so. Then set the TPS to .8 to .9v. At least that's what I understand and there's a thread here somewhere with the specifics on setting the screw.
How about the fuel pressure regulator? Sometimes when they go bad they leak through the vacuum line into the engine. O2 sensor? If that's bad it'll cause a rich condition. Check the MAP sensor and the ACT sensor. Then there's the coolant temp sensor for the computer (ECT). If that's not working it'll stay in enrichment mode and eat gas pretty bad but they rarely go bad...
My 87 F250 5.8 HO on the other hand... That's in the same boat but I'm slowly eliminating as many issues as I can. I'm hoping it won't be too long before I get to test mileage. It's my backup/plow truck and gets driven 50-60 miles a year...
If the TB screw has been messed with it's not the end of the world. Basically with the IAC out the screw should be set so the truck barely runs - think like 400rpm or so. Then set the TPS to .8 to .9v. At least that's what I understand and there's a thread here somewhere with the specifics on setting the screw.
How about the fuel pressure regulator? Sometimes when they go bad they leak through the vacuum line into the engine. O2 sensor? If that's bad it'll cause a rich condition. Check the MAP sensor and the ACT sensor. Then there's the coolant temp sensor for the computer (ECT). If that's not working it'll stay in enrichment mode and eat gas pretty bad but they rarely go bad...
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Joined: May 2013
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From: Idaho
Well i dont know about any one else but i have a 351M and after upgrading the ignition system, new battery and starter cables, new 3" Edelbrock elite series air cleaner new top end gaskets, driverside cylinder head out of junkyard that had been rebuilt, royal purple oil and filter, and hooker super comp headers i am getting unheard of gas mileage not to mention its got a leveling kit and 33x12.50 all-terrain tires and a 4x4
Senior User
Joined: Mar 2013
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From: Arkansas
Well i dont know about any one else but i have a 351M and after upgrading the ignition system, new battery and starter cables, new 3" Edelbrock elite series air cleaner new top end gaskets, driverside cylinder head out of junkyard that had been rebuilt, royal purple oil and filter, and hooker super comp headers i am getting unheard of gas mileage not to mention its got a leveling kit and 33x12.50 all-terrain tires and a 4x4
What tranny do you have? Do you have long tubes? Any fitment issues with the headers?