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Gas Mileage

Discussion in 'General Chevy & GM Tech Questions' started by rileyjr16, Oct 2, 2011.

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  1. rileyjr16

    rileyjr16 New Member 1000 Posts 100 Posts

    I have a 99 GMC with the 5.3. I got 180 miles on my tank and it is now just barely above 1/4 left. I filled it up Saturday morning. My tire pressure all around is 36psi and I am running stock265/75/16 Mastercraft A/Ts. I have a TBS and a Spectre CAI with a Hypertech30005 programmer. I'm starting to think this is an engine issue. Any ideas?
  2. Caddiac

    Caddiac New Member 1000 Posts 100 Posts

    What size is your fuel tank? My gut says that sucks but I do not know how bad it sucks. I gassed up today. It had 600 plus miles on the tank and it was on empty. That sucks, to know how bad is sucks, I have to tell you the tank size. LOL

    Have you seen a drop off in your milage recently? How many miles are on the engine? How long had you had the truck (roughly how many miles did it have when you got it and how many miles do you have now) ?
  3. rileyjr16

    rileyjr16 New Member 1000 Posts 100 Posts

    I got it with 201,000 now i guess it has around 216,000. The odometer is out. I run fuel injector cleaner through it about every 5 tanks. Oh 25 gallon tank
  4. 99'HEARTBEAT

    99'HEARTBEAT Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    In the Time that you have had your Truck, have you done any of the Following Maintenance,
    .....Change (Replaced)Filters Air and Fuel W/New ones
    .....A Cleaning of the MAF Sensor
    .....A Cleaning of the Throttle Body and Injector's
    .....A Set of New Plugs and Wires

    All of the Above items can help with Fuel Mileage and if its been awhile since these maintenance items have not been done....1 or 2 Clogged Fuel Injector's.....or Clogged Fuel Filter.....
    that in turn can and will Hurt your Fuel Mileage.......I would also look at having the Truck Scanned just to be sure No Trouble Codes are Stored......also What Rear Gears do you have??.......Regarding the Throttle Body Spacer, on the Late model Trucks, sorry but they do nothing for helping with gains of any kind.

    With the Speedometer out, the problem is most likely with the.....Stepper Motor for the Speedometer Gauge has gone Out/Bad, which is a Common Problem on the 99" to 2006" Trucks/SUV's Dash Cluster's........Regarding the Size of your Fuel tank, if you have a Short Bed, then you Fuel Tank would be a 26 Gal Tank.......and going with what you've mentioned above, let just say with a 1/4 tank of fuel left you used 20 Gals and went 180 miles.....that works out to about.....9.0 M.P.G.....not good.
    Last edited: Oct 3, 2011
  5. silveradotrailblazer

    silveradotrailblazer New Member 1000 Posts 100 Posts

    As Mike said, when was the last full maintaniace done? Air filter, clean MAF and throttlebody, fuel filter, spark plugs, (use A/C Delco's) plug wires, PCV valve etc...
  6. Caddiac

    Caddiac New Member 1000 Posts 100 Posts

    The odometer is out? How do you know you have been 180 miles if the ody is out? Based on the numbers, 180 miles and you used 3/4 of a 25 gallon tank that works out to be between 9.5 and 10 miles to the gallon. Even if you are off, you are not 50% off which is what you would need to get to 15 miles per gallon. I have to agree Silveradotrailblazer and 99Heartbeat. I would inspect the plug wires for cracks or evidence of arcing. I am not big on changing the plug wires unless they clearly are failing. Do you have any DTC codes? Take it to Autozone or Advance and they will pull the codes for free.
  7. murdog94

    murdog94 New Member 1000 Posts 100 Posts

    I would also like to know how much fuel you are putting in for that 1/4 of a tank gas guages arent an accurate guage of gallons in them. i have had trucks where 1/4 of a tank was actually closer to 10gallons of a 25 gal tank.
  8. rileyjr16

    rileyjr16 New Member 1000 Posts 100 Posts

    I went to where I hunt. 45 miles there, 45back. Did that 2 days in a row. Thats 180 miles. 37 miles are interstate on which I do 60-65. I have changed the air filter from stock to aftermarket as per the cold air intake. I put a transdapt throttle body spacer as well on it and programmed the engine. I changed the spark plugs and wires to AC delcos when I pulled 5 of the 8 boots from the wire. I have put 20$ in my truck, thats 5.3 gallons with 2 gallons left (aprox) and it filled up to 1/4 tank. Other people I know with the same truck are getting at least 375-400 miles to a tank. I don't pull a trailer or have a 4 wheeler in the back as I leave it at the camp.
  9. 99'HEARTBEAT

    99'HEARTBEAT Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    As mentioned by Caddiac and Myself, have you had the Truck Scan to see if any Trouble Codes are coming up??......and the A/C plugs you installed which one did you go with....on the Plug Wires(when I pulled 5 of the 8 boots from the wire.) not following you here??.....last have you done any of the Maintenance items mentioned above other than the Plugs/Wires??
    Last edited: Oct 6, 2011
  10. ChevyFan

    ChevyFan Administrator Staff Member 1000 Posts

    not sure if anyone asked but have driving conditions (speed or traffic congestion) changed?

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