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sudden drop in gas mileage

This is a discussion on sudden drop in gas mileage within the Technical / Maintenance forums, part of the General Discussion category; on my last tank of gas i had a major drop in mileage, about a 50% drop. i realize that ...

  1. #1
    V8 RWD!!!
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    sudden drop in gas mileage


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    on my last tank of gas i had a major drop in mileage, about a 50% drop. i realize that with colder weather and winter gas mileage will drop, but this seems like to much. the engine runs fine, exhaust smells the same, no stall, no bucking, oil pressure is normal (around 30psi). transmission shifts fine.

    here are the things that were different in the last tank
    oil and filter changed, valvoline 5w30, wix filter
    block warmer has been plugged in whenever possilbe
    starting fluid (ether) has been sprayed into the intake a few times
    4x4 has been used around town in snow, sometimes with more then needed throttle
    1 bottle of iso-heet in the gas tank

    air filter is pretty new, still looks very clean
    plugs and wires are about 6 months old
    tire pressure was at 35psi when i checked a few weeks back but i will check again. o2 sensor was changed this summer. takes a long time for the engine to get to operating temp, after the thermostat opens the temp drops a lot, temp seems to be around 175 most of the time. i have a locking gas cap, i don't think anyone is syphoning.

    give me any idea you have, something seems very wrong.

    1997 Ford Crown Victoria P71 281ci 128,330 miles, i wish my truck had a boxed frame
    1988 Chevy Suburban 1/2 ton, 350ci 4x4 146,500, 31x10.5x15s, mounting 33s next summer

  2. #2
    2nd Amendment Supporter mudbuddy's Avatar
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    Re: sudden drop in gas mileage

    why are you using ether that stuff is like dynamite you never know when your going to blow a head or two off the block or crack the rings or ring lands
    Last edited by mudbuddy; 12-05-2007 at 07:15 PM.

  3. #3
    V8 RWD!!!
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    Re: sudden drop in gas mileage

    Quote Originally Posted by mudbuddy
    why are you using ether that stuff is like dynamite you never know when your going to blow a head or two off the block or crack the rings or ring lands
    i realized that after i used it a few times, not going to use it anymore, unless it gets cold enough that i can't start the engine. forgot to mention, the belt was off the air pump for about 20-25 miles, i have since then put a new belt on. i disconnected the battery hoping that the computer will relearn a little better stratagy with the winter conditions, worth a shot. i guess i'll have to quit taking 3 block drives.

    1997 Ford Crown Victoria P71 281ci 128,330 miles, i wish my truck had a boxed frame
    1988 Chevy Suburban 1/2 ton, 350ci 4x4 146,500, 31x10.5x15s, mounting 33s next summer

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    Re: sudden drop in gas mileage

    how much idle time are you doing?

    1998 Chevy K1500 Z71 5.7L -True Duals w/ glasspacks, Airaid Intake, Nitto Terra Grappler 265/75/R16, 22 LED clearance lights, Dual Interstate Battery's, Billet grille Bug shield. No camper shell...

  5. #5
    V8 RWD!!!
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    Re: sudden drop in gas mileage

    i am trying to stay to a minimum with the idling, maybe 1-2 minutes. i have had a few times when traffic was basically stopped and spent 20 minutes to go 2 miles. engine never really seems to get warm/stay warm. in the last tank, the engine has seen operating temp maybe 4 times.

    1997 Ford Crown Victoria P71 281ci 128,330 miles, i wish my truck had a boxed frame
    1988 Chevy Suburban 1/2 ton, 350ci 4x4 146,500, 31x10.5x15s, mounting 33s next summer

  6. #6
    I Don't Like Food Anymore GMCSierra99's Avatar
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    Re: sudden drop in gas mileage

    Just out of curiosity what kind of O2 sensor did you get. If its a bosch it might of gone bad. They can suck and not last long.
    99 Sierra 5.3L, K&N CAI, Duel Exhaust, More go fast goodies to come.

  7. #7
    1 Ton Mafia 89liftedk1500's Avatar
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    Re: sudden drop in gas mileage

    3 of 12 polyurethane spring bushings installled...lol anyways what type of plugs and wires?what type of antifreeze mix are you running? and i doubt that ur air pump would make that much differance...and i would seafoam it....cap and rotor?

    and i thought my grand cherokee getting 10mpg was bad
    Last edited by 89liftedk1500; 12-05-2007 at 10:18 PM.
    93 C3500 Crewcab longbed soon to be closer to earth

  8. #8
    V8 RWD!!!
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    Re: sudden drop in gas mileage

    it was a bosch o2 sensor, i need to pick up some stuff at the dealership soon, i'll pick up an ac delco sensor then. i think i used champion plugs but i am not sure, they were copper and gapped correctly, don't remember the brand for plugs. antifreeze is the same as when i bought it, when i changed the thermostat i topped with 50/50 prestone.

    i gave up on the spring bushings after getting the right rears in. i decided against finishing because of concerns with bushing bind. wouldn't hurt to seafoam it against at some point.

    it seems to be a little so far on this tank, i'll take it easy on the throttle for a while and see how it does after 100 miles. i was fairly hard on it the last tank. mileage sucked the last tank but was a little above normal the tank before, maybe the gas pump stopped a gallon or two early. we'll see how this tank goes before i start to worry to much.

    1997 Ford Crown Victoria P71 281ci 128,330 miles, i wish my truck had a boxed frame
    1988 Chevy Suburban 1/2 ton, 350ci 4x4 146,500, 31x10.5x15s, mounting 33s next summer

  9. #9
    4/6 dropped Silvy on 22s
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    Re: sudden drop in gas mileage

    When I used to let the truck warm up for 10 minutes I noticed that I had to fill up more often. I have also experienced a big drop in fuel mileage, about 3 mpg since the weather got colder.

    When I put a new bosch 02 sensor in I did not notice any mileage increase either.

    Also check your tire pressure. It has probably dropped due to the colder temps. I know when I have even 3-4 psi less than 35psi on my tires, my mileage will dramatically increase. Every little thing helps. Good luck.

  10. #10
    1 Ton Mafia 89liftedk1500's Avatar
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    Re: sudden drop in gas mileage

    acdelco all the way on plugs....champions are junk
    93 C3500 Crewcab longbed soon to be closer to earth

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    Re: sudden drop in gas mileage

    I'm not sure on the older trucks, but I know mine won't go into closed loop (fuel) if the engine coolant sensor is acting up. It will continue to run in open loop mode and guzzle gas. The O2 sensor wouldn't matter because the computer won't use it's information while in open loop. Just a thought. Might be worth looking into. Good luck.
    1997 C1500 ECLB 5.7 Vortec G80 3.73 Rear
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    Diesel Nut justy's Avatar
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    Re: sudden drop in gas mileage

    175 seems cold I've always ran about 200-212F. Could be that your computer is saying, Oh the engine is cold yet. I'll still run in open loop.(runs very rich) And when that happens the engine is going to suck gas. You may have air in the cooling system. Something odd with your thermostat. Or push comes to shove. Put a cover on the radiator opening (over the grille). I have one on my diesel already.
    Last edited by justy; 12-06-2007 at 11:46 PM.
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    Re: sudden drop in gas mileage

    Quote Originally Posted by justy
    175 seems cold I've always ran about 200-212F. Could be that your computer is saying, Oh the engine is cold yet. I'll still run in open loop.(runs very rich) And when that happens the engine is going to suck gas. You may have air in the cooling system. Something odd with your thermostat. Or push comes to shove. Put a cover on the radiator opening (over the grille). I have one on my diesel already.
    This could be also my problem. Do you know that what the temperature should be when ecm gets closed loop?
    Last edited by itkonen; 12-07-2007 at 04:02 PM.
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    Registered User masintenn's Avatar
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    Re: sudden drop in gas mileage

    Letting the truck idle for extended periods to warm up will obviously kill mileage, but I bet your fuel filter is leaking to lose 50%.


    '96 Tahoe...5.7 Vortec...4wd...~205,000...Stock

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    Re: sudden drop in gas mileage

    You should have a 195F thermostat and the engne should toggle between 210F & 190F or very close to that. Do not buy a cheep stat get a Robertshaw,Stant or AC Delco the cheaper ones are usually running colder.
    Quote Originally Posted by itkonen
    This could be also my problem. Do you know that what the temperature should be when ecm gets closed loop?
    04 K1500 Silverado EXT 3:73 gears, 5.3L 265/75-16,Pace Edwards tonneau,

 

 

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