mistery missfire need help
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Freshman User
Joined: Feb 2009
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From: ohio
mistery missfire need help
Hello, I have 2000 ex v10 that has a random missfire but not all the time. If you let it idle all is fine for 30 sec. then you hear it. DA then fine for 20 sec then DA,DA,DA then fine for 30 to 45 sec. and the starts all over. When driving at a constant speed of 70 mph no miss at all unless you give it lite pedal to accell then it will miss.If you step on it to downshift no miss. after a few days of playing around i've found that if the motor labors under 3000 rpm it will miss at 2500 rpm the miss is more intermittent and at 3000 rpm the miss goes away. I have replaced plugs,cop's,injectors,fuel filter, cleaned maf sensor. and still have same problem. I have seem postings with simular problems but never a cure. Any direction you can offer will help 
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Freshman User
Joined: Feb 2009
Posts: 31
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From: ohio
both motorcraft and aftermarket. no different. I had autozone scan and they said 2 codes. One for random missfire, and one for primary circuit failure coil 5. I have 2 extra cop's that test good so I swapped out coil 5 but it made no difference. Also the cop I removed test good. That made me think shorted wire but never had a wire that only shorted sometimes. I would have to think a short is always a short. Thanks
Logistics Pro
Joined: Aug 2007
Posts: 3,974
Likes: 18
From: Millersburg, OH
both motorcraft and aftermarket. no different. I had autozone scan and they said 2 codes. One for random missfire, and one for primary circuit failure coil 5. I have 2 extra cop's that test good so I swapped out coil 5 but it made no difference. Also the cop I removed test good. That made me think shorted wire but never had a wire that only shorted sometimes. I would have to think a short is always a short. Thanks
Also check the PCV elbow that Krewat mentioned.
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Senior User
Joined: Dec 2008
Posts: 439
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From: Lubbock, Texas
Hello, I have 2000 ex v10 that has a random missfire but not all the time. If you let it idle all is fine for 30 sec. then you hear it. DA then fine for 20 sec then DA,DA,DA then fine for 30 to 45 sec. and the starts all over. When driving at a constant speed of 70 mph no miss at all unless you give it lite pedal to accell then it will miss.If you step on it to downshift no miss. after a few days of playing around i've found that if the motor labors under 3000 rpm it will miss at 2500 rpm the miss is more intermittent and at 3000 rpm the miss goes away. I have replaced plugs,cop's,injectors,fuel filter, cleaned maf sensor. and still have same problem. I have seem postings with simular problems but never a cure. Any direction you can offer will help
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Freshman User
Joined: Feb 2009
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From: ohio
thanks
When the coils were changed so were the boots. I have checked for vacuum leaks but haven't found any. Also tried new set of plugs-no luck. Interestingly problem seems to be on pass side. Just for s**t & giggles i sliped some cardboard between the wire harness and heater hose because they were touching and the truck seemed to not miss as bad. Any thoughts on that?
Senior User
Joined: Dec 2009
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sounds similar to what mine was doing a few weeks ago and it was a bad cop.. Not saying that is your problem but it does sound very similar.. Also you may want to replace the vacuum hose that was mentioned before that mounts under the throttle body.. I replaced mine with a hard plastic elbow and some strait hose on both sides of the elbow..
Posting Guru
Joined: Mar 2005
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From: Burnsville, MN
If you want to pursue your perception that the miss improves with the cardboard, there could be an argument that the harness is moving or vibrating with various motions, and the cardboard helped stabilize the harness. You might try a wire tie somewhere, and/or also plug and unplug any nearby fittings?
Tuned
Joined: Sep 2002
Posts: 408
Likes: 11
From: saline USA
My 2002 V10 occasionally gets crud on one of the plugs, it will intermittently miss. New Plugs, boots etc and it will run fine for about 5k then it starts again. I put two bottles of gumout injector cleaner in a tank of fuel and within 20 miles it stops and won't do it again for another 5k. I don't know if its ethanol, or an injector running a little rich, but the plug color is perfect and this is my third set of motorcraft plugs and every set has done this so I don't think its the plugs being bad, a coil usually goes or stops, and I cannot explain why the injector cleaner clears up the problem accept that its deposits from fuel some how.
Andrew.
Andrew.
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Freshman User
Joined: Feb 2009
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From: ohio
still fed-up
today I unpluged both o2 sensors and the truck ran a little better the miss now seemes to stop at about 2000 RPM. However to my surprise my check engine light never came on!! I would have thought that with no input form the o2 sensors the pcm would throw a code. any thoughts on this?
Logistics Pro
Joined: Aug 2007
Posts: 3,974
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From: Millersburg, OH
You will NOT get an CEL for O2 sensor issues just like you will not get a CEL for misfire issues. The system you have is basically and ODB-I system with an ODB-II interface. See the link below for more info.
https://www.ford-trucks.com/forums/5...t-come-on.html
https://www.ford-trucks.com/forums/5...t-come-on.html
Junior User
Joined: Oct 2009
Posts: 54
Likes: 0
From: Cheyenne, WY
both motorcraft and aftermarket. no different. I had autozone scan and they said 2 codes. One for random missfire, and one for primary circuit failure coil 5. I have 2 extra cop's that test good so I swapped out coil 5 but it made no difference. Also the cop I removed test good. That made me think shorted wire but never had a wire that only shorted sometimes. I would have to think a short is always a short. Thanks
Check to make sure the plug on the coil is making a good connection and that
the wires going to the coil aren't cut or rubbed-through. Check the whole
ignition harness to make sure it's not rubbed-through or cut somewhere, too. Beyond that, it could be a problem with a bad connection between the PCM and engine harness or a faulty PCM.
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