gm’s 4.3-liter vortec v6 - motor | automotive data ?· mately 48 psi. the main injector and fuel...

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  • 20 July 2000

    CURINGTHE FUEL

    INJECTOR

    BLUES

    CURINGTHE FUEL

    INJECTOR

    BLUESIn 1992, General Motorsintroduced a very uniquefuel injection system on its4.3-liter Vortec V6 truckengine. Dubbed CentralPort Injection, or CPI, thesystem looked and functionedmuch differently than previousdesigns. The CPI system utilizesone main injector that pressurizessix plastic tubes housed in the in-take manifold. Each tube termi-nates in a spring-loaded poppet

    BY JEFF MAST

    ERMAN

    Armed with a pr

    essure gauge, a

    good

    scope and a wo

    rking knowledge

    of its

    trouble spots,

    you can purge t

    he

    demons that pla

    gue GMs CPI sy

    stem.

    BY JEFF MAST

    ERMAN

    Armed with a pr

    essure gauge, a

    good

    scope and a wo

    rking knowledge

    of its

    trouble spots,

    you can purge t

    he

    demons that pla

    gue GMs CPI sy

    stem.

  • GMs 4.3-Liter Vortec V6GMs 4.3-Liter Vortec V6

    21July 2000

  • valve that supplies fuel to its cor-responding cylinder. The main in-jector is pulsed twice per crank-shaft revolution, with pressurizedfuel opening each poppet as sys-tem pressure approaches approxi-mately 48 psi.

    The main injector and fuelpressure regulator on the CPI sys-tem are housed in a molded plas-tic piece with the six plastic tubesattached. GM dealers service thisassembly as a single part number,and its quite expensive to replace.The good news is that replace-ment pressure regulators, as wellas fully reconditioned assemblies,are available via the aftermarket,which can save your customerssubstantial dollars on their repairbills.

    Common ProblemsLeaking pressure regulators arefairly common on the CPI sys-tem. Since the regulator ishoused in the intake, a ruptureddiaphragm often results in fuel

    being dumped directly into thecrankcase or passenger-side cylin-ders. Fortunately, the manifoldtuning valve (MTV) can be re-moved to allow easy inspection ofthe regulator if leakage is sus-pected.

    Fuel pressure leak-down withno external signs of leakage is an-other common problem on theCPI system. Like most injectionsystems, leak-down can result inlong crank times or a stumble af-ter restart. A quick way to isolatethe cause of a leak-down problemon this system is to cycle the keyand allow the fuel pump to runfor a couple of seconds. Once thepump stops, simply pinch off thesupply line. If the pressure holds,the cause is in the tank. If itdoesnt, the intake will have tocome off for a closer look at theinternals.

    Inadequate fuel pressure or vol-ume can also cause long cranktimes or a no-start condition. GMlists the pressure range for the

    22 July 2000

    CURING THE FUEL INJECTOR BLUES

    The manifold tuning valve (MTV) can be removed from the top of the intake toallow visual inspection of the pressure regulator area without removing theplenum. If theres fuel laying in the intake or if the area around the regulatoris very clean, the regulator diaphragm is probably leaking.

    Notice in this photo how the passenger side of the intake is much cleanerthan the drivers side area. This is partially due to PCV flow, but can also bean indication of a leaking fuel pressure regulator (arrow).

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    : Jef

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  • CPI system at 54 to 64. However,many techs have encounteredhard-start or no-start complaintswith pressure readings as high as56 to 57 psi on a cold engine. Thisproblem is amplified by the factthat many fuel pressure gaugeshave seen a rough life and can beoff by as much as 2 to 5 psi. So al-ways check the accuracy of yourgauge before working on a CPIsystem. Finally, remember thatthe fuel pressure shouldnt dropduring heavy acceleration. If itdoes, the volume of fuel being de-livered needs to be evaluated.

    Youll find that many customersdont replace their fuel filter as amaintenance item, and its not un-usual to see a filter thats rackedup 100,000 miles. Running forlong periods against a restrictedfilter can kill even the best of fuelpumps. It can also cause big-timeproblems for the CPI system.

    Testing has shown that evenslight changes in operating pres-sures can result in significant dif-

    ferences in the volume of fuelflowing through the poppetvalves.

    EGR WoesEGR valves have been known tostick open on CPI engines due tocarbon buildup. The result will bea rough idle, often accompaniedby a failed emissions test due tohigh HC levels. These enginesuse an electronically controlledEGR valve with a built-in positionsensor. Scan data can provide aquick check for this type of valve.With the key on and the engineoff, just compare the EGR De-sired percentage to EGR Actu-al. If the Actual percentage ishigher than the Desired, the valveneeds to be cleaned or replaced.

    The poppet valves on the CPIsystem have a tendency to stickopen. If this occurs, a largeamount of fuel will enter the com-bustion chamber. Fouled sparkplugs can be a tip-off to this prob-lem. If you look at the scan data,

    23July 2000

    The EGR valve on the Vortec V6 sticks open frequently as a result of carbonbuildup. While the intake is off, its always a good idea to remove the valve andclean off as much carbon as possible to prevent rough idling.

    The intricate design of the CPI plenum can make it difficult to use propane dur-ing a cylinder balance test to determine if a misfire is due to a lean cylinder.

  • youll find that the Long TermFuel Trim number will be nega-tive, as well.

    An improper spray pattern cancause one cylinder to run leanerthan the rest, which could resultin a rough idle and high HC dur-ing an idle emissions test. Per-forming a cylinder balance testcan help you isolate the problemcylinder. Once the cylinder isidentified, add propane at thethrottle body until the rpm peaks.Then repeat the test. If the weakcylinder now contributes, thepoppet valve for that cylindershould be suspect. One caveat:The intake design of the 4.3-literVortec V6 can prevent thepropane from being evenly dis-tributed among the cylinders.This can result in misleading test

    24 July 2000

    CURING THE FUEL INJECTOR BLUES

    This ASNU machine used by Linder Technical Services can be used to both testand recondition CPI injector assemblies. It has an adjustable pressure func-tion thats great for demonstrating the effect of fuel pressure changes onthe volume of fuel delivered to the engine during cranking.

    Left: Here, the pressure of the ASNU machine is adjusted to around the middle of the specified range of the CPI sys-tem. The tester pulses the main injector a number of times using a programmed frequency. The result is 54 milliliters(ml) of fuel delivered at each poppet valve. Right: Now the pressure is reduced to 54 psistill within spec, but at thebottom of the range. Notice how the volume of fuel delivered drops to 42 ml. Thats a 23% reduction in volume due tojust a 6-psi drop in fuel pressure. This can create a no-start or extended cranking times on a cold engine.

    Ph

    oto:

    Jim

    Lin

    der

    Ph

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    : Jim

    Lin

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  • results. When in doubt, removethe intake plenum.

    An off-the-vehicle injectortester is by far the best choice fortesting fuel flow on a CPI engine.If one is not available, theres analternative test that can be per-formed after the plenum is re-moved. Begin the test by remov-ing the coil wire from the distrib-utor and securely ground it toprevent the engine from starting.Leave the ignition primary con-nected so the main injector willpulse. With the intake plenum off,the weak cylinders plastic line cannow be pulled out of the intakeand the poppet valve placed in acontainer. Observe the spray pat-tern while an assistant cranks theengine briefly.

    Since the main injector is con-cealed by the intake, a noid lightcannot be used for no-start diag-nosis. However, there is a connec-tor behind the alternator that canbe used to check the power feedand the PCM-controlled groundside of the injector with yourscope. While the scope is not con-nected at the injector, grabbinga waveform at this location teststhe whole circuit. Note that GMuses a peak-and-hold driverarrangement on the CPI system,similar to its TBI setups. If thewaveform indicates a problem,the intake will have to be re-moved to check the rest of thewiring.

    With a working knowledge ofthe system and an accurate fuelpressure gauge, you can win thediagnostic battle with GM CPI.Just remember what makes thissystem so different from the morecommon ones you see every dayand youll be on your way to hav-ing found a cure for the fuel in-jector blues.

    26 July 2000

    CURING THE FUEL INJECTOR BLUES

    For a free copy of this article, write to: Fulfillment Dept.,

    MOTOR Magazine, 5600 CrooksRd., Troy, MI 48098. Additional

    copies are $2 each. Send check or money order.

    Once the intake plenum is removed from the top of the engine, the spray pat-tern of a suspect poppet valve can be observed. Use caution during this typeof test; excessive cranking can cause engine damage in a hurry.

    One look at this waveform tells you that the main injector on the CPI system isdriven by the PCM using the peak-and-hold method. The pattern was capturedby the scope connected to an external connector located behind the alternator.