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Worst MPG's after tune-up??

Discussion in 'Chevy Trucks' started by Mark13, Oct 23, 2009.

  1. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    Been a few weeks since a friend and I did a tune up on my 98. First tank after the tune up was 10.75mpg, then 12mpg, and the last 2 have been 11.5mpg. Before the tune-up I'd see 12-14mpg very frequently.

    The tune up consisted of:
    New plugs (Ac Delco Platinum)
    New wires (napa premium)
    Cap and Rotor are under 2yrs old.
    Clean Air Filter
    Oil Change with 5w30
    New Fluid in the trans, tcase, and both diffs.

    Hooked up his scanner and there are no trouble codes, cats are in good shape, truck idles +/- 5rpm of steady.

    Specs on truck:
    98 K1500 Ec/Sb
    3.73 gears
    285 A/t's on nbs wheels.
    Truck weight about 6,000lbs

    Rarely sees above 2100rpm.

  2. Sydenstricker Landscaping

    Sydenstricker Landscaping PlowSite Veteran
    Messages: 3,882

    Did you gap the plugs properly, or just take them out of the box and pop theml in??? That could have an affect on mileage
  3. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    The plugs we didn't gap. The box I beleive said gapless due to the possibility of scratching them.
  4. Milwaukee

    Milwaukee 2000 Club Member
    Messages: 2,180

    Did you remember to fix speedmeter. you need do that.

    I am sure you got 15 mpg due speed say 50 but you go 55 mph.
  5. badabing1512

    badabing1512 Senior Member
    Messages: 370

    X2 on that.
  6. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    I did the math correction when figuring out my milage at the pump. So the figure I used to do the MPG was correct even though my odometer reading was lower.
  7. sparky8370

    sparky8370 Senior Member
    Messages: 234

    Take the plow off until it snows.
  8. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    :D I wish that was the problem.

    I see 8-9mpg with the plow on.
  9. Sydenstricker Landscaping

    Sydenstricker Landscaping PlowSite Veteran
    Messages: 3,882

    I run the ac platinums in both my trucks and gapped them. Just gotta be a little easy with the gap gauge.
  10. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    Mark - you want to gap your plugs at .040". Probably time for a new Cap and Rotor. AC Delco. Have you cleaned your MAF, or cleaned the Throttle Body & IAC?

    What kind of air filter do you have?
  11. revtoyota

    revtoyota Member
    Messages: 50

    like the others said you need to gap the plugs never go by the box. might ever be surprised and get alot better mpg,
  12. marylandbigb

    marylandbigb Member
    Messages: 87

    did they sell u the correct ones???? remove the cap and look for carbon tracking
    Last edited: Oct 24, 2009
  13. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    I'll see if I have time this weekend to pull the plugs out and gap them.

    As for seeing if they are the correct plug I went by AC's book at the local farm and fleet where I bought them. So they are the right ones unless the book was incorrect.
  14. revtoyota

    revtoyota Member
    Messages: 50

    over the last 20 years of working on vehicles, I have seen plenty of books with wronge numbers. (ie) plugs, filters and all that stuff.
  15. RichG53

    RichG53 PlowSite.com Addict
    Messages: 1,135

    Talked to dealer parts guy about a year ago they now recommend this plug AC 41-985 gapped at .40 he told me they were using this plug the last 2-3 yrs.In stead of the NKGs. Could not find at Farm & Fleet ,went to Advance Auto... A bit Pricey......
  16. roybarber

    roybarber Junior Member
    from Ottawa
    Messages: 6

    Normally when you install the plugs. You drop the plug in the head, sometimes you crush the gap making it not firing correctly. Check the gaps but easy with the iridium or platinum tips
  17. Mark13

    Mark13 PlowSite Fanatic
    Messages: 5,796

    Had Napa check their computer and it's the same plug # on their computer as the book said at Farm and Fleet.
    I was going to get the plugs at napa but figured I'd check F&F and they had one box left. So I got them there since they were a bit cheaper then Napa.

    I'll check them later today or tomarrow. It's possible they could be gapped incorrectly or closed up some. They all were in their individual boxes inside one of the package of 8 boxes and didn't come out of the box until right before they went into the truck.

    Scanned my truck again and found a stored EGR code. Going to mess with that also and see if that could be the problem. I've had a problem with it a few months ago and after the check engine light came on and we scanned the truck I got 8-9mpg for 1 tank and then it went right back up to 12-14mpg so I didn' think a whole lot more of it.
    Last edited: Oct 25, 2009