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missing, weak spark

  #1  
Old 11-24-2007, 04:31 PM
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Default missing, weak spark

I have a 03 t/c 3.8. It just started missing one day.Things i have done. changed out plugs, wires, coil pack. Im getting spark but its weak on the 3 and 6. I thought it was a bad coil in the pack so i replaced the pack with new. But im getting the same isue. Is it possible that somthing els is giving that coil in the coil pack less voltage? What could cause just that coil to put out weak spark?
 
  #2  
Old 11-25-2007, 08:18 AM
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Default RE: missing, weak spark

How do you know it's weak spark? Seems you have your bases covered there.

Next thing is to check compression, leakdown, etc. Might not be a spark issue at all.

Might be a cam or crank sensor issue as well. Takes a 'scope to see if the signals are OK. Sometimes less costly to replace both sensors...but it's just a guess.

 
  #3  
Old 11-25-2007, 10:45 AM
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Default RE: missing, weak spark

i know its weak spark as the old plugs i pulled out are badly worn. And the others are fine. Also if i pull the boot off the plug while running you can see the diffrence. Could a cam or crank sensor cause just that coil in the coil pack to do that?
 
  #4  
Old 11-25-2007, 11:29 AM
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Default RE: missing, weak spark

Well, if the suspect plugs are worn you could have TOO MUCH spark, eroding the electrodes. Are you using Double Platinums (as OE) or Iridium plugs? Did the missfire go away when the plugs were new & gapped properly?


I have seen the two sensors cause poor running because their wave-forms (and hence the timing) affect the spark. Occasionally there would not be enough of a signal for the ECU to send a spark signal to the coils.

And the 3 & 6 plugs are the ones with the shortest wires, IIRC? Have you tried measuring the resistance of the new plug wires? Usually there is a range of resistance with the wires.

Do these suspect cylinders fire off the same coils? Perhaps there is a grounding issue.

Another way to see if its the wires would be to hook up the old wires from other cylinders (if you have them) and see if the spark energy improves.

Just thinking about what it could be, hope this helps.

 
  #5  
Old 11-26-2007, 08:50 AM
moe
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Default RE: missing, weak spark

i know of a flash for cyl. miss fire,but i think it was in the 3.3 .the spark controlcomes from a driver inside the pcm.moe
 
  #6  
Old 11-29-2007, 06:59 PM
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Default RE: missing, weak spark

ok, i tryed the other wires and still getting the same thing. When a driver goes out could it cause a coil to fire but just be weak? cuzz it is firing just very weak. Also when the cam and crank sensor goes out woudnt it throw up a fault code or mil. Or could they still work and just get weak enough to throw the timming out a wack enough to make it miss fire?
 
  #7  
Old 11-30-2007, 09:51 AM
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Default RE: missing, weak spark

Most ECUs/PCMs will allow a few missfires over time so unless the circuit was actually DEAD you may not see a code.

Same thing for the cam/crank sensors, if they are inop they will throw a code for sure (and you would have a no-start). Just out of kilter, and troubleshooting becomes tricky.

Dealers often have test PCMs they can bolt-in as troubleshooting aids. (Often factory troubleshooting procedures eventually get down to "replace the PCM with a known good")
 
  #8  
Old 11-30-2007, 03:06 PM
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Default RE: missing, weak spark

qick question, can the pcm adjust the spark out put?

well i guess ill replace the cam and crank sensors and see if that helps. I was just trying to troubleshoot the problem before replacing a bunch of stuff. But im at my wits end cuz i have never seen a problem like this before that wasnt fixed by new wires or a coil pack.

Let me go ahead and tell the full story about it so you can grasp a little more info about it. I bought it with 99k on it and it ran fine. No missing that i could pick up on by feel or hear. Ran great till about 109k. That is when it started missing real bad. So i pulled all the plugs and three where badly worn. One in front the other two in back. The other three where fine. Replaced them and wires. Same thing. Replaced coil pack. Same thing. Im still getting weak spark from them and i dont understand why. I would think that the pcm would tell it to fire or not to fire. Or does it adjust the spark?
 
  #9  
Old 12-15-2007, 09:55 AM
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Default RE: missing, weak spark

replaced the two sensors, no go. any other thing it could be that i could test?
 
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