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Engine Controls - On-Vehicle Adjustments: Other GMC Sierra 2500

Adjustments ~845 words

ENGINE MECHANICAL

Before performing any on-vehicle adjustments to fuel or ignition systems, ensure engine mechanical condition is okay.

VALVE CLEARANCE

Note. All engines are equipped with hydraulic lifters, except for 6.6L diesel engine. Adjustments are not required. The 6.6L diesel engine is equipped with mechanical roller lifters. Adjustment is not required as routine service. Adjustment is only performed after disassembly of engine components. See 6.6L V8 DIESEL in ENGINES.

IGNITION TIMING (GASOLINE)

Note. On all models, ignition timing is controlled by control module and is not adjustable.

CAMSHAFT RETARD OFFSET (5.0L & 5.7L)

Note. Distributor can be adjusted to prevent crossfire.

INJECTION PUMP TIMING/TDC OFFSET (DIESEL)

Note. Injection pump timing is not adjustable on 6.6L diesel engine.

INJECTION PUMP TIMING (6.5L DIESEL)

Original factory injection pump timing is usually marked with a static timing mark (a line scribed across both the pump mounting flange and front cover). This static timing mark should be used as a reference. If timing mark is not present on mounting flange, scribe a line to assist in how far injection pump needs to be rotated. To check and adjust injection timing, perform the following procedure.

  1. Start engine and allow it to reach normal operating temperature. Connect scan tool to Data Link Connector (DLC). Set scan tool to OUTPUT TEST/INJ PUMP mode. Activate TDC TIME SET. The DES INJ TIME value will read zero. NOTE: If engine stalls when TDC TIME SET is activated, turn engine off. Loosen injection pump nuts. Slightly rotate injection pump toward driver's side of vehicle. Tighten injection pump nuts. Repeat step 1 .
  2. With engine running at idle, ensure ACT INJ TIME displays 3.5 degrees. If timing needs adjustment, turn engine off. Adjust injection pump. A.039" (1.0 mm) pump movement equals about a 2 degree timing change.

TDC OFFSET LEARN PROCEDURE (6.5L DIESEL)

Note. DO NOT perform this procedure when only an injection pump is replaced.

This procedure is only used when one of the following conditions exist

  1. DTC P1214 is set.
  2. Engine has been replaced.
  3. Front engine cover has been replaced.
  4. Both PCM and injection pump have been replaced.
  5. CKP sensor has been replaced.
  1. Install scan tool. Start and warm engine to normal operating temperature. Using scan tool, clear DTCs. Turn engine off.
  2. Turn ignition on, with engine off. Hold accelerator pedal to Wide Open Throttle (WOT) position for a minimum of 45 seconds (this prepares the PCM to activate offset learn). Release accelerator pedal.
  3. Turn ignition off for 30 seconds. This step will power down the PCM. To verify if PCM powered down, use scan tool and access Data List. Display should read No Communication With Vehicle. If PCM has powered down, go to next step.
  4. Start engine. Using scan tool, check Engine Coolant Temperature (ECT) sensor reading. ECT sensor reading should be greater than 170°F (77°C). Warm engine if ECT is not as specified. PCM will automatically learn a new offset when coolant temperature is as specified and engine speed is less than 1500 RPM. A momentary stumble in engine RPM will indicate that the TDC offset has been activated.
  5. Using scan tool, check TDC offset. TDC offset should be between -0.25 and -0.75. If TDC offset is as specified, test is complete. If TDC offset is not as specified, go to next step.
  6. Turn engine off. Loosen injection pump flange nuts. Rotate injection pump to desired direction. A.039" (1.0 mm) pump movement equals about a 2-degree timing change. To achieve a positive reading, rotate pump toward driver's side. To achieve a negative reading, rotate pump toward passenger's side. Recheck TDC offset to achieve specified value. If TDC offset will not learn, go to next step.
  7. Check for the following: Coolant temperature should be greater than 170°F (77°C). Proper time intervals are observed for WOT and PCM power down. All PCM and injection timing stepper motor connections and for DTCs. Techline® terminal/equipment for the latest software version. Proper base installation of injection pump (engine shut-down ESO solenoid should be about straight up and down position). If TDC offset is stuck on a high (+) value, check for damaged camshaft driven gear key. If TDC offset is stuck on a high (-) value, check for damaged or improperly installed crankshaft position sensor. If all checks have been performed, check for a malfunctioning injection pump.

GASOLINE ENGINES

Note. DO NOT attempt to adjust idle mixture and idle speed. Both are controlled by Powertrain Control Module (PCM). Incorrect idle speeds are normally caused by dirty throttle plate or vacuum leaks. Ensure all vacuum components are functioning properly.

DIESEL ENGINES

Note. Diesel engines are equipped with electronic fuel injection in which PCM controls all fuel delivery modes based on inputs from APP, ECT and BARO sensors. Idle speed is not adjustable.

Note. Throttle position sensors are not adjustable.

DIESEL ENGINES & 8.1L GASOLINE ENGINE

Throttle control is electronic with no mechanical link from engine to accelerator pedal. PCM monitors position of accelerator pedal based on signals from a nonadjustable Accelerator Pedal Position (APP) module. APP module is mounted above accelerator pedal.

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