GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Engine Control Unit Function Parameters

Function

Enable Parameter

Result or Use

Decel Fuel Cut-Off (DFCO)
    • MAP < 25 kPa
    • RPM > 2000
    • Time > 30 Seconds (1997)
    • Time > 20 Seconds (1998)
    • TPS <= 1%
    • Exit DFCO > 31 kPa
    • The catalyst temperatures may exceed maximum limits during extended decel maneuvers.
    • When the criteria is met, the low pressure lock-off will be cycled off to shut off all fuel and cool the catalysts.When the criteria is met, the low pressure lock-off will be cycled off to shut off all fuel and cool the catalysts.
    • DFCO turns off the LPL until mode is exited.

Decel Open Loop
    • MAP < 25 kPa
    • RPM > 1100
    • Time > 0 Seconds
    • TPS <= 1%
    • Exit DFCO > 31 kPa
    • The vehicle will operate in open loop as soon as a deceleration is detected that meets the criteria.
    • The vehicle will transition into DFCO if this decel time reaches 30 seconds.

Speed Limiting

VSS > 97 mph

RPM > 3000
    • Speed limiting is achieved by closing the MCV valve at 97 mph and does not reopen until 94 mph.
    • The vehicle operates in open loop while in speed limiting.

RPM Limiting

RPM > 5500
    • RPM limiting is achieved by closing the MCV valve at 5500 RPM and does not reopen until 5475 RPM.
    • The vehicle operates in open loop while in RPM limiting.

Fuel Switch-Over
    • FSU Voltage < 1.75 Volts
    • OBD Fault
    • Wide Open MCV (< 4500 RPM and 0.45 second at WOT, 100 ms at idle or 2 seconds at crank)
    • 8 second cranktime
    • The vehicle will switch over to gasoline if an OBD fault is detected, if crank time exceeds eight seconds, or if a wide open MCV is detected for the amount of time noted.
    • For 1997 vehicles, the FSU will cause a switch-over if voltage (indicating fuel level) drops below 1.75 volts (4 136 kPa at 21°C [600 psi at 70°F]).
    • The vehicle will not restart on CNG until voltage exceeds 3.00 volts (7 583 kPa at 21°C [1,100 psi at 70°F]) The vehicle will not restart on CNG until voltage exceeds 3.00 volts (7 583 kPa at 21°C [1,100 psi at 70°F]).
    • For 1998 vehicles, the FPS will cause a switch-over if voltage (indicating fuel pressure) drops below 0.85 volt (2 412 kPa at 21°C [350 psi at 70°F]).
    •  The vehicle will not restart on CNG until voltage exceeds 1.5 volts (6 890 kPa at 21°C [1000 psi at 70°F]).

Open Loop
    • Start-up
    • OBD failure of both O2 sensors
    • Speed Limiting
    • RPM Limiting
    • Deceleration
    • Block Learn and EGO multiplier will both be disabled, and EGO multiplier will be returned to 1.000.
    • Fueling will be based on the block learn table as it exists prior to going open loop.

Closed Loop
    • ECT = Time
    • O2 Switch = Time
    • For the vehicle to enter closed loop operation, the ECU looks at the ECT at start-up and will hold off closed loop operation for 20-90 seconds, depending on temperature.
    •  In addition to ECT time, the ECU will look for an HO2 sensor switch and add an additional 7-35 seconds, depending on HO2 activity.

Start Prime Pulse MCV
    • RPM min. > 25
    • RPM max. < 450
    • Delay Time: 400 ms
    • Duration: 200 ms
    • Must exceed 100 RPM increase every 4 ms in order to disable the pulse

The ECU will deliver a prime pulse after every complete ECU shutdown (if the criteria is met) by opening the MCV wide open in order to aid in starting.

Lock-Offs
    • RPM min. > 25
    • Key on
    • The ECU will open the HPL for 1 second after a complete shutdown and a key-on sequence.
    •  After a 25 RPM signal is received, the LPL and the HPL will be opened as long as an RPM signal is present.

Warm-Up Cycle
    • ECT at key off > 70°C (160°F)
    • Delta between ECT at start and ECT at key off > 40°C (104°F)

For diagnostic use only.

Gasoline Starts or Cold Start Strategy

10°F or every 100 starts

Will switch to CNG operation when the criteria below is met.

    • Fuel Level > 600 psi
    • RPM > 1300 (1997)
    • RPM > 1400 (1998)
    • TPS < 6% for 400 ms
    • 30 mph < VSS < 100 mph (1997)
    • 40 mph < VSS < 100 mph (1998)
    • Gasoline start strategy used for gasoline injector durability.
    • On gasoline start codes 58 and 41 run (while in gasoline operation). All others are delayed until five seconds after switch-over occurs.
    • Abort Criteria:
       - TPS open > 6% during 500 ms switch-over period.
       - % error desired gas to actual gas for 1.5 seconds.

Important:: For 1998 vehicles, the gasoline fuel injectors will be actuated for four pulses during every start.

Code Types

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    • Type A: Emissions related. One cycle to set the MIL and DTC.
    • Type B: Emissions related. Two consecutive driving cycles to set the MIL and DTC.
    • Type C: NOT USED. Non-emission related. No MIL , but the SES light will turn on.
    • Type D: Non-emission related. No MIL or SES light set.