HESITATION ON ACCELERATION CHECK MAP HOSE FOR BLOCKAGE

SOME 1980 DEFI EQUIPPED VEHICLES MAY EXPERIENCE A HESITATION DUE TO A RESTRICTION IN THE VACUUM SUPPLY TO THE MAP SENSOR. ON THESE VEHICLES, THE ECM USES RATE OF CHANGE IN MAP READING TO PROVIDE ACCELERATION ENRICHMENT. IF MAP CHANGES QUICKLY, THE ECM DECIDES THAT ACCELERATION ENRICHMENT IS CALLED FOR AND PROVIDES ADDITIONAL FUEL TO THE ENGINE. IF THERE IS A BLOCKAGE OR RESTRICTION IN THE MAP LINE, THE RESTRICTION MAY ACT AS A DELAY VALVE CREATING SLOW MAP RESPONSE. TO DIAGNOSE, TEE A VACUUM GAGE INTO THE MAP HOSE AT THE MAP SENSOR. THE VACUUM GAGE SHOULD RESPOND QUICKLY TO CHANGES IN ENGINE LOAD. IF THE GAGE SHOWS SLOW RESPONSE TO ENGINE VACUUM CHANGES, THE CAUSE IS A RESTRICTION IN THE MAP HOSE OR THROTTLE BODY MAP PORT. TO CORRECT: 1. CHECK MAP HOSE FOR RESTRICTION DUE TO FOREIGN MATERIAL, COLLAPSED OR PINCHED MAP HOSE. REPAIR OR REPLACE AS NEEDED. 2. REMOVE THROTTLE BODY AND CHECK THROTTLE BODY MAP PORT FOR CARBON BUILD-UP OR RESTRICTION (SEE FIGURE 1). CLEAN OR REPAIR AS NECESSARY. 3. AFTER THE CAUSE OF THE RESTRICTION IS REPAIRED, RECHECK THE VACUUM RESPONSE AT THE MAP SENSOR AS PREVIOUSLY DESCRIBED TO ENSURE THAT THE RESTRICTION IS CLEAR AND THAT THE MAP CAN RESPOND TO CHANGES IN ENGINE VACUUM. THROTTLE BODY (DEFI) GENERAL DESCRIPTION THE THROTTLE BODY, FIGURE 6C-227, USED ON THE 6.0 LITER V8 ENGINE, IS A NEW SYSTEM OF AIR AND FUEL CONTROL WHEREBY THOSE COMPONENTS (THROTTLE VALVES, FUEL INJECTORS AND FUEL PRESSURE REGULATOR) SUPPLYING AIR AND FUEL FOR COMBUSTION FOR ALL HOUSED IN ONE MAJOR UNIT THE THROTTLE BODY ASSEMBLY. THE ECM PROGRAM ELECTRONICALLY SIGNALS THE FUEL INJECTORS IN THE THROTTLE BODY ASSEMBLY TO PROVIDE THE CORRECT QUANTITY OF FUEL FOR A WIDE RANGE OF OPERATING CONDITIONS. SEVERAL SENSORS ARE USED TO DETERMINE WHICH OPERATING CONDITIONS EXIST AND THE ECM THEN SIGNALS THE INJECTORS TO PROVIDE THE PRECISE AMOUNT OF FUEL REQUIRED. THE THROTTLE BODY ASSEMBLY IS CENTRALLY LOCATED ON THE INTAKE MANIFOLD WHERE, FOR SIMPLICITY, AIR AND FUEL ARE DISTRIBUTED THROUGH TWO BORES IN THE THROTTLE BODY TO THE INTAKE MANIFOLD SIMILAR TO A NORMALLY CARBURETED ENGINE. AIR FOR COMBUSTION IS CONTROLLED BY TWO THROTTLE VALVES, ONE FOR EACH BORE, CONNECTED TO THE ACCELERATOR PEDAL LINKAGE. FUEL FOR COMBUSTION IS SUPPLIED THROUGH TWO FUEL INJECTORS, ONE FOR EACH BORE, MOUNTED ON THE THROTTLE BODY ASSEMBLY WITH THE METERING TIP LOCATED DIRECTLY ABOVE THE THROTTLE VALVES, FIGURE 6C-228. THE INJECTORS ARE ELECTRICALLY "PULSED" OR "TIMED" OPEN OR CLOSED BY THE ELECTRONIC CONTROL MODULE (ECM) AND NORMALLY ARE ACTUATED ALTERNATELY ACCORDING TO THE ENGINE FIRING ORDER. THE ECM RECEIVES INPUTS FROM THE VARIOUS SENSORS CONCERNING ENGINE OPERATING CONDITIONS, PERFORMS HIGH- SPEED CALCULATIONS OF ENGINE FUEL REQUIREMENTS AND "PULSES" OR "TIMES" THE INJECTORS, OPEN OR CLOSED, FOR PRECISE DELIVERY OF FUEL TO EACH BORE. AN ELECTRICALLY OPERATED SOLENOID VALVE IN THE INJECTOR IS USED TO CONTROL FUEL DELIVERY OUT OF THE INJECTOR NOZZLE. WHEN THERE IS NO FLOW OF ELECTRICAL CURRENT FROM THE ECM TO THE SOLENOID, SPRING FORCE CLOSES THE METERING VALVE (BALL-TYPE) IN THE INJECTOR NOZZLE PREVENTING FUEL FLOW OUT THE NOZZLE. WHEN THE SOLENOID VALVE IS ENERGIZED, THE SPRING-LOADED METERING VALVE MOVES TO THE FULL OPEN POSITION. SINCE THE PRESSURE DIFFERENTIAL ACROSS THE VALVE IS CONSTANT, THE VOLUME OF FUEL FLOW IS CHANGED BY VARYING THE TIME THE INJECTOR IS HELD OPEN. THIS FUNCTION IS PERFORMED BY THE ECM. IN ADDITION, AN IDLE SPEED CONROL (ISC) IS MOUNTED ON THE THROTTLE BODY ASSEMBLY FOR CONTROL OF FAST AND SLOW IDLE SPEEDS. INPUTS TO THE ISC ACTUATOR MOTOR COME FROM THE ECM AND ARE DETERMINED BY THE IDLE SPEED REQUIRED FOR A PARTICULAR OPERATING CONDITION. FOR EASE OF SERVICE, ALPHABETICAL CODE LETTERS ARE INCLUDED ON THE THROTTLE BODY ASSEMBLY AT EXTERNAL TUBE LOCATIONS TO AID IN IDENTIFICATION OF VACUUM HOSE CONNECTIONS. THE ASSEMBLY IDENTIFICATION IS ROLL-STAMPED VERTICALLY ON A FLAT AREA LOCATED ON THE FRONT OF THE THROTTLE BODY, FIGURE 6C-229. IF REPLACING THE THROTTLE BODY ASSEMBLY, FOLLOW THE MANUFACTURER'S INSTRUCTIONS CONTAINED.

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.