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1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

This is a discussion on 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated. within the Technical / Maintenance forums, part of the General Discussion category; this truck runs like brand new - perfectly - plenty of power - idles good - starts good.....and now the ...

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    Exclamation 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.


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    this truck runs like brand new - perfectly - plenty of power - idles good - starts good.....and now the problem.

    After the truck gets up to temp, it backfires and dies, like it has lost ignition.....I can wait a few minutes and it will start again but them does the same thing....it does it going down the road or idling......i have changed the plugs, wires....it has individual coil packs, but i do not think thats it since it runs great for a while....it does show an engine overspeed light just before it dies.....could this be a sensor going bad when it heats up and cutting my ignition????? please help! need this truck to work! thanks!!!!!!!

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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    That L21 is a rare beast. Sounds like a tps going bad or possibly a cam sensor.
    CCLB dually, LS-1 Injectors, Airaid Intake, Magnaflow 2.5 dual in/out, Derale Cooling Tube Pan, B&W 24K super cooler[


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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    Out of curiousity. Anything special about the L21? What did it come in? Is there any potential for it? (I prefer big blocks, thats why I am asking)
    1995 K2500 Ext Cab 5.7 5spd NV4500(RPO MW3) 4x4 8600GVW, 3.42 rear end, New Edelbrok 3704 Performer TBI Intake, ZOOM 30014 Performance Clutch, 265/75/R16 8 Lug
    2001 GMC 2500HD Ext Cab Long bed, 6.0 Stock, 4L80E, 265/75/R16 8 Lug

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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    The L21 was in some Top Kick trucks and P12 chassis motorhomes. Its got a Forged Rotating assembly. Forged pistons, Crank, Rods, a pretty good start. The Coil Near PLug ignition was the experiment to the L18 and the LS series igntions of today. It prob has a 4 speed allison auto behind it. But may have a standard. But other then the forged rotating assembly and the coil near plug igntion its basically an L29 vortec. Oh the timing cover and crankshaft sensor are different as well.
    CCLB dually, LS-1 Injectors, Airaid Intake, Magnaflow 2.5 dual in/out, Derale Cooling Tube Pan, B&W 24K super cooler[


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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    The L21 is basically an L29 with CNP (coil near plug) ignition and ETC(electronic throttle control). It uses that same year Corvette/F-body PCM. It is rumoured to have a forged crank, and possible forged pistons. It is also rumoured that L21 blocks are the same as L29 blocks except that the L21 blocks have closer tolerances, so the tighter L21 blocks are used for L21 use, while the slightly looser blocks are used for lighter duty L29 applications. This engine replaced the 427 7.0 GEN IV or GEN V BBC. It is used in 1998-2000 P30 vans, 98-2000 Topkick/Kodiac trucks and in some motorhome chassis.
    the ETC allows for a fast idle function for any PTO(power tak-off) functions that may be needed on teh medium duty trucks. The idle is controlled via the cruise control stalk controls. here is a link on how to uses them.
    http://www.gmupfitter.com/publicat/bull/bull24.pdf

    The L21 is the very 1st truck engine from GM that uses CNP and ETC. It gets its cam position signal from a sensor that is located on a conventional distributer minus its cap. It gets its crank position sensor in the same location as the L29 but uses a 24x reluctor wheel(L29 is 4x) and uses an L21 specific crank position sensor.

    This post has some good pics of an L21
    http://www.pacificp.com/forum/viewto...t=24x+reluctor

    the L21 was a test bed for the new for L18 8.1 496 GEN VII BBC.

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    PAuly
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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    * It also came in the very last OBS 3500HD's. Most sources say the last 3500HD was 2002; but I've seen an auction with verified VIN # for a 2003 with the 8.1

    Not that all this helps the original poster much; the bad thing is not too many people know much about the 8.1. It's got some key differences between the Vortec 7.4 that make it unique. For example, I might say you should check the ignition module, but I have no idea what the 8.1 has for one or if that coil pack setup even uses one.

    The "engine overspeed" might be an indicator of some sensor going nuts, or it just may be coincidence.

    One thing that is always good to do when troubleshooting EFI engines is to pull a fuel pressure test. If it were a Vortec 7.4, you should be looking for 58-60 or so PSI. Not sure if 8.1 is the same.

    Richard

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    Re: 1999 GMC Topkick L21 7.4L Backfire and quit mysteriously..me frustrated.

    If the truck quits when hot, and loses spark (not fuel) I'd suspect a faulty crankshaft position sensor. The spark is timed using this sensor and they usually fail hot. On the Vortec engines, the camshaft sensor in the distributor (or the oil pump drive, as in your L21) is just for fuel injector timing. You can pull your CKP sensor and you may see cracks on it. If you do, it's done for sure. However, it may still be failing without any visible faults.
    Gov't/ASE Certified Master Automotive Technician w/ Red Seal
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