Bad mpg
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New User
Joined: Feb 2019
Posts: 19
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From: Mesa Arizona
Bad mpg
I have a 1980 4x4 5.0 np435 and I’m struggling to get 4 mpg driving careful. It has new plugs and wires, distributor cap, ignition module, air filter and pcv valve. None of these things were an attempt to fix gas mileage but just cause they were bad. I’ve read threads of guys with the same setup getting 14 mpg. What are some things that could cause this?
Cargo Master
Joined: Jan 2019
Posts: 3,369
Likes: 16
From: Caraway, AR
What's your method for calculating fuel economy? I'd imagine if you were getting four miles a gallon you'd be able to taste the fuel coming from the exhaust inside the cab while driving 60mph into a headwind.
What size tire, how aggressive the tread? What ratio is in the front and rear end. Are you driving in 4x4 all the time, as in your stuck in 4L and the hubs haven't been unlocked since '82. Brakes drag? How much additional weight is on the truck? Haul a lot of tools and gear? Is your timing way retarded and the carb set pig rich? How well does the truck run? Is this a daily driver or does it set for awhile? What's the likely hood you have a leak in your tank. The gas is evaporating or being siphoned from the bowl vent on the carb or boiling out in the desert heat?
Lot of things can affect fuel economy. Got a picture of your truck and your engine bay just so we can get a rough idea of what we're looking at?
While you wait for the others to wake up, start reading this thread, it has a lot of good fuel economy info.
What size tire, how aggressive the tread? What ratio is in the front and rear end. Are you driving in 4x4 all the time, as in your stuck in 4L and the hubs haven't been unlocked since '82. Brakes drag? How much additional weight is on the truck? Haul a lot of tools and gear? Is your timing way retarded and the carb set pig rich? How well does the truck run? Is this a daily driver or does it set for awhile? What's the likely hood you have a leak in your tank. The gas is evaporating or being siphoned from the bowl vent on the carb or boiling out in the desert heat?
Lot of things can affect fuel economy. Got a picture of your truck and your engine bay just so we can get a rough idea of what we're looking at?
While you wait for the others to wake up, start reading this thread, it has a lot of good fuel economy info.
Fleet Mechanic
Joined: Jul 2003
Posts: 1,844
Likes: 129
From: Atlantic Canada
Is this truck a short box or long? Reg cab or extra? Lifted or stock height? Are the tires stock size (and if so which)?
Tuned
Joined: Nov 2016
Posts: 269
Likes: 13
From: Shelbyville
I had have to guess that your odometer must be reading wrong for some reason. I can't imagine a 5.0 getting that kind of mileage for any reason other than a massive fuel leak.
My 79 with a 7.5 auto and 4x4 averages 12mpg if I drive careful, 8mpg if I decide to burn rubber at every red light.
My 84 with a 5.8 manual and 4x4 averages 12 mpg so far (have only had it a few weeks) and it in fact does have a fuel line leak, slowly dripping in my driveway. AND runs extremely rich at idle. So it should go up to that 14 maybe 15 number once those issues are addressed.
I can't imagine any reason why the engine is actually consuming that much gasoline.
Have a bad manual fuel pump and gas is getting into the crankcase? If the diaphragm is damaged it will pump fuel straight into your oil.
Have any fuel leaks?
Use a gps to make sure odometer is counting correctly, I've seen some that even though the speedometer is accurate the gears in the odometer miss every other tooth.
My 79 with a 7.5 auto and 4x4 averages 12mpg if I drive careful, 8mpg if I decide to burn rubber at every red light.
My 84 with a 5.8 manual and 4x4 averages 12 mpg so far (have only had it a few weeks) and it in fact does have a fuel line leak, slowly dripping in my driveway. AND runs extremely rich at idle. So it should go up to that 14 maybe 15 number once those issues are addressed.
I can't imagine any reason why the engine is actually consuming that much gasoline.
Have a bad manual fuel pump and gas is getting into the crankcase? If the diaphragm is damaged it will pump fuel straight into your oil.
Have any fuel leaks?
Use a gps to make sure odometer is counting correctly, I've seen some that even though the speedometer is accurate the gears in the odometer miss every other tooth.
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New User
Joined: Feb 2019
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From: Mesa Arizona
I will for sure use a gps to check it again I’m driving in 2 hi for sure. It does struggle to go 65 mph but it will do it. Nothing getting into the oil and it does smell like crap while driving. I’ll have to check the gearing to see what it is. I’ll try and check if the fuel pump is bad. Brakes aren’t dragging and if they are it isn’t noticeable driving it coasts pretty well. I’m not carrying any tools are gear. The timing is advanced by like 5 I believe. Thank you guys for the great response. I’ll keep this updated till I figure it out.
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Tuned
Joined: Nov 2016
Posts: 269
Likes: 13
From: Shelbyville
I agree with Franklin2. You should definitely get that timing into spec.
Can you define smells like crap? Burning oil? Running rich?
Do you have a tach? What rpm is the engine at when driving at 65mph? IMO around 2200-2500rpm would be healthy for mpg.
Can you define smells like crap? Burning oil? Running rich?
Do you have a tach? What rpm is the engine at when driving at 65mph? IMO around 2200-2500rpm would be healthy for mpg.
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