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2006 Saab 9-3 Aero Misfiring

I am having a problem with my 2006 Saab 9-3 Aero Combi Wagon w/130k miles
When idling, driving up hills, and accelerating to pass, the engine seems to miss multiple cylinders.
I’ve had all spark plugs replaced within the last year, and all the cylinder coils were checked after bringing the car in for this problem (one was replaced).
I took the car in yesterday to address this problem and the mechanic stated that he didn’t know what to do.
The check engine light is on, and the mechanic told me that the readout states that their might have been a problem with the gas cap, but they checked it out and it looked sound.
Any ideas? Could this be a fuel pressure problem? spark plug wires?

You need another mechanic.

First of all, it is a Saab and it is out of production because the company was shut down. There are resources for Saab service, however. See the link;

The cars are quite picky about the spark plug used and if it is not the recommended manufacturer, which is NGK platinum tipped plugs for your Aero, replace them with the correct plug. If the only code read by the mechanic is “loose gas cap” and tightening had no effect, why wasn’t it replaced? You don’t have plug wires so there is no need to replace them. The car has a “coil on plug” either individually or a cassette that contains all 4 coils. You must have the former if only one coil was replaced. How did he know which coil to replace? It might be another coil.

You need to find a better mechanic.

So the first mechanic replaced one of the coils because he said it was firing improperly, causing carbon buildup where it shouldn’t have been. That was the first mechanic. The second was the one who did nothing.
Would a loose gas cap cause a misfire?
I’ll ask about the plugs

I’m assuming this is the code your mechanic read:

If the gas cap isn’t the problem, then you have a major leak in your evaporative emissions system. Have the system smoke tested to find the leak, then have it fixed. It could be related to your misfire.

The mechanic should have run a compression check while the spark plugs were out just to make reasonably sure there are no serious mechanical issues with the engine.

What is unknown is what this carbon buildup on one of the spark plugs was. That could be minor or could be major but it’s water under the bridge now.

There’s not enough information known to be even halfway definitive about the cause of this problem but yes it could be fuel pressure related.

It should be kept in mind that for some reason many mechanics are scared of SAABs and apparently so because they’re a bit of a quirky car. That could be the reason why they’re kind of glossing things over and passing the buck instead of admitting any fear of delving into what they feel is a very mysterious unknown.

Assuming compression is good I would recommend a thorough scan by a Euro garage in your area along with a fuel pressure test and possibly a vacuum test.
A vacuum test will reveal if there’s an engine compression problem but it won’t show you which cylinder, or plural, is the problem if a problem actually exists in that area.