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 35 (HVAC SYSTEMS - MANUAL)PERFORMANCE TESTSHEATING PERFORMANCE
1999 GMC Forward Control 5.7 R, Automatic
Heating Performance
1999 GMC Forward Control 5.7 R, AutomaticSECTION Heating Performance
WARNING: This page is about a different car, the 2002 Buick Regal and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- If sent here from Too Cold In Vehicle test, go to next step. If not sent here, see TOO COLD IN VEHICLE , under SYMPTOM TESTS.
- Start engine and allow to idle. If engine reaches normal operating temperature, go to next step. If engine does not reach normal operating temperature, go to step 9.
- Allow engine to idle. Select FLOOR mode, minimum blower speed, and maximum temperature setting. Feel temperature of inlet and outlet hoses at heater core. If inlet hose feels warmer than outlet hose, go to step 7. If inlet hose does not feel warmer than outlet hose, go to next step.
- Install thermometer into center instrument panel air outlet. Secure thermometer to Select VENT mode, maximum blower speed, and maximum temperature setting. Record temperatures at center instrument panel air outlet and heater core outlet hose. If temperatures are similar, go to next step. If temperatures are not similar, go to step 6.
- Inspect cowl, recirculation door and HVAC module case for cold air leaks. If leaks are found, repair as necessary. When repairs are complete, go to step 10.
- Check temperature door operation. If problem is found, repair as necessary. When repairs are complete, go to step 10.
- Turn engine off. Back-flush heater core. Start engine. Select FLOOR mode, minimum blower speed, and maximum temperature setting. Feel temperature of inlet and outlet hoses at heater core. If inlet hose feels warmer than outlet hose, go to next step. If inlet hose does not feel warmer than outlet hose, go to step 10.
- Replace heater core. See HEATER CORE under REMOVAL & INSTALLATION.
- Repair low engine temperature concern.
- Operate system to verify repair. If system operates correctly, system is okay. If system does not operate correctly, go to step 2.
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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.