INFORMATION ON ENGINE OVERHEAT/LOW POWER. GMC Truck GMC Suburban

SUBJECT: INFORMATION ON ENGINE OVERHEAT/LOW POWER

MODELS: 1990-93 CHEVROLET AND GMC TRUCK C/K, R/V, G 2-3 MODELS WITH 7.4L ENGINE (RPO L19 - VIN N)

DURING MOST WARM WEATHER (OVER 70 DEGREES F) HIGHWAY DRIVING AND WHILE IN SLOW MOVING URBAN TRAFFIC, THE NORMAL 7.4L ENGINE COOLING SYSTEM TEMPERATURE IS 200-240 DEGREES F. DURING EXTENDED PERIODS OF IDLE IN DRIVE RANGE WITH AIR CONDITIONING ON, SYSTEM TEMPERATURE MAY EXCEED THIS RANGE. SHIFTING TO NEUTRAL AND INCREASING ENGINE SPEED WILL REDUCE THE TEMPERATURE TO THE NORMAL RANGE.

TOWING ON LONG GRADES MAY CAUSE THE COOLANT TEMPERATURE TO RISE. THE "CHECK GAGES" LIGHT WILL COME ON AT 247 DEGREES F AND THIS IS CONSIDERED THE BEST GUIDE TO FOLLOW REGARDING SUSPECTED OVERHEAT. IF AN OWNER REPORTS THE CHECK GAGES LIGHT DID COME ON FOR COOLING SYSTEM OVERHEAT, A ROAD TEST WITH A TECH 1 SCANNER SHOULD BE PERFORMED TO VERIFY THE COOLANT TEMPERATURE IS EXCEEDING 245 DEGREES F.

IF THE TECH 1 DOES CONFIRM OVERHEAT, A VISUAL CHECK FOR THE FOLLOWING CAUSES SHOULD BE MADE.

ADD ON OBSTRUCTIONS IN FRONT OF THE RADIATOR/CONDENSER BUG SCREENS EXTRA COOLERS SPARE TIRE OR BIKE RACKS EXTRA LIGHTS HEAVY BUILDUP OF BUGS OR DEBRIS IN THE COOLING FINS DAMAGE TO THE COOLING FINS RESTRICTING AIRFLOW

COOLANT IS THE RADIATOR FULL OF COOLANT TO THE CAP SEALING LIP? IS THE COOLANT CLEAN OR HAS IT BEEN REPLACED IF THE TRUCK HAS OVER 30,000 MILES? DOES THE COOLANT HAVE THE PROPER WATER MIX RATIO (TOO LITTLE WATER IS WORSE THAN TOO MUCH FOR GOOD HEAT TRANSFER)? IF THE COOLANT HAS BEEN REPLACED, DOES IT CONFORM TO GM SPEC 6038- M (NO PROPYLENE GLYCOL)? IS THE RECOVERY JUG LEVEL CORRECT AND NOT OVER OR UNDER FILLED? IS THE RECOVERY JUG TO RADIATOR NECK HOSE AIR TIGHT SO NO AIR CAN BE DRAWN INTO THE SYSTEM DURING COOL DOWN? DOES THE SYSTEM AND RADIATOR CAP BOTH HOLD A STEADY 15# PRESSURE? IS THERE CORROSION INSIDE THE RADIATOR AROUND THE TUBE OPENINGS, (A SIGN OF REDUCED EFFICIENCY)?

THERMOSTAT AND FANS DOES THE THERMOSTAT BEGIN TO OPEN AT 190 DEGREES F USING THE TECH 1 AND FEELING THE UPPER HOSE? DOES THE VISCOUS FAN ENGAGE AT 220-230 DEGREES F USING THE TECH 1, (LISTEN FOR SOUND CHANGE AND SEE DIAGNOSIS PROCEDURE IN SECTION 6B1-4 OF THE SERVICE MANUAL)? DOES THE FAN HAVE THE CORRECT CLEARANCE TO THE SHROUD AND ARE ALL THE CORRECT SHROUDS IN PLACE? DOES THE ELECTRIC FAN BLOW AIR REARWARD TOWARD THE RADIATOR? DOES THE ELECTRIC FAN COME ON AT 210-220 DEGREES F (VISUAL CHECK) USING THE TECH 1?

ENGINE TEMPERATURE WHILE TOWING CAN ALSO BE AFFECTED BY GASOLINE QUALITY AND OVERLOADING. THE ENGINE ANTI-KNOCK SENSOR MONITORS COMBUSTION AND WILL RETARD THE IGNITION SYSTEM SPARK TIMING AS NEEDED TO CONTROL DETONATION. IF LOW OCTANE GAS IS BEING USED AND/OR THE TRUCK IS OVERLOADED, OR TOWING HIGH FRONTAL AREA TRAILERS AT ABOVE LEGAL POSTED SPEEDS, THE SPARK TIMING MAY BE RETARDED. AS SPARK TIMING IS RETARDED, COMBUSTION CHAMBER TEMPERATURES GO UP, INCREASING THE COOLING SYSTEM HEAT LOAD. AS THE COOLANT TEMPERATURE RISES, THE ENGINE CONTROL MODULE IS PROGRAMMED TO RETARD SPARK TIMING REGARDLESS OF A KNOCK SENSOR SIGNAL. THIS WILL FEED THE CYCLE OF POWER LOSS AND LEAD TO OVERHEAT.

IF ALL OF THE ABOVE CHECKS ARE FOUND TO BE CORRECT, A VISCOUS FAN CLUTCH WITH A LOWER ENGAGEMENT TEMPERATURE WILL PROVIDE INCREASED AIRFLOW THROUGH THE RADIATOR SOONER. THE OWNER SHOULD BE ADVISED BEFORE INSTALLATION THAT HE MAY HEAR FAN NOISE ALL THE TIME WHEN TOWING AND THE EXTRA DRAG OF THE FAN RUNNING MORE MAY CAUSE A SLIGHT REDUCTION OF MILES PER GALLON.

IF THE OVERHEAT CONDITION PERSISTS, CONTACT TECHNICAL ASSISTANCE FOR A REVIEW OF THE FACTORS INVOLVED.

SERVICE PARTS INFORMATION

P/N DESCRIPTION QTY 15671897 FAN CLUTCH 1

PARTS ARE CURRENTLY AVAILABLE FROM GMSPO.

FIGURES: 0

CAPTIONS:

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.