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99 suburban 5.7 vortec stumble just off idle

This is a discussion on 99 suburban 5.7 vortec stumble just off idle within the Technical / Maintenance forums, part of the General Discussion category; mornin all, As the title says, 99 suburban 5.7 vortec engine. slightly hard to start, idles smooth, but has a ...

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    99 suburban 5.7 vortec stumble just off idle


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    mornin all,
    As the title says, 99 suburban 5.7 vortec engine.

    slightly hard to start, idles smooth, but has a severe stumble as soon as you dip in to the throttle. if you feather the throttle, you do not experience it, you have to get 15-20% into it to make it stumble.
    This only happens at idle, not at any other point, so the only time you see it is at a stop.

    I have removed the throttle body, and cleaned it. The IAC got a good cleaning as well, and since the TPS was giving suspect readings (dropped out in spots) it got replaced, and the new one has been checked, with good results.
    I have also been checking for vac leaks, and am unable to find any.

    Any suggestions from the masses?

    Rob

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    Loves GMT400 haters someotherguy's Avatar
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    Re: 99 suburban 5.7 vortec stumble just off idle

    Rule #1 when troubleshooting an EFI engine, check the fuel pressure. Easy to do on the Vortec - small block has the test fitting right next to the black plastic square "Vortec" box. On the small block you want to see a minimum of 58psi. Let us know whatcha get.

    Richard

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    Re: 99 suburban 5.7 vortec stumble just off idle

    I am on pump #3 this time with an OEM pump. kind of aware of the normal issues with fuel pressure, this seems different since once past the stumble, it pulls very strong at any throttle setting all the way to redline.

    that being said, I will check it.

    Rob

    ps, just ran it up to a shop, he dropped the scanner on it (free of charge), and no codes were showing. but I was able to demonstrate it to him, he suspects the cap/rotor/plug wires.

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    Re: 99 suburban 5.7 vortec stumble just off idle

    If I'd know you were gonna have it scanned, I'd suggested you have him check the timing while you were at it - since on the Vortec it requires a scanner that can read the cam retard.

    Richard

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    Re: 99 suburban 5.7 vortec stumble just off idle

    Quote Originally Posted by helowrench View Post
    I am on pump #3 this time with an OEM pump. kind of aware of the normal issues with fuel pressure, this seems different since once past the stumble, it pulls very strong at any throttle setting all the way to redline.

    that being said, I will check it.

    Rob

    ps, just ran it up to a shop, he dropped the scanner on it (free of charge), and no codes were showing. but I was able to demonstrate it to him, he suspects the cap/rotor/plug wires.
    I am having the same problem in my 97 'burban, it won't do it all the time but I get the stumble. I replaced the cap,rotor,plugs & wires and it is still doing it.

    I am stumped and so is the GM tech here at my college.
    Jeff...

    97 GMC 'Burban w/t, 5.7 Vortec, True dual exhuast, 265/75/16 Adventuro M/Ts

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    Re: 99 suburban 5.7 vortec stumble just off idle

    Quote Originally Posted by JGresko View Post
    I am having the same problem in my 97 'burban, it won't do it all the time but I get the stumble. I replaced the cap,rotor,plugs & wires and it is still doing it.

    I am stumped and so is the GM tech here at my college.
    Maybe it's time for a new multecII injector spider.
    2003 GMC Sierra RCSB 1500 2wd, 5.3L, 3.73s w/ g80 locker

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    Re: 99 suburban 5.7 vortec stumble just off idle

    It has now progressed to a no start situation.
    I can hear the pump going, and have verified spark (ouch)
    Tonight I will check the pressure, and post the results.

    rob

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    Re: 99 suburban 5.7 vortec stumble just off idle

    Also check for pressure bleed off after the pump turns off. It should hold pressure overnight, at least for several hours. Bleed off can point to a bad check valve in the pump or a leaking pressure regulator. Of course make sure your gauge isn't bleeding any pressure.

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    Re: 99 suburban 5.7 vortec stumble just off idle

    Quote Originally Posted by 99-2door View Post
    Also check for pressure bleed off after the pump turns off. It should hold pressure overnight, at least for several hours. Bleed off can point to a bad check valve in the pump or a leaking pressure regulator. Of course make sure your gauge isn't bleeding any pressure.
    my gauge is a european car unit for Bosch K-Jet, which gives me a positive shut off valve, and much higher pressure reading capabilities.

    Rob

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    Re: 99 suburban 5.7 vortec stumble just off idle

    fuel pressure test shows 60psi engine off, key on.
    stays dead on 60psi while cranking, still no start.
    no quick bleed down, will check after 1 hr.

    I guess it is not the pump, filter, or regulator.

    any ideas where to look next?

    Rob

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    Re: 99 suburban 5.7 vortec stumble just off idle

    What all did you do between posts #1 and #7?
    08 HHR LS - 2.2 | 5 speed | 30 MPG

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    Re: 99 suburban 5.7 vortec stumble just off idle

    replaced tps, smoothed out a little, but still had a wierd stumble

    had the codes run, nothing to show

    drove it for a few days, it got harder to start, until today whe no start at all.

    checked fuel pressure, with the above results.

    now batt is dead, low amp charger is on it.

    Rob

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    Re: 99 suburban 5.7 vortec stumble just off idle

    are there any methods to ops checking the air temp sensor, and coolant temp sensor?
    Last edited by helowrench; 09-17-2009 at 05:39 PM.

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    Re: 99 suburban 5.7 vortec stumble just off idle

    just checked, no fuses blown

    Rob
    Last edited by helowrench; 09-17-2009 at 05:51 PM.

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    Re: 99 suburban 5.7 vortec stumble just off idle

    do you have spark?

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    Re: 99 suburban 5.7 vortec stumble just off idle

    I checked spark with a timing light, but not at the plug.

    late last night I went back to basics, and pulled the cap and rotor.

    I found .090"-.100" had burned off from the business end of the rotor.
    I will be getting a new one on the way home today, and will post my results.

    Rob

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    Re: 99 suburban 5.7 vortec stumble just off idle

    now to stop the naysayers
    yes, I did come back to post the fix.


    survey says
    cap and rotor replaced, it now starts instantly, idles very smooth, and pulls clean at any throttle setting from any rpm to any rpm.

    BTW why the @#$% is this cap and rotor worth a minimum of $45 ???????????????

    is there any older style "stand up" cap and rotor combo (and wire set) that I can interchange with?

    Rob
    Last edited by helowrench; 09-19-2009 at 05:00 PM.

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    Re: 99 suburban 5.7 vortec stumble just off idle

    Quote Originally Posted by helowrench View Post

    BTW why the @#$% is this cap and rotor worth a minimum of $45 ???????????????

    is there any older style "stand up" cap and rotor combo (and wire set) that I can interchange with?

    Rob


    1. The newer EFI systems have more complicated parts. We replaced the cap and rotor for our '96 Tahoe and my '90 GMC about two years ago, Tahoe rang up $50 and the GMC got $20.

    2. No.
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    Re: 99 suburban 5.7 vortec stumble just off idle

    I have got the stumble going on as well. Never in problems starting or anything just a stumble and lower RPM's if I dump the throttle. I recently replace the spark plugs, cable, fuel filter, and fuel pump. Still got it. Mine is 1999 2door tahoe. Man is it frustrating. Any one have suggestions beside what has been said in this thread. I will try and check the fuel pressure soon. Truck drives great otherwise just this stumble.

 

 

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