03 bonneville ssei multiple random misfire - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-02-2015, 08:15 PM   #1
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Default 03 bonneville ssei multiple random misfire

I have an 03 Bonneville that is showing a p0300 random misfire. I've replaced the plugs and wires and ohmed the coil packs and they check out ok. Im still getting the random misfire and it runs really rough and will eventually stall. What else can I look at to try to fix this problem? Also when I start the car when it'* cold it runs fine for a while but when it warms up the engine runs really rough and then eventually stalls. Thanks for any help anyone can give.
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Old 04-02-2015, 09:19 PM   #2
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Sounds like I might have a plugged cat.
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Old 04-02-2015, 09:36 PM   #3
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Sounds like I might have a plugged cat.
Thanks. I'll check that.
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Old 04-03-2015, 06:09 AM   #4
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Or low fuel pressure.
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Old 04-03-2015, 08:18 AM   #5
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Or low fuel pressure.
Fuel pressure is good. Right around 50 psi when running. 45 psi with key on not running.
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Old 04-03-2015, 02:24 PM   #6
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When you changed the plugs, were any of the old ones wet or fouled? I assume wires are going to the correct plugs?

Ohming the coil packs, only comes into play if the reading is off......need to check coil output with a spark tester......

If it checks out, on all coil terminals, then since pressure seems ok, it could be an injector problem.....also try disconnecting the MAF, and see if it runs better....

Any loss of coolant?

Otherwise, you could have a back pressure problem.....
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Old 04-03-2015, 02:33 PM   #7
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Originally Posted by Tech II View Post
When you changed the plugs, were any of the old ones wet or fouled? I assume wires are going to the correct plugs?

Ohming the coil packs, only comes into play if the reading is off......need to check coil output with a spark tester......

If it checks out, on all coil terminals, then since pressure seems ok, it could be an injector problem.....also try disconnecting the MAF, and see if it runs better....

Any loss of coolant?

Otherwise, you could have a back pressure problem.....
The old plugs were not fouled or wet. They actually looked like they were just put in not too long ago. I put new ones in anyway. Wires are going to the correct cylinders. If I disconnect the MAF, what does that tell me? Injectors? If that doesn't work I'll disconnect the cat and see if that is plugged. Thanks.
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Old 04-03-2015, 03:56 PM   #8
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The old plugs were not fouled or wet. They actually looked like they were just put in not too long ago. I put new ones in anyway. Wires are going to the correct cylinders. If I disconnect the MAF, what does that tell me? Injectors? If that doesn't work I'll disconnect the cat and see if that is plugged. Thanks.
Nope not the cat converter. Unbolted it and engine still stalled. Runs really good when cold but when it gets warmed up it stops. Checked contacts on icm they look good. Going to get a spark tester and I'll let you know afterwards. And it doesnt appear to be using coolant. Thanks.
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Old 04-03-2015, 07:36 PM   #9
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I'm wondering if the IAC isn't closing. Engine runs fine when cold because the PCM is commanding the IAC to open, but once the engine is warm, the PCM wants to close it but its not closing, thus causing a "vacuum" leak and P0300 and stalling conditions.
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Old 04-03-2015, 07:54 PM   #10
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Ok here'* what I did. I checked the coils with a spark tester and found that the coil on cylinders 3 & 6 are intermittent spark. I swapped the 3 & 6 coil for 5 & 2 coil and of course switched the wires as well and found that 3 & 6 are still intermittent spark. That tells me the problem is the ICM or before that? Does that sound right? Thanks for all your responses.
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