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Saab P0304....I'm stuck, please help! #5234475 10/09/19 05:29 AM
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Hoofhearted Offline OP
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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.


2017 Ford Transit 250
2008 Saab 9.3 2.0T
2006 Saab 9.3 Aero
2005 Saab 9.5 Arc
2005 Saturn Vue 3.5
2004 Saab 9.3 Aero "BCB Stage 2"
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234486 10/09/19 05:57 AM
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Nukeman7 Offline
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I would suspect a bad connection in the wiring harness circuit supplying the current or trigger signal to coil #4. I had a similar problem on a Honda and it turned out to be a bad quick disconnect plug going into the coil from the harness. I spliced in a new QD plug and the problem was resolved.

Last edited by Nukeman7; 10/09/19 05:57 AM.
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234488 10/09/19 05:57 AM
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ET16 Offline
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What's the brand of the coil? Saabs are picky with coils. Must be OEM.

Re: Saab P0304....I'm stuck, please help! [Re: Nukeman7] #5234493 10/09/19 06:06 AM
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Hoofhearted Offline OP
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How would i be able to tell if the wiring harness is bad?


2017 Ford Transit 250
2008 Saab 9.3 2.0T
2006 Saab 9.3 Aero
2005 Saab 9.5 Arc
2005 Saturn Vue 3.5
2004 Saab 9.3 Aero "BCB Stage 2"
Re: Saab P0304....I'm stuck, please help! [Re: ET16] #5234494 10/09/19 06:07 AM
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Hoofhearted Offline OP
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Looks to be a "Genuine GM Coil part# 12629037


2017 Ford Transit 250
2008 Saab 9.3 2.0T
2006 Saab 9.3 Aero
2005 Saab 9.5 Arc
2005 Saturn Vue 3.5
2004 Saab 9.3 Aero "BCB Stage 2"
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234556 10/09/19 07:21 AM
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meep Offline
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Mechanic is being pretty thorough. Out of curiosity, are you running highest octane fuel? Other thought I have is a slightly chipped/cracked valve or other cause for a predetonation hotspot. If a higher octane fuel reduces the symptom, that'd be a theory. Otherwise, if the coil isn't being driven properly, one can see why he'd be thinking ECU. If he could put an oscilloscope on the coil feed while driving it, he might be able to tell.... driving down the road with wires out the hood into the window... probably needs a buddy?


2018 F150
2015 crv (wifey!)
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234572 10/09/19 07:37 AM
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atikovi Offline
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Need to find a mechanic that has and knows how to use a Pico scope type tool to monitor the electrical systems. Cam and crank sensors, coil and injector drivers, etc.

Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234585 10/09/19 07:52 AM
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Trav Offline
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Try NGK LFR7AIX and gap them at 0.8mm instead of the OE 1.0mm, look at the coil connector carefully and test its wire for shorting either a dead short or intermittent to ground (common with these), move the wire around while testing. Check the connections, terminals and wiring carefully at the ecm. IIRC this one does not use an ionization module so the ignition coils are wired directly to the ECM.
I just finished the big 3 wiring upgrade and eliminated the main battery switch on one of mine yesterday (also an 06 but 2.0T) after being plagued by all sorts of weird electrical gremlins, this should fix them once and for all.


ASE L1, Master. Deutsch Meisterbrief.
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234602 10/09/19 08:06 AM
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mk378 Offline
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I guess it is difficult to remove the injectors because the usual procedure would be to swap the injector over to another cylinder and see if the misfire follows. Does a cylinder drop test at idle show running on all 6 cylinders?

Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234716 10/09/19 09:47 AM
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What fires the coils? On GM cars, there's an Ignition Control Module (ICM) that fires the coils. That ICM is usually the culprit for not firing one of the coils --- or one of the coil packs if it's a waste spark ignition.

Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234720 10/09/19 09:49 AM
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Farnsworth Offline
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Looks like your data spotted it, low vacuum. Codes are misunderstood. First step would have been to check for vacuum leaks, which is a bit of physical work compared to reading electronic screens. Since firing was checked, it is most likely an intake leak, even like a brake booster or it may be a cracked hose. The low vacuum reading has to be fixed first. Notice the tech notes in second article starts with checking for vacuum leaks.

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/

https://www.engine-codes.com/p0304.html 2cents

Re: Saab P0304....I'm stuck, please help! [Re: Farnsworth] #5234739 10/09/19 10:10 AM
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Trav Offline
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Forced induction engines and diesels produce low vacuum at anything above idle, this engine IIRC uses an electric vacuum pump the 4 cyl uses a mechanical one driven off a cam shaft on the rear of the head, the other cam drives the PS pump.
Engine vacuum is not a great diagnostic tool on these engines but it should be a little higher I would guess at idle as you say,


ASE L1, Master. Deutsch Meisterbrief.
Re: Saab P0304....I'm stuck, please help! [Re: Hoofhearted] #5234756 10/09/19 10:18 AM
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Mech needs a lab scope. Most any misfire (especially problem ones) should start with relative compression, then if it passes that hook up probes to the injector and coil to see what it actually going on in real time.


2005 Lincoln Aviator 4.6 DOHC
2000 Honda Accord 2.3
2001 Hyundai Elantra 2.0
1979 Ford F-150 351M
Re: Saab P0304....I'm stuck, please help! [Re: meep] #5234757 10/09/19 10:19 AM
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Hoofhearted Offline OP
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Ok, Thank you for this information. I will find a mechanic who has this


2017 Ford Transit 250
2008 Saab 9.3 2.0T
2006 Saab 9.3 Aero
2005 Saab 9.5 Arc
2005 Saturn Vue 3.5
2004 Saab 9.3 Aero "BCB Stage 2"
Re: Saab P0304....I'm stuck, please help! [Re: mk378] #5234760 10/09/19 10:21 AM
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Hoofhearted Offline OP
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I don't know if he did a drop down test on all 6 cylinders as I didn't see this performed on the invoice but I will ask. Thank you.


2017 Ford Transit 250
2008 Saab 9.3 2.0T
2006 Saab 9.3 Aero
2005 Saab 9.5 Arc
2005 Saturn Vue 3.5
2004 Saab 9.3 Aero "BCB Stage 2"
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