SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P0205-P0351DTC P0303DTC P0303: CYLINDER NO. 3 MISFIRE DETECTED (3.0L "V" BODY)CIRCUIT DESCRIPTION
1999 GMC Forward Control 5.7 R, Automatic

Circuit Description

1999 GMC Forward Control 5.7 R, AutomaticSECTION Circuit Description

Crankshaft Position (CKP) sensor sends pulses to ECM as reluctor wheel teeth rotate past sensor. Besides using these pulses to synchronize ignition and fuel injector operation, ECM also times interval between each pulse and compares each new time interval with previous one to determine when an excessive change in crankshaft speed has occurred.

A misfire causes an unexpected change in crankshaft speed. A certain amount of acceleration/deceleration is expected between each firing stroke, but if crankshaft speed changes more than expected amount, ECM will interpret this as a misfire. ECM continuously calculates crankshaft position from CKP sensor signals.

This information is used to determine which cylinder is misfiring so ECM can increment appropriate misfire counter and set appropriate DTC, if necessary. If ECM is able to determine a specific cylinder is misfiring, DTC for that cylinder will set. If misfire is random and on multiple cylinders, DTC P0300 will set. Conditions for setting DTC:

  • Misfire more than legislated limit (amount of misfire ranging from 1.0-12.5 percent).
  • Condition must be met for at least 5 camshaft revolutions.

RENDER: 1.0x

NO RELATED

Recommended Tools & Savings

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Affiliate disclosure: We may earn a commission at no extra cost to you.

Shop Manual-Ready ToolsFree Quote CheckQuote Shield Pro

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.