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Misfire under load by n7vrz
Started on: 10-08-2013 12:48 PM
Replies: 5 (197 views)
Last post by: Silicoan86 on 10-08-2013 09:43 PM
n7vrz
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From: Dixon Springs, TN
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Report this Post10-08-2013 12:48 PM Click Here to See the Profile for n7vrzClick Here to Email n7vrzSend a Private Message to n7vrzEdit/Delete MessageReply w/QuoteDirect Link to This Post
I need some help diagnosing this.
3.4DOHC with a misfire under load that just started recently. The misfire is not there when idling or part throttle. Just when under load.
I am not all that familiar with electronic ignitions so bear with me if you make a suggestion and I don't see where you're going with it.
My assumptions since it idles and no misfire under part throttle:
1. The plugs and wires are okay
2. There is no problem with the valve train
3. Fuel delivery is good
I have a garage I can get the car into but not enough room to drop the cradle. I also don't have a lot of free money to throw at it. I'm retired and on a fixed income. So if you want me to change out the coils/ICM/etc., expect a months wait between items depending on cost. I also don't have test equipment beyond a VOM and a power probe.
So, suggestions?
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Tweeder
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Report this Post10-08-2013 12:59 PM Click Here to See the Profile for TweederSend a Private Message to TweederEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by n7vrz:

I need some help diagnosing this.
3.4DOHC with a misfire under load that just started recently. The misfire is not there when idling or part throttle. Just when under load.
I am not all that familiar with electronic ignitions so bear with me if you make a suggestion and I don't see where you're going with it.
My assumptions since it idles and no misfire under part throttle:
1. The plugs and wires are okay
2. There is no problem with the valve train
3. Fuel delivery is good
I have a garage I can get the car into but not enough room to drop the cradle. I also don't have a lot of free money to throw at it. I'm retired and on a fixed income. So if you want me to change out the coils/ICM/etc., expect a months wait between items depending on cost. I also don't have test equipment beyond a VOM and a power probe.
So, suggestions?


My ls motor did that once, turned out to be a bad coil.

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86 SE Convertible 3800sc 4t65e HD.

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2.5
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Report this Post10-08-2013 05:26 PM Click Here to See the Profile for 2.5Send a Private Message to 2.5Edit/Delete MessageReply w/QuoteDirect Link to This Post
When any car I've had did it, it was wires. Id double check them, they dont have to look bad to be bad.
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Dave E Bouy
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Report this Post10-08-2013 08:18 PM Click Here to See the Profile for Dave E BouyClick Here to Email Dave E BouySend a Private Message to Dave E BouyEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by 2.5:

When any car I've had did it, it was wires. Id double check them, they dont have to look bad to be bad.


x2

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David Hambleton
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Report this Post10-08-2013 09:42 PM Click Here to See the Profile for David HambletonClick Here to Email David HambletonSend a Private Message to David HambletonEdit/Delete MessageReply w/QuoteDirect Link to This Post
Here's a process you could try: Pull one plug wire at a time & test for the miss under load. You may be able to tell which cylinder is missing if it doesn't seem worse when that plug is disconnected. Try a different plug in that cylinder, since plugs can malfunction under load. If that doesn't fix it, try a different wire for the same reason.
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Silicoan86
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Report this Post10-08-2013 09:43 PM Click Here to See the Profile for Silicoan86Click Here to Email Silicoan86Send a Private Message to Silicoan86Edit/Delete MessageReply w/QuoteDirect Link to This Post
I had the same problems last week with my 3.4 DOHC Fiero. Major misfire, but only under load. Turned out that the #2 spark plug wire was corroded from rain water dripping off the back window and getting into the plug wells. I replaced the wire and the car is now running better than it has all summer.

I also made a quick, temporary cover to prevent water from getting back in there. Not pretty, but it'll keep water out till I come up with a more permanent solution.

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