I have a 2001 Olds Alero 3.4l V6 with approx 280k on it and its misfiring. I had to have the harmonic balancer replaced in the vehicle due a very loud squeaking noise. After I had the harmonic balancer replaced the engine has started misfiring, the misfires are only happening when between either 2300 - 3k rpm, light accleration, cruise control in use when driving. So far the following work has been done on the car to try and resolve the misfire. The information below is from the mechanic

1) Replaced both coil packs
2) Spark plug wires checked and ok
3) Checked and replaced fuel filter
4) Throttlebody service
5) Fuel Injectors cleaned
6) Possible Bad Gas..- Gassed up @ Shell (US), and Petro Canada. ( both top tier fuel ranking)

So now without stepping on my mechanics toes I would love to know if any of the following could contribute to this problem

1) Timing is off
2) Bad Spark plugs
3) Bad spark plug wires
4) PCM or someother sensor
5) O2 Sensor


any help is appreciated.

 

It's hard to know without knowing more service history, but if the plugs and wires haven't been replaced in the last 5 years or 100K kms, replace them. The most common cause of misfire is bad plugs and/or wires.
By the way, the PCM is not a sensor. It is the Powertrain Control Module, the "brain" that is responsible for engine operation.
Is the misfire bad enough to cause the check engine light to flash while it is misfiring? If so, there will be a stored code that could help pinpoint a cause.

 

The mechanic has had more time to investigate and we have verified its not the following

Bad Spark plugs - replaced spark plugs
Bad spark plug wires - salt water test

O2 Sensor - Is going to check but does not believe...

The news now is that it is a random misfire with no codes. Its not enough to trigger a code some how... When you remove the EGR the problems go away. When you reinstall the EGR the problem again rearises.

 

Then it seems like the EGR would be the culprit...let us know if a new EGR fixes it.

 

EGR and O2 sensor replaced.