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Rough idle when cold

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by Sir Koscoe, Feb 13, 2013.

  1. Sir Koscoe

    Sir Koscoe Member

    Now that I have the break issues fixed it's on to my next project. In the mornings when it is cold out the truck idles rough. Roughly like I am pressing the gas and letting off until it gets warm then it runs fine. Could this be a dirty MAF and/or TB? My brother said his 99 did this same thing and cleaning the TB helped. I know it will be a good idea to clean them anyway since the truck has 184k on it. Just wanted your thoughts. Thanks.
  2. j cat

    j cat Rockstar 4 Years 1000 Posts

    using throttle body cleaner and removing the IAC and then clean the parts will help idle issues.I use a tooth brush and a cloth to clean and wipe out the carbon deposits as much as possible.
    the PCV and the crankcase hoses need to be checked/replaced. the ECT could be defective @180,ooomi I would replace it.

    pcm uses MAP/MAF/ECT to adjust cold start idle,,, a leak/bad PCV in the crankcase venting or other vacuum leak throws off these programed PCM idle control. also leaks in the air intake after the MAF.

    I would get crc MAF cleaner and clean the small maf sensing wires. just a small amount of spray on the wires is all that is needed. do not touch the wires.

    hold the throttle body open to dry out the throttlebody cleaner before starting. If its cold out use a hair dryer to evap the solvent before starting.
  3. Sir Koscoe

    Sir Koscoe Member


    Thanks Cat. The truck runs good after it warms up, so i will clean everything when it gets a bit warmer out unless the issue becomes worse.

    Dumb question, but will this affect my MPG's?
  4. j cat

    j cat Rockstar 4 Years 1000 Posts

    lets say you have a failure with the crankcase vent system which causes a loss of the calibrated measured Vacuum of an amount GM used to program the PCM. then the PCM will see that the engine is with a lean condition now it opens the injectors for a longer time to correct to get the 14.7 to 1 air to fuel ratio.

    this causes lower MPG.

    after the cleaning and checking for any vac failures the engine should idle smooth and with the maf cleaned faster throttle response
  5. Sir Koscoe

    Sir Koscoe Member


    Awesome. I can't tell if the truck has a vac leak of a small miss. You can hear it while I am on the throttle. I need to give the engine bat a really good check once it warms up outside. I don't want to freeze my boys off since I have no garage of my own to work in.
  6. j cat

    j cat Rockstar 4 Years 1000 Posts

    looks like the nasty cold is not coming back too soon 40's up here. the 3 ft of snow we got last week is down to about a foot. nice weather now.
  7. Sir Koscoe

    Sir Koscoe Member

    my buddy and his family drive pulling trucks so I could work in his garage, but he and his wife are having thei first baby this week so I don't want to bother him.

    it snowed here yesterday and a lady asked me hwo the roads were, it maybe snowed 1/8" inch. People get too bent out of shape when they see snow.
  8. topshelfproducts

    topshelfproducts New Member

    I have a 99 with the same issues, cleaned maf and everything else, put in new intake manifold gaskets as I have heard that can cause the issue, new plugs and wires, new pcv valve and fuel filter, new air filter, still have the same issue. Seems this is a pretty common problem after reading through the forums, I have decided might be easier to live with it at this point...unless anyone can point to a resolution that has solved without fail. The dealer said it could be multiple sensors etc, but I hate throwing money blindly at the problem.
  9. j cat

    j cat Rockstar 4 Years 1000 Posts

    in your case I would replace the ECT costs about 20.oo . @ 100,ooo plus miles they start to go out of cal.spec.ECT is a big part of the cold start PCM process.

    if the PCM thinks its 70 deg f when its 20 deg f, cold start is not good.
  10. shibby2oo8

    shibby2oo8 Member 2 Years 100 Posts

    Instead of shotgunning track down somebody with a scanner and watch the sensor data in real time while also monitoring engine temp with a temp gun. Between the maf, iac, tps and upstream sensors you should be able to tell if it is a vacuum leak. I have said this before and will prob say it again but the intakes do warp so just putting gaskets on do not fix the issue. Between high miles, not torqueing the bolts properly and plastic fatigue in the shop we always recomend the dorman intake kit. All of the vortec engines have this problem as well as the 3800's and all ford v6's with plastic plenums.

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