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Old 02-27-2007, 02:41 PM   #1
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Default Misfire at Low RPM

Hey everybody, just a note on how helpful this forum has been. I don't post because I'm still learning about my new Bonnie (drove a '91 for years), but I've troubleshot most of my problems from threads here and experimentation on my own.

This weekend I started getting a slight misfire at low rpm as I was driving around town which got worse until I it threw a code... P0304 (#4 cylinder misfire). I recently changed the plugs, but hadn't gotten to the wires yet so this was the obvious place to start.

Wires are installed and aren't contacting the 02 sensor or any other components, and the plugs look ok but they are Bosch which I hadn't heard anything bad about until this forum. (Like I said, I'm still learning about her.)

It is still misfiring slightly at Low RPM (under load and at higher RPM she runs like a dream and you can't even tell if there is a misfire or not.)

My next action will be to swap out the spark plugs and reapply some dialectric jelly to the connection. Meanwhile I'll check the coils and probably replace the ignition switch.

I'm going out of town for the next couple of days and won't be able to get to her until Thursday, so if I can hear some expert'* opinions on the direction I'm headed and if there is anything fishy about what I've said I'd appreciate it.

I need to update my signature block, I now have 62000 on her. (Road trips!)
I'll post a pic after I clean her up.
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Old 02-27-2007, 03:16 PM   #2
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You are correct, Bosch plugs have caused similar problems in other members cars here.

I would start buy installing some different (even cheaper) plugs in the car to see if it improved.

The next thing to look at is wires, I did not fully understand your post and whether or not you did swap out wires already. If its not the plugs or wires we can look at other possibilities (coils, etc) but lets start there.
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Old 02-27-2007, 03:28 PM   #3
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Wires are new as of Saturday.

I'll start with the plugs and see what happens.

Thanks.
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Old 02-27-2007, 03:29 PM   #4
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Coil pack testing procedure is in Techinfo. Please also carefully inspect your coil posts for rust/corrosion.
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Old 02-27-2007, 03:50 PM   #5
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Got the article.

The posts are clean.
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Old 02-27-2007, 04:15 PM   #6
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Slim, what is the rationale behind the decision to change out the ignition switch?
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Old 02-27-2007, 04:21 PM   #7
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Quote:
Originally Posted by lugnut
Slim, what is the rationale behind the decision to change out the ignition switch?
VERY good question... i didnt catch this
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Old 02-27-2007, 05:52 PM   #8
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I agree, it is a VERY good question and one that I'm glad you caught.

I honestly have no rationale of my own, but I heard from a buddy that an ignition switch was causing a misfire in his car a while ago. (Not a 100% reliable source, and it doesn't make much sense to me, but if a change of plugs doesn't work and the coils check out I'm willing to try anything. I'm a mechanical guy and I've heard of/dealt with stranger electrical phenomenom that have given me major headaches.)

Judging from the reaction it'* something that I shouldn't have to worry about...

One other thing that I failed to ask in the original post. I'm wondering if it could be a ground issue? I've checked the ground from the negative cable to the frame and it seems to be in good condition and making good contact, but I don't have a schematic and don't know if there is anywhere else that I need to check. (Don't think that it would be related to the techinfo article on ground BUS connectors.)

Thanks again, this forum is a lifesaver.
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Old 02-27-2007, 06:53 PM   #9
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You wouldn't be the first to crack a plug during installation. Take a close look at #4.
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Old 02-27-2007, 09:02 PM   #10
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Did you just inspect the grounding strap to the frame from the battery, or did you remove it? The rust and other problems will be on the threads of the bolt, and in the body. You will not be able to see them by just looking at the bolt from above.
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