2001 Chevy S-10 No.4 cyl misfire

2001 CHEVROLET S-10
55,000 MILES • 6 CYL • 4WD • AUTOMATIC
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J407
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My ses light will come on for couple days then go off.I have checked with obd2 scanner and it is always no.4 cyl misfire.I have changed plugs,cap,rotar,wires,done comp check and it was all with-in 5 lbs of each other.Changed no 4 wire and plug still pops the light mostly when driving down highway and always about 2000 to 2100 rpms when it throws the check engine light.It runs fine and idles as it should .Ant ideas?

updated 4-30-10
well you answered my question and was told it was probably the spider injection system and I replaced it with an up-graded version and well after doing all of that the no.4 cyl mis fire code still pops up when going down the highway. the code was cleared also after install of the spider injection.I lost the info for my login and password so could you foward this to the right people to see if they have any other answers to this problem.The truck only has 55,000 miles on it

updated 5-1-10
Checked plugs and they look the same (clean and not black or sooted up) The light will go off after driving around city a few days but if you drive it back on hwy for a while at around 2000 rpm the light will pop back on. If the computer was messed up with that inj. wouldnt the light be on all the time?
Apr 1, 2010 at 2:23 PM
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2CARPRO JACK
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Sounds like you have an injector going out.Those CPI style fuel injections systems had big problems with injectors.The updated injector comes as a whole unit (for all 6 clinders) but it eliminates the extra valve that usually causes the problems.
Apr 2, 2010 at 1:13 AM
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2CARPRO JACK
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Sometimes when injectors short out, they can take out the injector driver (trigger) in the computer.Double check that the plug is burning the same as the others(color). It may need to go on a scanner to watch and see what the injector is doing.Also try spraying carb cleaner at teh intake port for that cylinder to see if it has a vacuum leak
Apr 30, 2010 at 6:52 AM
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2CARPRO JACK
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Most likely, yes.maybe a crank angle sensor issue.That is what tells the PCM the timing of the cylinders and when they should be firing.Is there any noticeable difference in the way it runs when the light comes on? you can email me directly at jackson@2carpros.com
May 3, 2010 at 12:41 AM
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J407
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When the light comes on the truck seems to run at say 2500 rpms to run at 70 mph instead of say 2000 or 2100 rpms to go 70mph. I guess what I am trying to say is seems like its reving a little higher to go down the road. But as far as the way it idles and runs it seems to do fine.No miss or sputtering .This really has been a issue as I am pretty meticulous about trying to keep my vehicles in top running cond. Could a scanner tell if the crank angle sensor would be bad as we have one at work if I could talk mechanic into hooking it up after hours.Its a pretty good scanner probably cost $2,0000 oo or so.
Thank you so much for trying to help me figure this issue out.
Thanks again
Jim
May 3, 2010 at 11:57 PM
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2CARPRO JACK
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Having a real scanner on it may help determine what is going on.You can watch the short term/long term fuel trim to see if it is trying to compensate for cylinder 4.Yes they are expensive, but sometimes you need more than just a code reader to fix these new cars.i wouldnt suggest buying one yourself, that would be a lotof trips to the local mechanic.
May 4, 2010 at 12:55 AM
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J407
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Here is the readings off of the obd 2 scanner if that might help when code is set
throtle position is 23.1%
air flow is 37.03 %
map sensor 65kpa
coolant temp 195 f
short term fuel trim 1 -3.4%
long term fuel trim 1 13.2%
short term fuel trim 2 3.1 %
long term fuel trim 2 11.7 %
short term fuel trim 3 -72.9 %
long term fuel trim 3 24.2 %
long term fuel trim 4 97.6 %
short term fuel trim 4 47.6%
It set at 2100 rpm
May 22, 2010 at 1:45 PM
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2CARPRO JACK
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I would swap #4 plug and wire with another to see if the miss will follow it. Sometimes a plug can get cracked and it will cause a misfire.I think the RPM difference may be the"limp" mode the PCM is in when it has a misfire code, causing the trans to stay out of lock up on the converter.
May 23, 2010 at 8:14 AM
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J407
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Already did that and also the wire. Same results light comes on as I said
May 24, 2010 at 2:00 PM
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2CARPRO JACK
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Looking at the short term on #4 makes me wonder If you may have a slight intake leak.Since all parts are new and changed around, it must be something mechanical with the engine since it is trying to compensate #4 so much.Has it ever given any other code than #4 misfire? what code number is it exactly?
May 25, 2010 at 12:58 AM
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J407
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It is P304 and I have sprayed carb cleaner around the intake 3 or 4 times with no change in the engine speed. I switchedthe no 4 plug with a brand new plug and have switched the wire with a differnt wire and still comes up with a no 4 cyl misfire on the obd 2 code reader.The light will go off after driving it on the highway, but you might have to drive it around city driving a day or so .It is really agervating after changing everything and it is still throwing this code.Just want my truck to be right as when I do drive it is usually on long highway trips and dont want it breaking down
May 25, 2010 at 9:40 AM
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2CARPRO JACK
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A misfire code wont cause it to break down, but I understand your frustration, sometimes these things dont make any sense.I was referring to an internal intake leak.I take it that the engine was misfiring before all the tune up parts were installed? Did the updated injector come with a new pressure regulator? Maybe try a cylinder leak down test on #4 to be sure the valves are sealing tight, its a little more accurate to the engine condition than a comp test
May 26, 2010 at 1:07 AM
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