Correcting Drivability Problems

Procedure for correcting driveability problems with Callaway Twin Turbo Corvettes, assuming there are no “hard” trouble codes:

  1. Perform a careful visual inspection of the following, looking for air leaks in particular (Because 1990-1991 models has no MAF, air leaks before the throttle body will not cause drivability problems although they will cause a performance loss.)
    • Is the MAF securely clamped in the airfilter housing? (1988-1989 models)
    • Is the rubber boot securely fastened to the MAF and the casting that goes into the “wonder bar”? (1988-1989 models)
    • Is the compressor discharge tube on the left side turbocharger (pipe that goes up left side of motor and into the bottom of the left side intercooler) securely clamped to the turbocharger? This is a potential problem area because the clamp can slip to a position where it is not securing the pipe to the turbo but looks as though it is secure. There is not enough airflow at idle for this to show up as an air leak, but at high speed it will.
    • Are both the CCP purge hose and vacuum control hose connected at the check valves below and to the front of the left side intercooler? Are they connected at the throttle body?
    • Are the breather hoses connected at the “T” under the throttle inlet casting? Is the hose from this “T” connected to the next “T,” and then are the hoses intact to the throttle body and the air-oil separator? (The gold cadmium plated can in front of the power steering pump.)
    • Make sure that the spark plug primary and secondary wires behind the distributor are not inter-twined.
    • Make sure that the ignition timing is set at 6 degrees BTDC with the bypass wire disconnected. If the timing is severely retarded (anything after TDC), remove the distributor from the engine and check the drive gear for excessive wear.
    • Check the air filter for signs of having been wet, or if it is seriously dirty. Replace it for either reason.

  2. Does the boost gauge show 12″ Hg of pressure at idle (higher at higher altitude)? If it doesn’t, then hook a vacuum gauge to a port on the intake manifold to confirm that the engine is making reduced vacuum. Important note: the boost gauge is calibrated in absolute pressure, so 12″ of pressure on the boost gauge corresponds to 18″ of vacuum on a diagnostic vacuum gauge. If this is the case, then diagnose as you would any engine that has low vacuum (i.e., air leak, inlet or exhaust restriction, etc.). If the boost gauge is showing pressure higher than the vacuum gauge attached above, trace the nylon hose to the firewall, then from inside to the MicroFueler and, finally, to the boost gauge to see where it is leaking. On 1987 models, it also must be checked from the “T” at the brake booster down to the left side wastegate, and then across the bottom of the car to the right side wastegate. The most likely places for leaks are at any of the connections in the first six inches of this hose, or at the “T” in front of the MicroFueler module. This hose will hold a vacuum if all is as it is supposed to be. See schematic on car vacuum hose routing sticker.
  3. Connect a “TECH-1” or similar scan tool to the car and take a snap shot at full throttle acceleration with the center of the data stream being the 2-3 shift. On a “TECH-1”, this would be when to push the button. Is the oxygen sensor reading spending most of its time (say 4 counts out of 5) in the range of 750-950 mV? If it is significantly lower, then the operation of the MicroFueler must be checked. The MicroFueler is the black electronic module mounted above the panel over the driver’s knees. It senses engine RPM and boost pressure and drives the two injectors that are located in the “ram’s horn” throttle inlet casting. With the exception of the fuel it receives from the fuel rail and the tach signal from the white wire to the distributor cap, the MicroFueler is independent of the car’s fuel and ignition systems. To check the MicroFueler, disconnect the connectors at the two injectors on the front of the engine. With the engine running, disconnect the vacuum hose from the plenum that runs to the MicroFueler, and apply regulated air pressure to it. UNDER NO CIRCUMSTANCES SHOULD SHOP AIR BE USED. AIR PRESSURES OVER 20 PSI WILL DAMAGE THE TRANSDUCER INSIDE THE MICROFUELER. Put an injector test light on each of the connectors disconnected above. The light should blink. As the boost pressure or RPM are raised, the blinking rate should increase. Test both injectors. Remove the pressure from the hose and reconnect the injectors. Re-apply the pressure and see if the idle changes (pressure to the fuel pressure regulator might also be being increased). Feel or listen to each of the two injectors to see that they are clicking. If the test light does not light or if the injectors are not being driven, than there is a wiring problem or the MicroFueler is defective and must be replaced. Check the wiring per the pinouts listed below. Otherwise the MicroFueler is not causing the lean condition.

    Microfueler harness pinouts:
    #1 & #4 – empty
    #2 – ground (brown wire)
    #3 – black injector wire
    #5 – system voltage, key on hot
    #6 – black injector wire
    #7 – tach signal (black wire) with engine running test light to ground should blink brightly.
    #8 – white injector wire (paired with #3)
    #9 – white injector wire (paired with #6)

  4. Check for proper fuel pressure. The procedure is the same as that outlined in the Chevrolet shop manual except the blue cap on the “T” on the fuel rail is the proper place to hook the gauge up. The car should be driven with the gauge taped to the windshield as the volume of fuel the car requires at wide open throttle is substantially greater than that required at idle. The fuel pressures should be as outlined in the shop manual (approx. 37 psi at idle, 48 psi at wide open throttle) but, as the car goes into boost, fuel pressure should increase as manifold pressure increases, to a maximum of 56 psi. Diagnose low fuel pressure per the Chevrolet shop manual.