misfire number 4 on 2001 Impala

Hello All:
I just got back from a weekend round trip of 400 miles of highway driving. I averaged about 80MPH, and the car got 27.3MPG average for two fill ups. Overall, the car is performing great, BUT it occasionally
misses at 3700RPM.
I had to merge that required a WOT, and at 3700RPM the car bucked and the check engine light was on. I drove the car for approx 100 miles, and stopped at an AutoZone to pull the codes. The reader said "Misfire detected number 4". He cleared the codes, and said it is probably a spark plug wire????
I drove another 120 miles with no problem, then did a brief fast acceleration to 3000 RPM, and no problems.
On the way home (approx 200 miles) the car performed well. I slowly crept up the speed, and sure enought at 3700RPM the car bucked, and the Check Engine Light came on (approx 110MPH!).
So, another stop at AutoZone and the same code is read; :Misfire detected number 4".
The plugs were replaced about 19K miles ago with the same AC Delco types.
So, I really do not think it is plug wires. The car gets great gas mileage, idles smoothly, and only misfires at 3700RPM.
The car has 59K miles, and is a 3.8 V6.
Any ideas?
Thanks for any help, and have a nice day,
Bill
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Is this 3700 at 110 mph it misses or 3700 rpms at any speed?

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I have not tried a WOT from a dead stop, but to mee, it seems that it is a 3700RPM issue (I have not gone above 3700RPM). Thanks for any advice, Bill Shep wrote:

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You know you are at about the speed limiter at 110. Thats why I asked if it only happens at that speed.

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Thnaks for the reply, but it is not speed related.
Here is what I did today; WOT from 35MPG to approx 75MPH. The car accelerates FAST until the tach goes into the high 3000RPM range. Then you feel the car hesitate, then the Check Engine Light blinks while the tach dips back and forth between 2000 and 4000RPM. I back off the throttle, and the check engine light stops blinking and stays lit.
I scan the codes (guy at local auto parts store lets me scan them myself), and get "P304". I clear the codes and go back out. Anything BELOW 3700RPM and the car is just fine; as you approach 3500RPM, you can tell power is rolling off, but no jerks, or hesitation is felt. If you stay under 3700RPM, no check engine lights, no codes set.
Everyone is telling me spark plug wires, but I find this hard to believe. They are about $56 from GM, and if I replace them myself, I guess I will try it. Every car I have ever owned went approx 100K miles before their first spark plug wire set were replaced, but maybe this car is just different?
Should I just order a set of wires from GM, or is there any faith in NAPA replacements? Do I have to tip the motor over to replace the wires?
Thanks for any advice.
Shep wrote:

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Short of an accurate diagnosis such as testing the wires with an ohmmeter, I would replace them. Replace them with oe if possible, I would also pull the plug maybe you cracked the insulator on installation.

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If $ is really tight & your labor is really cheap, you might swap #4 plug & wire with another cylinder's & see if it now misfires on the other-than-#4. Luck. sdlomi2
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Likewise, swap the coil with another one. Just don't do both swaps at the same time.
--

-Mike-
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The guy at the auto parts store made a good point; the fault code is ALWAYS plug number four. Therefore, it is probably spark plug number four, or spark plug wire number four. If it were coil packs, it would also fault the plug number that is in series with number four (number 1, I believe).
I guess I was gun shy because I have heard about plugged catalytic converters causing misfire codes, but now I am fairly certain that it is a plug, or wire. Since the plugs have less than 20K miles on them, and the wires are original, I'll invest in a set of GM wires (I think it is part number 12192468). Hopefully I will not have to rock the motor forward to replace these wires!
Thanks for your help. I'll report back next week with my results.
Bill
Mike Marlow wrote:

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A dirty fuel injector will also cause misfire codes. I was getting P038, I think, and good gas mileage. After paying $120 labor at a GM dealer to replace #4 spark plug and spark plug wire!!!! the code and misfire returned. They power flushed the fuel injectors and it fixed the misfire and vehicle ran much better. They explained that the pintles are really small as they are trying for gas mileage and even a small deposit of carbon on the pintle will cause problems.
The second service was free and I got $55 back from the first service after complaining that the labor charge seemed excessive :)
Jim

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I replaced the plug wires on my 2001 Impala with 60K miles. It had a misfire number 4 P304 code only when I was at 3700RPM.
The plug wire for cylinder number 4 must have been damaged when I had the plugs replaced about 9K miles ago. Here is the story; I had a heck of a time getting the plug boots off the plugs. They are like glued on. Anyhow, I took the back three wires off first (they really need to come out 3 at a time due to the routing and wire looming). Then I matched up the wires with the new ones, and routed and loomed them back like the orignal. When I went to put number 4's wire on, I could not seat it? I felt the top of the plug, and I could feel rubber, thin wire, and the spark plug's top connector. So, I think that wire had been damaged (all remaining 5 wires came off properly).
In any case, I just test drove the car, with WOTs from a dead stop to 85MPH on the back roads: NO Misfire, NO Check Engine Lights, just smooth power all the way up to 5000RPM shift points!
It is not too bad of a job, and you probably could change the back plugs without rotating the motor?
Thank you all for your help.
Bill George Jetson wrote:

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Had the same problem the MAF sensor was dirty. Clean the MAF sensor.
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