Random Misfire

Saleen304

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I am getting random and specific misfire codes on my 2005. Codes are either P0300 (random) or specific to a cylinder (P0305, P0303, P0306, P0304). Seems like cylinder 5 gets the code most often. I have tried the following:



1. Replaced all coils with new motorcraft coils

2. Replaced new brisk plugs with new Ht0s gapped at 0.030"

3. Checked tune (Lito)

4. Replaced EVAP purge valve

5. Checked for vacuum leaks (16.4 Hg at idle)

6. Replaced cam position sensors

7. Replaced crank position sensor

8. Checked grounds for PCM and crank position sensor ground shield

9. Had MAF checked (PMAS HPx)

10. Checked injectors (Bosch 1000cc injectors were installed Aug 2015). I replaced them with 60lb/hr injectors I had available. Adjusted tune, issue did not go away.

11. Disconnected vct solenoids. Problem did not go away.

12. Compression check. Ave 199. 1- 200, 2- 208, 3- 199, 4- 196, 5- 199, 6- 192, 7- 200, 8- 207



Problem showed up this year at a HPDE event in May and have been fighting it all year. I could replicate it on the street by holding 4000 rpm in 3rd gear for 4-5 seconds and the check engine light would start flashing and I could feel the misfire. Would take 10 sec or so to go away. Lately it comes up more often even if I just drive normally. Datalog shows it holding steady at 14.7 A/F and then go lean when the misfire happens. This does not occur at wot. Has anyone had an issue similar? What are my next steps?
 
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redfirepearlgt

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Check AC on the alternator. If its in excess of about 250mV AC the alternator full wave rectifier diodes are likely beginning to fail. COP's with AC induced is like trying to run 87 octane in 13:1 compression engine. LOL!
 

Saleen304

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Check AC on the alternator. If its in excess of about 250mV AC the alternator full wave rectifier diodes are likely beginning to fail. COP's with AC induced is like trying to run 87 octane in 13:1 compression engine. LOL!



Any tips on how to do that? Alternator was new (motorcraft) in April of this year and is the updated 3 phase Denso unit. I am running lots of high amperage stuff. Stereo amp, he fans, EMP pump (30 amps alone), and electric water pump.
 

redfirepearlgt

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Just as you checked the DC voltage, you will check for AC voltage with your voltmeter set to measure AC. If the voltage reads 250mV or higher there is a strong possibility that a rectifier diode or more than one is failing.

All alternators as a rule are 3 phase AC generators with full wave rectification that converts the AC to DC voltage. If one of the diodes in the rectifier circuit fail the result is either AC voltage from one of the phases or pulsing DC which is nothing more than square wave AC vice sinewave AC to keep the electrical theory simple. There is also additional flitering of an LC circuit that could fail and allow AC ripple to reside ont he DC outputs as well. Either way simply set your multimeter/voltmeter to measure AC voltage. Hook it across the battery with the car running and measure to see if any AC exists.

This is just a check to help eliminate distracters. It may well test fine. But until you know it's still a viable fault to consider.
 

05stroker

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Does it break up or miss that you can tell? Sounds like what happened to my kids car. Turned out to be the drivers in the PCM.
 

Saleen304

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Does it break up or miss that you can tell? Sounds like what happened to my kids car. Turned out to be the drivers in the PCM.



Yes I can feel it. Lito said my next step might be pcm.
 

redfirepearlgt

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Does it break up or miss that you can tell? Sounds like what happened to my kids car. Turned out to be the drivers in the PCM.

I was thinking that as a last resort but was hoping he'd find a flaky Alternator causing the issue. Didn't yours go out after it was repaired and you ended up having to get another one from a junk yard or something along those lines?
 
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05stroker

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I was thinking that as a last resort but was hoping he'd find a flaky Alternator causing the issue. Didn't yours go out after it was repaired and you ended up having to get another one from a junk yard or something along those lines?

Yes I did replace it. I'm not sure they ever really fixed the other one.
 

06monera96

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Currently having the same issues with my 05.

But what i found is that this only happens at night during 15c or below weather.
And only on certain tunes.

But smell raw gas at times under wot pulls.
Not sure yet if it the plugs going bad from using octane booster when at the drag strip (once or twice a month)
Or the pcm drivers are dying.
 

Saleen304

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Just as you checked the DC voltage, you will check for AC voltage with your voltmeter set to measure AC. If the voltage reads 250mV or higher there is a strong possibility that a rectifier diode or more than one is failing.

All alternators as a rule are 3 phase AC generators with full wave rectification that converts the AC to DC voltage. If one of the diodes in the rectifier circuit fail the result is either AC voltage from one of the phases or pulsing DC which is nothing more than square wave AC vice sinewave AC to keep the electrical theory simple. There is also additional flitering of an LC circuit that could fail and allow AC ripple to reside ont he DC outputs as well. Either way simply set your multimeter/voltmeter to measure AC voltage. Hook it across the battery with the car running and measure to see if any AC exists.

This is just a check to help eliminate distracters. It may well test fine. But until you know it's still a viable fault to consider.



Checked AC voltage. Engine off is 26.4, engine on is 30.6.
 

Saleen304

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Does it break up or miss that you can tell? Sounds like what happened to my kids car. Turned out to be the drivers in the PCM.



What did you end up going with for PCM? New ones are $530 and show it's compatible with 2005-2007 MY. When I look at used ones they are $125-$200, but there are different part numbers for each MY. Can I just grab a manual trans pcm from a 2007 and use it? Is there something special you have to do with the PATS or is that not related to PCM?
 

05stroker

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What did you end up going with for PCM? New ones are $530 and show it's compatible with 2005-2007 MY. When I look at used ones they are $125-$200, but there are different part numbers for each MY. Can I just grab a manual trans pcm from a 2007 and use it? Is there something special you have to do with the PATS or is that not related to PCM?

You have to find a used one. Ford no longer makes them. I paid $150 for a used one. I just made sure it was for an auto and had Manuel send us a revised tune for the different PCM code. You should be able to get one with your same code though if you are ordering one. We were in a bind and did not have that option at the time.
 

Saleen304

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You have to find a used one. Ford no longer makes them. I paid $150 for a used one. I just made sure it was for an auto and had Manuel send us a revised tune for the different PCM code. You should be able to get one with your same code though if you are ordering one. We were in a bind and did not have that option at the time.



So I need to get a 2005 MT computer. A 2007 MT won't work? I only ask since 2005 was the first year and there seems to be more and more issues so I would like to put in one that is a bit newer. My computer doesn't have a part number on it. Just says show car.

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Looking at car-part.com there are two options for 2005 with MT and 4 options for 2007 with MT.

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Saleen304

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Never mind. I am blind. I found out it is the 5R3A-ACC.
 

redfirepearlgt

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Checked AC voltage. Engine off is 26.4, engine on is 30.6.

30.6 I am going to assume is millivolts AC. The reading with the car off is erroneous since the alternator is not generating any voltage.

Was hoping it would have been an alternator for you but glad you isolated it down to the PCM and found it. Big ticket items are always the last thing rather than one of the first things to blame for the failure.
 
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Saleen304

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You have to find a used one. Ford no longer makes them. I paid $150 for a used one. I just made sure it was for an auto and had Manuel send us a revised tune for the different PCM code. You should be able to get one with your same code though if you are ordering one. We were in a bind and did not have that option at the time.



Reading the service manual it says I will need to reprogram the PATS if a new PCM is installed. Is that true with a used PCM? What about VIN? I thought VIN was also part of PCM.
 

05stroker

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Reading the service manual it says I will need to reprogram the PATS if a new PCM is installed. Is that true with a used PCM? What about VIN? I thought VIN was also part of PCM.

Those would be good questions for Manuel or your tuner.
 

Saleen304

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Those would be good questions for Manuel or your tuner.



Yeah, I have an email to him, but I think I found a better solution. There is a company called SIA Electronics that will rebuild it with lifetime warranty for $161. They said they see this issue with the ignition portion all of the time.
 

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