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Super Mechanic
Super Mechanic, Master technician
Category: Chevy
Satisfied Customers: 17220
Experience:  29 yrs. exp., ASE master, cert. in G.M. electrical, a/c, susspension, brakes, electronic steering
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1995 chevy blazer: rotor..coil..control module..tps, egr valve

Customer Question

hi i have a 1995 chevy blazer with a code of p0300. i replaced everything. the plugs wires cap and rotor, coil and ign control module, tps, egr valve, and idle air control valve, and is still misfiring like crazy.
Submitted: 6 years ago.
Category: Chevy
Expert:  Super Mechanic replied 6 years ago.

Hello and welcome to Just, Super mechanic here. About your vehicle.

There has been a service bulletin for that type of concern. Here is what it states.

Engine - Hard/No Start/Miss/Rough Running

File In Section: 6E - Engine Fuel & Emission

Bulletin No: 56-63-06

Date: October, 1995

Supplemental Diagnostics for Central Multiport Fuel Injection (CMFI) Engine Hard or No Start, Miss or Rough Running (Replace Fuel Pump or Fuel Injector) Models:
1992-95 Chevrolet and GMC Truck S/T; M/L Models
1992-94 Oldsmobile Bravada
with 4.3L CPI Engine (VIN W - RPO L35)


Some 4.3L CMFI engines may be hard to start, will not start, or may miss or run rough.



The fuel pump/system may not be delivering the required fuel pressure to the CMFI injector (is the fuel filter plugged or due for replacement?) or, the CMFI unit may have a fault.




If the engine cranks but does not start or is slow to start, first check for sufficient fuel in the gas tank. Then using Kent-Moore tool J 34730-20 or equivalent check for fuel injector pulse. Use J 26792 (ST-125) spark tester or equivalent to check for ignition spark. If the injector pulse or ignition spark is not present, this bulletin may not apply. Use the appropriate procedure to diagnose and correct as required.

If sufficient fuel, spark and injector pulse signal are present, the fuel pump should be checked for current draw.




Verify that the battery connections are sound and that it has a minimum of 12 volts. Using the 10 amp scale of an ammeter, connect one lead to the fuel pump prime terminal (see appropriate manual for location) and the other end to the battery B+ terminal. If the ammeter shows less than 7 amps draw, with the fuel level in the tank clearly above the empty mark (low fuel level can cause low current draw readings), the fuel pump should be replaced. If 7 amps or more is indicated, disconnect the ammeter leads and install fuel pressure gauge J 34730-1A, following the appropriate service manual procedure. While watching the fuel pressure gauge, turn the ignition switch on. The fuel pump will run for two seconds. The gauge should indicate 380-440 kPa (55-64 psi). When the pump stops after two seconds, the pressure may drop slightly but then stabilize and should not bleed down. If the pressure did not bleed down, skip the next step.

If the specified pressure was met but bleeds down:



Install Tech 1, raise vehicle to gain access to fuel feed line forward of the in line fuel filter. Using the Tech 1, command the fuel pump on. The fuel pump should run for two seconds and stop. After the fuel pump stops, immediately pinch the flexible fuel line using pinch off pliers J 41413-50, part of kit J 41413 (EVAP Pressure Cart).

If fuel pressure holds steady after pinching the fuel line, remove the fuel tank sender and check the fuel pump pulse damper (pulsator) for damage to its seals. If the pulsator seals are intact, replace the fuel pump.


If the pressure bleeds down after pinching the fuel feed line, inspect the fuel feed line between the pinch pliers and the CMFI unit for a fuel leak. If a leak is found, repair or replace as necessary. If no leak is found, replace the CMFI unit.

If the specified pressure did not bleed down after 2 seconds:




Using injector tester J 39021 with adapter J 34730-225, activate the injector while observing the pressure gauge. It should drop to between 275 and 296 kPa (39-43 psi).

Important: Do not activate the injector more than twice without starting the engine. This is to prevent flooding the engine with fuel which can cause hard starting.

If a single activation of the injector resulted in the indicated gauge pressure falling below 275 kPa (39 psi), replace the injector. If no drop in pressure was observed, refer to the injector coil test in the appropriate service manual. The correct injector coil voltage and resistance range have been revised (Reference bulletin 51-63-02). It is 3.3-4.8 volts and 1.9-2.1 ohm's resistance when measured at temperatures between 50°F and 95°F

If the system passed the above tests, start the engine and observe the pressure gauge. The pressure should be 5-10 psi lower with the engine at idle speed. While observing the pressure gauge, quickly open and close the throttle. The pressure should have approached the key on test pressure of 380-440 kPa (55-64 psi). If it did not, replace the CMFI unit.



To test the injector poppet nozzles, remove the upper intake manifold and Install injector poppet nozzle testers J 34730-230 following the service manual procedure.

Turn the ignition switch on. Energize the injector. This will verify if each poppet nozzle opened by checking for the presence of fuel in each tester. If one or more of the nozzles did not open, a miss will result and the CMFI unit should be replaced.




If the symptom persists, replace the fuel pump only if the amp draw is less than 8 amps.