98 silverado p0300 miltiple misfire replaced distributor,plugs,wires,egr,fuel pump and fuel filter,o2 sensors,cleaned mass air flow, replaced injector spider assy, im stumped on this

Check engine light on ? Code's ? Scan data from a scan tool , MAF - mass air flow sensor , map , fuel trims , O2 sensor etc....

Multiple misfires can be caused by a few different things , intake manifold vacuum leaks , low fuel pressure , basic things like plug wires an spark plugs needing to be changed if hasn't been done already . How many miles on the vehicle ? If over 100,000 K timing chain an gear wear can also cause this . The powertrain control module (PCM) uses information from the ignition control (IC) module and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring the variations in the crankshaft rotation speed for each cylinder the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause the three-way catalytic converter to overheat under certain conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for 3-way catalytic converter overheating are present.
If the misfire is random, check for the following conditions:
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System grounds

Ensure all connections are clean and properly tightened.

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Mass air flow sensor

A mass air flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition. Try operating the vehicle within the Failure Records conditions with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the Mass Airflow Sensor Replacement .

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Damaged accessory drive belt or driven accessory

A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.

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Air induction system

Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, exhaust gas recirculation (EGR) valve, and intake manifold mounting surfaces.

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Fuel pressure

Perform a fuel system pressure test. A malfunctioning fuel pump, a plugged filter, or a malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis .

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Fuel injectors

Refer to Fuel Injector Solenoid Coil Test .

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Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire, refer to Alcohol/Contaminants-in-Fuel Diagnosis .

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EGR system

Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

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Extended idle

Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to set, review the Freeze Frame/Failure Records for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80?°C (176?°F), the condition described above is very likely the cause of the DTC to set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s.

P0300 Random misfire - lean condition , vacuum leak , low fuel pressure , fuel filter plugged partially , mass air flow sensor .
If the misfire is random, check for the following conditions:
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System Grounds: Ensure all connections are clean and properly tightened.

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MAF: A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.

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Air Induction System: Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty crankcase ventilation valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.

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Fuel Pressure: Perform a fuel system pressure test. A faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.

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Injector(s): Perform injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O rings.

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EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

have you cleared the codes between fault reads
clean the maf first .....replace the maf if faulty
coil packs ?
injectors

run a compression test to check for loose valve seat insert . no 1 cylinder
other wise it is a fuel pressure problem and faulty injector/s

Clean mass air flow sensor with a nonchlronated brake cleaner,install new air filter and check all vacuum lines for leaks then check for vacuum leak around or at intake manifold.Code P0140 is for mass air flow sensor code P0300 is a multiple misfire which more than likely is a vacuum leak possibly intake manifold gaskets leaking.

MULTIPLE MISFIRE CAN BE CAUSED BY.CLOGGED FUEL INJECTORS.ADD A CAN OF SEAFOAM TO A 3/4 TO FULL TANK OF GAS.CHANGE FUEL FILTER, IF ALL READY CHANGED SPARK PLUGS AND WIRES.A WEAK FUEL PUMP AND A CONTAMINATED MASS AIR FLOW SENSOR WILL CAUSE LEAN MISFIRE CONDITIONS.

Hi.

P0300 is random misfire, and can be caused by a number of problem. First check if there are more codes stored in the PCM.

If you have only P0300, check spark plugs, distributor (depending on year distribution coils) and high voltage wires from distributor to spark plugs. Those are the most common cause for random misfire.

If distribution is OK, check for ignition problems. A too rich or too lean mixture can result in misfire. Often the problem is not reported by the PCM with a lean or rich code based on the O2 sensors' input.
Check fuel system, injectors, filter and sensors in air intake and air intake itself, vacuum system etc.

Check with particular attention the EGR system, that is a common fault resulting in random misfire. Chrysler Sebring built in the second half of the 90s are commonly suffering from EGR problems (there are several Chrysler service bulletins on this).

P0300 is often accompanied by symptoms such as engine running rough or shutting down. There are more faults, apart the ones listed that can result in P0300.

Regards.

Ginko.

Most likely you do not have bad O2 sensor or coil sience you have P0300 code.
Check for vacuum leaks first , this is very common problem(vacuum leak) , lean mixture will cause misfire. I'm tallking about V6 engine , 4cyl have same problems but for misfire it may have bad coil (they under recall now).
Also check condition of air mass sensor , if it's dirty ,
buy air flow sensor cleaner and clean it according to user manual on the can.(air mass sensor can cause
O2 sensor code)
Check vacuum hose to fuel pressure regulator.
There are many more things to check , but that for mechanic shops after you try this.

Have the drive gear on the distributor checked, these are known to wear and cause a timing issue.