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Old 09-16-2007, 06:39 PM
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4.3 miss

My 4.3 in a 1989 Chev 1/2 ton has a horrible miss with backfire at anything over mid throttle. This is not constant. It happens when cold and when hot. I replaced fuel filter,cap ,rotor ,leads, plugs. O2 sensor. Is the coil a likely suspect and if not ,what next. When it is running correctly it is smooth. When it is acting up I can give it full throttle and it is hitting about 10% I don,t do this because of the insane backfireing. I let off the gas and it is fine at part throttle. The plugs are a good color and all the same from hole to hole. I am running out of ideas.
Thanks , Clint
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Old 09-17-2007, 02:09 PM
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Is the check engine light on? If it is you can pull the code out and that may help point the way.

Usually they just die, but I have seen a bad ignition module cause misfires and general bad behavior. At the end of their life they overheat and turn erratic. Upon replacement, be sure to clean where the old one was and be sure to put the dialectic grease under it or you will be replacing it again soon, as it will overheat and fail fairly quickly. If it is a high millage truck, it may not be a bad idea to change the coil too since you already threw all those new ignition parts on it.
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Old 09-17-2007, 10:15 PM
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4.3 miss

Well I put a new coil in it and when I fired her up it was missing at throttle and it backfired even louder yet it runs well at half throttle or lower.
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Old 09-17-2007, 11:13 PM
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You said you changed the fuel filter but did you check the fuel pressure? This very well may be an electrical problem but it could also be a fuel problem.
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