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  1. #1

    Default Throwing $$$ at a rough idle

    I've got a 2007 Silverado 2500HD 6.0 Vortec with a PO300 random misfire/rough idle issue. 150k miles on the truck. I've read most threads on the subject (much appreciation!), and will try some more solutions today.

    The truck runs rough at idle, though smoothes out as the rpm's rise. One time, it did completely misfire and limped back to my shop. My plumber was driving it, and he didn't relay any more info than that. It often gave PO300 codes, which I would clear each time it happened. It is beyond the "normal" 6.0 rumble which many have mentioned. It is not currently throwing any codes, though it still idles rough.

    Money I've thrown at the problem without real success:

    replaced spark plugs and wires
    replaced air filter and cleaned mass air flow sensor
    visually checked for vacuum leaks and wiring issues
    replaced all ignition coil packs
    replaced 4 o2 sensors (no codes after this though I haven't driven very much since)
    had a new muffler installed... which lead my to this post....

    I had the muffler replaced yesterday and the catalytic convertors checked. One appeared to be ok, and the other was completely gone, with chunks rattling in the muffler. The muffler shop called and checked GM part cost. The driver side was $539, and the passenger side $1,050!!!(I'm in CA so they have to use approved cats :( which apparently only GM is. The muffler guy didn't think that replacing the missing cat would help the rough idle. I think it would help, not remedy the problem. My thought is that if the driver side 4 cylinders are running without the cat , and the passenger bank is running with a cat and back pressure, then the left and right banks will be at odds with each other.

    I'm at the end of my rope on this, and today will try solutions other here have posted:

    checking idle air control
    check for manifold or vacuum leaks with carb cleaner, and torque bolts
    check for gas in fpr hose (help please, what is fpr hose)
    clean the ground for ecm
    check exhaust manifold and torque
    check vacuum line at fuel regulator

    If anyone can tell me how to do the crankshaft sensor variation relearn, I would really appreciate it.

    If these don't work, I'll take it to the dealer to reprogram pcm.


  2. #2


    Yikes! Sounds like you've got the basics covered and then some. Let us know how it goes.

  3. #3
    TRPLXL2's Avatar
    Join Date
    Jan 2009
    Norwalk, Ohio
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    The Crankshaft sensor relearn has to be done with a scan tool i.e. Snap on mt2500 or Tech 2 or equivelant.

    fpr hose-fuel pressure regulator hose/line

  4. #4


    Check the fuel pressure. These motors just like the 5.7L Vortecs are very sensitive to fuel pressure.

    97 Chevrolet Suburban K1500 5.7L
    78 GMC High Sierra K1500
    Fat, drunk, and stupid is no way to go thru life.

  5. #5
    Sr. Apprentice
    Join Date
    Oct 2006
    Sheldon, Iowa



  6. #6


    Sounds kinda silly, but swap out the fuel filter.

    Had a similiar issue and this was my actual resolution.

    Cheap fix.
    Pavement sucks... :grrrrrr:

    1994 K1500 4x4 Ext. Cab
    PCM chip and Xenon 8000k bulbs bored over .050"
    balanced Crank, 10:1 compression
    Flat top pistons,cam,ported heads
    Option Racing Cold Air Intake
    Holley Throttle Body Spacer
    Accell Igition, 45kv coil
    FloTech Headers, no CATs, dual exhuast
    Goodyear Silent Armor A/T 305/70-16
    Pioneer SupertunerIII CD USB Bluetooth
    Kicker ZX200.2 ZX Amp
    Kicker Audio COMP 10" Subwoofer

  7. #7


    OK, so I've now also done:
    a very thorough carful smoke test with the a friends pressurized smoke machine. No leaks
    checked MAF sensor and cleaned throttle body and intake as far as I could get with deep creep.
    removed one cylinder head cover and checked for anything visibly broken (there is a TSB naming a problem with the valve stem heads breaking off where the retainer clip holds them).
    torqued intake and exhaust manifolds
    removed ECM and checked all pins for signs of oxidation or burns, opened ECM and checked for any water intrusion or signs of short
    removed new spark plugs to check condition (all looked equally good)
    rechecked wiring for shorts
    changed oil
    replaced MAP sensor (it was packed with grease)
    checked fuel pressure
    I also replaced either the crankshaft or camshaft sensor...

    Done within the last month:
    replaced spark plugs and wires
    replaced air filter and cleaned mass air flow sensor
    visually checked for vacuum leaks and wiring issues
    replaced all ignition coil packs
    replaced 4 o2 sensors (no codes after this though I haven't driven very much since)
    had a new muffler installed... which lead my to this post....

    I have an appointment on Wednesday for the dealer to reprogram ECM and do the crankshaft relearn.

    Man, this thing is killing me!

  8. #8


    Quote Originally Posted by Boonduff View Post
    Check the fuel pressure. These motors just like the 5.7L Vortecs are very sensitive to fuel pressure.
    Yep. I'd bet dollars to doughnuts you have a fuel pump going bad. Low fuel pressure will cause these symptoms and will cause it to throw O2 sensor codes when in all likelihood they were fine. Also, a fuel pump can begin to go bad, i.e. have lower than normal pressure for a long time before they crap out for good.

    You may know this already, but if not, there is a valve right on the side of the intake manifold, it looks similar to a tire stem valve. That is where you can connect a gauge to tell you how much pressure your pump is putting out. You want to be somewhere in the neighborhood of (someone please correct me if I am wrong) 40psi. Check that ASAP before you go nuts!
    -2001 Suburban 2500 LT 8.1
    -2000 Silverado 1500 LT Z71

    Gone but not forgotten:
    -2007 TrailBlazer SS
    -2004 Cadillac CTS-V
    -1998 Silverado 1500 LT Z71
    -1989 Pontiac Formula WS6

  9. #9


    Quote Originally Posted by TRPLXL2 View Post
    I gotta ask...............Did you pull the intake and throttle body OFF the motor before cleaning it, or did you shoot that stuff right into the motor? If so you can add more problems to your list, also did you ever do a compression check on this engine? Burnt valve or bent is a possibility? Or vacuum leak..

    Edit: Realistically for the amount of money you have dumped in this motor, you basically could have bought another 6.0 and been done with it......
    Yes, I pulled the throttle body off to clean the intake manifold. I cleaned the length of the entry duct all the way back to where it reduces in size.

    Your last statement is foolish on so many levels.

    First of all, at what point would you replace the engine? When it first had a rough idle, or would you replace the spark plugs? If that didn't fix it, would you then buy a new motor, or replace the wires? Keep what point would you make the jump from a purchasing a $10-$100 part that seems likely to fix the problem, to shelling out thousands for "another" 6.0?

    What kind of engine should I replace it with? A pullout? Whats to say the pullout doesn't have a similar, if not worse problem? I used to pull, test, pressure wash, and pallet engines for a salvage yard in my early twenties. If it turned over and fired, it was a good pullout, sold with a 30 day warranty. Let me assure you that a large percentage of those engines weren't in as good of condition as my 6.0 being discussed. I surely wouldn't want to buy pullout.

    OK, next a long block???, Last I checked, they didn't come with most, if not any of what I've replaced.

    What if the problem was the ECM? The Fuel pump?, A shorted wire? A vacuum leak? and on and on.....

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