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Discussion Starter #1
I just brought my 2006 Saab 9.3 Aero with the 2.8 liter 6 cylinder turbo (126,000 miles) into the shop because the check engine light came on and it has been running fairly rough since. The shop who specializes in Saab repair used a Tech 2 diagnostic scan tool and found a P0304 cylinder #4 misfire. Below is the list of diagnostic testing that was performed.

**Remove and replaced coil on cylinder#4 which was in two pieces.
**Moved plug to adjacent cylinder and misfire stayed in cylinder#4
**Number 4 injector is shut off after 30 seconds via engine computer to save converter from damage.
**Completed injector volume test to confirm even fuel displacement on all cylinders.
**Checked for sticking injector using stethoscope and found to be fine.
**Test drove while watching live computer data. MAF, fuel trims, etc...All normal...MAF 7-145 gr/sec
**View valves fully opening and closing using digital baroscope inside cylinders.
**Vacuum test shows low vacuum 10-13" mercury.
**Compression test normal @ 150-160 all cylinders.
**Perform running compression test - slightly low readings on front bank of all 3 cylinders.
**Catalytic converter is not plugged.
**Cam timing is proper.
**Performed a smoke test on intake to check for vacuum leaks.

Mechanics notes:
" I was initially suspecting a faulty ECU but now believe there could be something internal going on but not sure at this point."

Has anyone experienced this issue or know what my next step should be. Did the mechanic miss something or possibly overlook something that could be the culprit. Would an engine tear down be the next step? The engine starts up great and sounds normal and does not make any noises that would indicate something internal would be wrong, i still drive the car and it really only misfires under acceleration and hardly noticeable at idle. Please help!!! Any advice would be greatly appreciated.
 

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It looks as most of the usual suspects were eliminated.

Do they have any insight into the reason the ECU was sending a signal to shut down the #4 injector?
 

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Have you double checked wires between ECU and coil and injector (with multimeter)?
 

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Discussion Starter #4
It looks as most of the usual suspects were eliminated.

Do they have any insight into the reason the ECU was sending a signal to shut down the #4 injector?
I just asked my mechanic and he told me he really didn't know. He also sounded a little confused on the phone when I asked him. I don't think he has much experience with the newer Saab cars, he told me this stuff doesn't happen on the older Saab cars so that's why he doesn't know.
 

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Discussion Starter #5
Have you double checked wires between ECU and coil and injector (with multimeter)?
I have not personally, I would of hoped my mechanic did but now I'm starting to wonder about him. I do have a multi meter and I can definitely check. What should my setting be on the multi meter and what reading should I expect. Thank you!!!
 

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Ohms and when you check wires, it should be near zero.
EDIT: Check also between wire and ground. It should be OL (over limit) at any setting.
 

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Well, the front bank of cylinders is a bit notable.
Im not too familiar with these V6 motors.
What does the coolant look like? Low level? Do a block test with a chemical tester-could be a head gasket.
Also post the freeze frame data for when the code sets.
Other basics are battery connections and battery health and voltage drop in the system too from battery to block /ecu?
Do you have any tuner/hot rod/aftermarket stuff on it?
 

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I imagine that injector gets shut down due to a huge misfire detected by the ecu. Maybe the ecu can no longer switch that coil on and off. I think its called the coil driver. It’s possible the mechanic used a power probe and applied power to a sensitive circuit...but that doesn’t explain the low vac. Do these motors have an EGR valve that could be stuck open? Or a PCV? Has any work been done prior? You know if that coil was crap, it couldve been misfiring long enough to send a bunch of raw fuel into the catalytic converter plugging it up. You can test back pressure with a special vac fitting screwed into the o2sensor bung hole (haha).
 

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you sure those are the only codes? You could probably use a mobile phone Torq tester for current data...tech 2 isn’t always needed.
 

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The only thing I don't see is changing the #4 Spark plug. If it's dead, there's no spark, which I think would be the same as the coil not firing regarding the code. They did the coil, but if the plug is dead, that won't make a difference.
 
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