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I have a '96 Seville STS and it's all of a sudden giving a P0300 misfire code. It feels like it's running on 7 cylinders. I've changed plugs and wires. I found 1 wire cut slightly but I still have the same problem. Still runs awful and giving the P0300 code. It seems like the car was missing when the engine was cold but as soon as you drove for 1/4 mile, it ran fine. If it's one of the 4 coils, how can I check them to see which one and if it even IS a coil. I've heard it could also be a vacuum leak or compression since the P0300 is a "Random Misfire" instead of cylinder specific. How possible could that be?
 

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It sounds like a bad coil pack. Some OBDII cars will pinpoint the bad cylinder by setting a code like PO304 for cylinder #4. The '96 does not do this. PO300 is all that you get.

Using a Tech2 or ignition diagnostic tools you can find a bad coil in a couple of minutes. I assume you do not have access to these. You can always just buy 1 coil pack and start swapping them out. Replace the first one...test...if the miss is still there replace the original coil....and swap out the next coil. Repeat until you work thru all the coils.

As for PO300. Here is the factory diagnostic description.

Circuit Description
This diagnostic test is used to detect misfires in any cylinder. A misfire occurs when there is a complete lack of or poor combustion in the cylinder. A misfire will cause a fluctuation in crankshaft revolution speed. The PCM uses the time between 4X reference pulses to determine when these fluctuations occur. At lower engine speeds, fluctuations in crankshaft speed are measured each reference pulse to detect misfire. At higher engine speeds, fluctuations in crankshaft speed are monitored over two engine revolutions and fluctuations in crankshaft speed at each half revolution are compared to compute a balance number. The 4X reference pulses originate in the ignition control module and vary with RPM.

Conditions for Setting the DTC
Test Conditions
DTC(s) P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0340 and P0502 are not current or have not set since last key ON.
Coolant is between -7°C (19°F) and 128°C (262°F).
Engine speed running for at least 3.19 seconds.
Engine speed is less than 6375 rpm.
Traction control is not present.
Cylinder shut down is not present
Ignition voltage is between 9 and 16 volts.
Throttle angle 0.3 degrees or more.
Vehicle speed is less than 45 mph.
During a 12.5 millisecond time period throttle angle stays between 0.6 and 1 degrees.
During a 100 millisecond time period throttle angle stays between 0.96 and 1.28 degrees.
Delta MAP diagnostic not in progress.
Number of consecutive positive delta revolution mode indexes less than 20.
Number of consecutive negative delta revolution mode indexes less than 20.
A/C compressor clutch has not changed states in the last 100 milliseconds.
Slip speed is at least -64 rpm.
18 engine cycles have occurred since any of above conditions and were not met.
Rough road indication is not considered present.
16 engine cycles have occurred since power-up.
16 engine cycles have occurred since both the reference mode and revolution mode were disabled.
8 engine cycles have occurred since a CAM error, or an ignition system error.
Diagnostic tool not connected.
10 engine cycles have occurred since desired torque less than 95.3%.
Failure Condition
A misfire has been detected.

Action Taken When the DTC Sets
The PCM will turn on the SERVICE ENGINE SOON MIL under the following conditions.

Impending catalytic converter damage
If a cylinder misfires at a sufficiently high rate for at least 200 engine revolutions, a catalytic converter damaging misfire level is considered present and the Service Engine Soon MIL will flash until the catalyst damaging condition is no longer present, at which time the MIL will remain on steady.
Disable TCC if either the number of emission misfires or number of catalyst damaging misfires exceeds 2.
Excessive exhaust emissions
If a cylinder misfires 15 times in 200 revolutions for 5 out of 16 200 revolution tests in each of two consecutive trips (engine run cycles which meet specific criteria), an excessive exhaust emissions condition is considered present and the Service Engine Soon MIL will be turned on.
Disable TCC if either the number of emission misfires or number of catalyst damaging misfires exceeds 2.
PCM turns ON the ENGINE MISFIRE message.

The PCM will record operating conditions at the time the DTC sets. This information will be stored in the Freeze Frame and Failure Records.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF after three consecutive drive trips within the same driving parameters that set the DTC and the diagnostic runs and does not fail.
A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
A History DTC will clear after eighty drive cycles within the same driving parameters that set the DTC with no failures of any emission related diagnostic test.
Use a scan tool to clear DTCs.
Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Clearing Diagnostic Trouble Codes in PCM Description and Operation.
Diagnostic Aids
If more than one cylinder showed a history of misfire, look for something in common between the cylinders such as, shared coil, etc. If only one cylinder showed a history of misfire look for a possible vacuum leak at that cylinder, etc.
If fault cannot be duplicated, wet secondary ignition system and operate vehicle under conditions that the DTC set.
It is possible to set DTC P0300 if the vehicle has run out of or is very low on fuel.
To quickly determine the cylinder(s) that are misfiring, use the scan tool and observe Misfiring Cyl. in the Misfire data list. This will display the primary and secondary misfiring cylinders.
Due to the potentially destructive nature that engine misfire has on the exhaust catalyst always check for proper catalytic converter operation after a misfire has been repaired. This can be accomplished in the following ways:
Drive the vehicle under the Test Conditions for DTC P0420 and ensure that the DTC runs and passes.
In IM240 areas (or those properly equipped) perform the Catalytic Converter Service Bay Test.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.

If other DTCs are set, diagnose them first because other problems may be causing the misfire.

Checking if multiple cylinders are misfiring. If there is a single cylinder misfiring, the diagnosis is much easier.

Checking for causes of a single cylinder misfire.


Logan
 

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nas_fan said:
I have a '96 Seville STS and it's all of a sudden giving a P0300 misfire code. It feels like it's running on 7 cylinders. I've changed plugs and wires. I found 1 wire cut slightly but I still have the same problem. Still runs awful and giving the P0300 code. It seems like the car was missing when the engine was cold but as soon as you drove for 1/4 mile, it ran fine. If it's one of the 4 coils, how can I check them to see which one and if it even IS a coil. I've heard it could also be a vacuum leak or compression since the P0300 is a "Random Misfire" instead of cylinder specific. How possible could that be?
Rather than swapping packs until you find the right one, can't you pull plug wires one at a time ? Say you pull the # 1 wire and it runs worse, that's not the problem. Put it back and move to the next wire. When you pull one and there's no change, that's the problem . Just a thought.
Bob
 

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Discussion Starter #5
bob2231 said:
Rather than swapping packs until you find the right one, can't you pull plug wires one at a time ? Say you pull the # 1 wire and it runs worse, that's not the problem. Put it back and move to the next wire. When you pull one and there's no change, that's the problem . Just a thought.
Bob
I thought of that, but it didn't work. There's was no change when I pulled any plugs. Don't know why. I fixed it though. I bought a coil and swapped until I found the bad one. Runs great now. All the plugs were getting spark. I guess just not enough spark coming out of the bad coil.:)
 

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nas_fan said:
I thought of that, but it didn't work. There's was no change when I pulled any plugs. Don't know why. I fixed it though. I bought a coil and swapped until I found the bad one. Runs great now. All the plugs were getting spark. I guess just not enough spark coming out of the bad coil.:)
At least you got it fixed, and fairly cheap too. That's good.
Bob
 

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THIS IS FANTASTIC!!!!!! :bouncy:

I have been trying to figure what to do my 98 which gets the same code every now and then. I posted something about it earlier, but to no avail.

Thank you so much! :worship:
 
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