P0300 Random Misfire advice?

[Jason]

Car – 2004 Saab 9-3 Aero Convertible 2.0T
– roughly 181K miles and oil change about 5-6K miles ago

Fault Code – p0300 Random cylinder misifre

Symptoms – heavy misfire/spudder under load (WOT)

What i’ve done – Initially, I purchased one ignition coil and swapped it through the cylinders until the symptoms went away. About a week later it came back, and rather than fooling around with one coil again, I purchased another three coils and replaced them all. This time, the symptoms went away completely. Reset the CEL and all was good for a few weeks until it came back after about 2.5k (ish) miles. This time around, I replaced the spark plugs, which were all fouled at the time. Reset the CEL again, but it came back after about 20 minutes of driving. Went back online to find the next diagnosis checkpoint, which was suggested to check for intake leaks. I bought some carb cleaner and proceeded to give my best attempt at listening for idle changes when spraying, but I couldn’t really hear any obvious changes. The car idles a little rough (between 700-780 RPM) and It wasn’t clear if the carb cleaner was making a difference anywhere. I’ve since replaced the upstream o2 sensor as well; thinking that a faulty sensor might be making the ECU provide the incorrect fuel ratio which might cause a misfire. CEL is back again.

What i’m looking for – any advice on what to try next or anything that i’m missing. I’m fairly new to working on cars and i’m pretty much teaching myself how to do things as I go (learning from internet videos and online repair manuals). If there’s anything i’m missing or doing wrong please let me know. thanks

Topic

[Don]

I would recommend checking your fuel pressure. If you don’t have a fuel pressure gauge you can rent one at a lot of the bigger auto parts stores. Also it’s possible the crankshaft sensor is starting to fail but you would likely get more than just a misfire code.

[Jason]

[quote=”machinist85″ post=164725]I would recommend checking your fuel pressure. If you don’t have a fuel pressure gauge you can rent one at a lot of the bigger auto parts stores. Also it’s possible the crankshaft sensor is starting to fail but you would likely get more than just a misfire code.[/quote]

At some point i did get a crankshaft position a sensor code, but it went away shortly after. Maybe i should look into that?

[Jim Davis]

Im with machinist85 on this, simple first, check the fuel pressure and see what you have. I think it should be about 40 psi, but check with a service manual.

[Author]

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