Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 150 (ENGINE CONTROL SYSTEM - TROUBLE SHOOTING - NO CODES)SYMPTOM DIAGNOSISCUTS OUT, MISSESIGNITION SYSTEM CHECKS
1999 GMC Forward Control 5.7 R, Automatic
Ignition System Checks
1999 GMC Forward Control 5.7 R, AutomaticSECTION Ignition System Checks
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
Check for an ignition system misfire.
CAUTION:
Do not perform this test for more than 2 minutes in order to prevent damage to the catalytic converter.
- Start the engine.
- Remove one ignition coil at a time using insulated pliers.
- If there is an RPM drop on all cylinders (equal to within 50 RPM), see ROUGH, UNSTABLE, OR INCORRECT IDLE AND STALLING .
- Check for spark on the suspected cylinders with Spark Tester (J 26792), or equivalent, if there is no RPM drop on one or more cylinders.
- If there is no spark, see ELECTRONIC IGNITION SYSTEM DIAGNOSIS under IGNITION SYSTEMS in SYSTEM & COMPONENT TESTING article.
- If there is spark, inspect the spark plugs for correct operation and good ignition system performance.
Check the ignition system for moisture, dust, cracks, burns, etc.
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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.