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wrong gap = poor mileage?

Discussion in 'Chevy Trucks' started by highlander316, Feb 16, 2010.

  1. highlander316

    highlander316 Senior Member
    Messages: 578

    ok I put plugs in my 97 454 last year. When I did, Autozone computer said .060 gap. However, later on I read that GM issued a bulletin stating the gap should actually be .040. Could this actually make a huge difference? Thanks.
     
  2. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    The older 7.4 Vortec engines don't take favorably to closing the gap up that much I've discovered. Closing it up does improve idle smoothness and even helps throttle response some but it also sacrifices fuel mileage if it's tightened up too much.

    I've found .050 to be about the best all around trade off for max ignition component life, least amount of mis-fires, and idle smoothness without affecting MPG's too much.
     
  3. highlander316

    highlander316 Senior Member
    Messages: 578

    do you think it'll be worth it to close it up that much? Throttle response is kinda iffy, and the truck does misfire a little.
     
  4. Evanbrendel

    Evanbrendel Senior Member
    Messages: 181

    what about a tbi 7.4? getting ready to do a tune up sorry for the hijack
     
  5. 70monte

    70monte Senior Member
    Messages: 468

    I gapped my plugs at .045 on my 98 K3500 7.4L on the advice of many people on another forum. I don't know if it actually idles any smoother or not and the mileage is not much different. Sometimes I think it ran better at the .060 gap. I'm running the AC-Delco Rapidfire #12 instead of the factory Delco Platinums.

    Wayne
     
  6. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Have to assume you meant open them up. And yes I'd open them up to .050. Not that it will help any mis-fires you're experiencing because it likely won't, sounds like you have another issue causing that. But it should bring MPG's back up to acceptable numbers. If it hasn't had a cap & rotor, plug wires or an injector cleaning done in a long time it's probably well overdue.
     
  7. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    The TBI equipped trucks use the old HEI ignition system and don't like to fire anything wider than about a .045 gap max without overheating the coil. So on those it's best to stick with the originally recommended. Also stick with the old reliable and recommended .035 CR43TS A/C Delco plugs, they work the best on the TBI stuff.

    What you can do that will wake them up a little is to bump the ignition timing up to 8 deg from the OEM spec of 4 deg. Doing so adds a little response and power but still won't help MPG's. It is however important that the engine is in a good state of health before adding a little timing or else you may find detonation that wasn't previously there before. Bumping it up on a truck with excessive oil consumption/carbon buildup or other internal issue will cause it. But you can give it a try, if it detonates you can always reset it back. Unless the timing tab is rusted off. :D
     
  8. Evanbrendel

    Evanbrendel Senior Member
    Messages: 181

    hey thanks for the info its a newer dealer installed motor so ill try that and im doing a complete tune up so ill deffinatly use the ac delcos
     
  9. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    Evan - I put some fresh CR43TS in my TBI 350. Made a nice improvement. I installed them at .045. After Mike's advice I pulled them out and tightened them down to .035. Idles much smoother.
     
  10. highlander316

    highlander316 Senior Member
    Messages: 578

    sorry I should have stated I have them set at .060. For the life of me, I can't remember if I did cap and rotor last year. What's the easiest/best way to do an injector cleaning?
     
  11. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Close them up to .050 then. Should find noticeable improvements as I mentioned and the secondary ignition components will last longer too. If you can recall if the C&R was done simply pop the cap off and have a look. If it's been on there for too long you'll be able to tell right away as the Vortec caps don't have a long service life, they get rough inside fairly quickly.

    The easiest and best way for an injector clean is have a shop do it that has the injector cleaning tool. About the best bag for the buck as there's no need for any major disassembly so it isn't labor intensive, should cost less than $150 or so for the service. I do my own trucks about every two years just as preventative maintenance and still notice an improvement after they're done. So you can imagine how helpful it is on one that's never been done.


    At tune-up time it's also wise to clean the throttle body and IAC valve too as they're an often overlooked item. 2COR has a thread in the Chevy section with some info on that.
     
  12. highlander316

    highlander316 Senior Member
    Messages: 578


    alright sounds good. I'll have my buddy who works at a dealership hook it to the machine when I take it for inspection. I had it done on a 99 F250 (5.4 v8) I had years back. Made a world of difference. Thanks for your help.

    One other question on a different topic. I have an 02 Trans Am with the LS1. It's been sitting since beginning of December. I put Stabil in the tank. However, is there anything I should do motor wise before I start the car in another month or so? Is there potential for rust/corrosion on the cylinder walls since the motor hasn't been run in a while? Thanks for all your help. :waving::waving:
     
  13. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    A few months of inactivity is of no concern, especially since you treated the gas properly which is the most important thing to do with todays gasline. It's when they set for a year or more is when extra steps should be taken but otherwise just crank it up and go. :drinkup:
     
  14. 2COR517

    2COR517 PlowSite Fanatic
    Messages: 7,115

    Saved you the search;)

    http://www.plowsite.com/showthread.php?t=82292
     
  15. highlander316

    highlander316 Senior Member
    Messages: 578

    haha will do. Have some suspension upgrades to do before she hits the road. :D:D Thanks for the help!
     
  16. highlander316

    highlander316 Senior Member
    Messages: 578