This car has been a nightmare... The latest issue is the SES light is flashing periodically and the engine runs very ruff at these times. It used to chug going up hills unless gas really pushed down, but when the ses light is on it chugs at all speeds.

My mechanic (who is a great mechanic, just not cheap..lol) ran diagnostic and it showed cylinder 3 had misfired over 300,000 times. The rest of the diagnostics led him to change the injector to cyl. 3, which worked for a couple of days and now it is back peiodically.

Any thoughts? Here is some of the history on this car:
This is the 3rd engine in this car at 133,000 miles as one blew when I was visiting my parents and my brother'* mech. put in a junkyard engine with 55,000 miles which blew about 3 mos. later on Christmas Day. It was under warranty for 3 months and a junkyard engine with 28,000 miles was put in about a year ago. The first engine may have been caused by ?transmission fluid in the oil, it dev. a knock after a fast lube place changed the oil. Mechanic changed oil and said it was definately not oil in there. 1st junkyard engine gave out on highway with no notice, still had oil pressure and would crank but shut down as soon as engine was warm. The mech. Claimed I over revved it, but that was not the case.

Any help please????? Is it time to junk it?
Thanks,
Elizabeth

The flashing light is a misfire. Check the plug on #3, and check the plug wires, change if needed. Also test the coils.

If you come up empty handed after Dan'* advice read the following info. It may help

Document ID# 432115
2000 Pontiac Bonneville


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DTC P0300 Engine Misfire Detected

Circuit Description
The powertrain control module (PCM) has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the ignition control module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring. If 2 percent or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 11 or more misfires are detected during any 10 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible 3-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Running the DTC
None of the following DTCs are set:
Vehicle speed sensor (VSS)
Throttle position (TP) sensor
Engine coolant temperature (ECT) sensor
Crankshaft position (CKP) sensor
Camshaft position (CMP) sensor
Manifold absolute pressure (MAP) sensor
Mass air flow (MAF) sensor
The engine speed is between 550 and 5800 RPM.
The system voltage is between 9 and 18 volts.
The ECT indicates an engine temperature between -6°C (20°F) and 120°C (248°F).

Conditions for Setting the DTC
The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause 3-way catalytic converter damage or emissions levels to exceed mandated standards.

Action Taken When the DTC Sets
The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed unless three way converter damage is possible in which case the light will illuminate after the first failure.
If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.
Conditions for Clearing the MIL/DTC
The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
The DTC can be cleared by using a scan tool.
Diagnostic Aids
The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as DTC status Failed Since Code Clear, the misfire history counters (Misfire History Cyl #1-#6) will still contain a value that represents the level of misfire detected on each cylinder. The scan tool displayed misfire counter values (Misfire History Cyl. #1 through #6) can be useful in determining whether the misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil - 1/4, 2/5, 3/6), or is random. If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:

A misfire DTC may set if components that affect the crankshaft position sensor have been recently replaced, and the CKP System Variation Learn Procedure has not been performed. If the diagnostic table does not identify a problem then perform the CKP System Variation Learn Procedure .

If the misfire is random, check for the following conditions:

Crankshaft Position System Variation Refer to CKP System Variation Learn Procedure . The crankshaft position system variation compensating values are stored in the PCM non-volatile memory after a learn procedure has been performed. If the actual crankshaft position variation does not match the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set. The crankshaft position system variation learn procedure is required when any of the following service procedures have been performed:
The PCM has been replaced.
DTC P1336 has set.
The engine has been replaced.
The crankshaft has been replaced.
The harmonic balancer has been replaced.
The crankshaft position sensor has been replaced.
Any engine repairs which disturb the crankshaft/harmonic balancer to crankshaft position sensor relationship.
Check the secondary wires associated with the affected cylinder pair for disconnected ignition wires or for excessive resistance. If any spark plug wire resistance is more than the specified value, replace the affected spark plug wires. VIN 1 Spark Plug Wires Resistance
3280-4921 ohms per meter (1000-1500 ohms per foot)

VIN K Spark Plug Wires Resistance
3280-4921 ohms per meter (1000-1500 ohms per foot)

A damaged or malfunctioning ignition coil. Check for cracks, carbon tracking, or other damage. Also check the coil secondary resistance. The secondary resistance should be within the specified range. Ignition Coils Secondary Resistance
5000-8000 ohms.

Switch ignition coils and retest. If the misfire follows the coil, replace the ignition coil.
Check the system grounds. Ensure all the connections are clean and properly tightened.
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. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor.
If the PCM stops receiving data from the EBCM/EBTCM, DTC P0300 can set due to a loss of rough road data. Check for stored ABS/TCS DTCs, especially DTCs related to a serial data malfunction. Refer to Diagnostic System Check - ABS in Antilock Brake System .
Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, an incorrectly installed or malfunctioning crankcase ventilation valve, or for vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces.
Perform a fuel system pressure test. A malfunctioning fuel pump, plugged fuel filter, or malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Pressure Test (VIN K) or to Fuel System Pressure Test (VIN 1)
Check the fuel injectors. Refer to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Between 10-35 Degrees C (50-95 Degrees F) or to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Outside 10-35 Degrees C (50-95 Degrees F)
Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Check the EGR system for a leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.
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 be set, review the freeze frame and fail records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures less than 80°C (176°F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 gm/*.

Important:
If the level of misfire was sufficient to cause possible catalyst damage (if the MIL was flashing), ensure that the DTC P0420 test is completed and passed after verifying the misfire repair.


Reviewing the Failure 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.

do you have an alarm system>? If so GM bulletin states improperly installed alarm system may cause Misfire. If you have an alarm I can post you bulletin to read.

Hey, thanks for the replies. When he ran the diagnostics, the codes (I do not remember what he told me) led him to believe it was not the plug or the plug wires, but in the injector (which was replaced). I only have the factory alarm on the car, is this what you are talking about or an add on alarm?

I will give my mechanic all this info. as he says he "loves a challenge" lol. It has been running fine, with no misfires since Saturday.....

aftermarket installations. I have copied bulletin for any one interested

2000 Pontiac Bonneville


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Info - Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems #02-06-05-004A - (Jan 9, 2004)
Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems
2004 and Prior GM Passenger Cars and Light Duty Trucks

2003-2004 HUMMER H2

2004 and Prior Isuzu Light Duty Trucks

This bulletin is being revised to add the 2004 model year and include specific DTCs in the title line. Please discard Corporate Bulletin Number 02-06-05-004 (Section 06 - Engine/Propulsion System).

General Motors Engineering, in an effort to determine the root cause of catalytic converter damage, has determined that aftermarket alarm systems incorrectly installed in vehicles have the potential to cause misfire codes and damage to the converter. These alarm systems use a circuit interrupt which utilizes the ignition circuit on the vehicles.

These alarm systems utilize mechanical relays and normal vehicle movement can trigger these relays to engage and disengage the ignition circuit while the vehicle is in motion. These disruptions of the ignition circuit, which occur in milliseconds, may cause more fuel to be commanded. Over time, this dumping of fuel on and off again can cause misfire codes and ultimately damage the converter assembly.


Important
Engineering could not identify any alarms that utilize solid state circuitry that would eliminate this concern. Because of this, it has been determined that all alarm systems must be routed through the starter circuit in order to avoid this condition.


Dealers must be aware of this issue and take note of the wiring on vehicles with alarm systems that come in for repair, particularly for catalytic converter damage that seem to have no known root cause.



GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these 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 your GM dealer for information on whether your vehicle may benefit from the information.
WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION


© Copyright General Motors Corporation. All Rights Reserved.

Check for corrosion under the plug wire where it connects to the coil. Super easy to do and a visual will identify a problem if corrosion is the problem. I has the exact problem except it was a misfire on #5 and ended up simply being rust/ corrossion on the connection between the coil and the plug wire.

I would suggest checking the injector plug for corrosion. Also test the plug for proper signal. Odd that two injectors would fail on the same cylinder.