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Discussion Starter · #1 ·
My 04 gt has been misfiring under load and at cruise above 2500 rpm. Replaced coils and plugs (.054) replaced fuel pump (injectors and rails are in excellent shape but fuel pres was low side) the car spent 2 weeks in the shop the best results came after clearing the computers flash memory then rebooting ( not just unhooking the bat ) fellas at the shop are as stumped ad i am. Most data on scan checks ok except for a super wierd fuel presure indication of over 4,000 psi which is impossible... I have all the data on ltft stft etc so if you have an idea but need a spevific number ive got it just ask... Codes being thrown are p0316 p0300 and somtimes p0304 p0300 and p0307.... This is a post for a real mustang guru, light wieghts need not reply lol...
Thanks in advance for anny input
 

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try replacing the FRPS on the fuel rail and check the wiring around the sensor. Sounds like the electronics inside it may have failed causing the erratic fuel pressure reading
 

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Discussion Starter · #3 ·
I was looking for data on values to check on the frps. ie resistance or voltage values, something i can check with a multimeter, and cant find any. Wouldnt a bum frps throw a code ?
 

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Discussion Starter · #4 ·
The shop thinks it may be an issue with the computer memory, my thought is to search for a chip or tuner
 

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Has the alternator been tested for excessive AC ripple?

Do you have underdrive pulleys?

What has been done to rule out excessive EGR flow?

Have you tested the fuel pressure damper?

Better to monitor fuel pressure as reported by the PCM. The sensor may be fine but the A/D converters in the PCM could be bad. Even better to compare the PCM fuel pressure with an external gauge (disconnect and plug the vacuum reference line for testing only). We want to see if the PCM reported fuel pressure is close to the external gauge.
 

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Discussion Starter · #6 ·
Swapped frps and still getting misfires. To rule out alternator the mechanic drove with the alternator unhooked, also with the egr solenoid unhooked (seprate test drives) thanks wmburns i,ll chavk that stuff out in the next day or so and keep you posted.... Oh and yes i do have underdrives, i was under the impression that they only affect the alternator at idle, am i mistaken?
 

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Underdrive pulleys will sometimes cause an unstable CKP signal. The symptom is random misfires. Bolt on UD pulleys tend to be more likely to cause an unstable CKP signal.

Disconnecting the battery as a functional test of the alternator proves nothing. Nor does it tells us ANYTHING about excessive AC ripple. FYI, excessive AC ripple will affect the CMP signal.

To anybody reading this. Don't disconnect the battey from a running car. The battery is required to excite the alternator's field coil. No battery = no alternator.
 

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Discussion Starter · #9 ·
So i tested the alternator and it passed ripple at idle was 142mV 22a at high rpm it was 180mV 24a, and all the grounds seem good, and the wiring harness is in great shape. What is that part in the pcm, the a/d converter? If the problem is with the pcm how can i get a good surfire diagnosis? Im tired of throwing money at this with no results.
 

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I personally do not know the actual value for excessive AC ripple. Nor do I have a professional alternator test tool. For that, I take it to someone that has the tools and can give simple good/no good answers.

Several questions were asked which were not answered. What have you done to rule out an EGR problem? What about underdrive pulleys? Moisture in the spark plug wells?

Consider replacing the all of the COP boots. Autozone (Durlast) makes a set of COP boot insultators that cost less then a single COP.

I have doubts this problem can be reasonably diagnosed without an ODB2 scanner. Data logging of the fuel pressure, MAF, load, TPS, LTFT for starters.

The gold standard to use to diagnose ignition problems is an ignition O-scope which is out of the reach of the home mechanic. If you want sure fire results, consider the services of a professional.
 

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Discussion Starter · #11 · (Edited)
I have an scan tool here is the freeze frame on a p0308 it also had a 300 but no data:
Calc load: 23.9
ect: 181deg
stftB1: 3.9
ltftB1: 7
stftB2: 0
ltftB2: 7
Rpm: 2054
Speed: 47mph
Spark adv: 36 deg
Iat. 39 deg
MAF: 2.33 lbs/min

Tps: 21.9
O2 senB1S1: .390v
stft B1S1: 3.9
o2 senB1s2: .320v
Stft B1S2: NA
o2sen B2S1: .860v
Stft B2S1: 0
O2sen B2B2: .625v
Stft B2S2: NA
I had it at a shop for a while, they swapped cops new plugs... Trim were lean we changed fuel pump and now trims are rich any input greatly apreciated, thanks for the help so far
 

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What was the last mod done? When/How did this start? Suddenly, gradually? Did you sort the codes out one by one? Usually if a code is given...it's a great place to start... Just throwing that out there.
 

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Discussion Starter · #13 · (Edited)
Ok to recap been getting p0316 302,303,308, and300. Tried all the usuall tricks, 8 fresh coils, and plugs gapped to .054, No luck. Took it to the shop, they ruled out a vacum leak, a/c ripple, cam n crank sensor, fuel regulator, fuel rails, fuel injectors. They thought the fuel pump was the problem, replaced it no luck, however the fuel trims were lean at missfire before the new pump, after the misfires are rich at misfire. I have my own obdII scanner and if you have a question about trims or anything i have the data just ask... Please help desprate
 

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do you have access to an SCT flasher and a laptop? You can datalog ford specific parameters with it and at a higher rate than most OBD2 scanners. Wondering what the short term fuel trims look like on the upstream (bank 1) sensors. The bank 2 downstream of the cats aren't important. Need to see a history of them, not just a snap shot. If I get some time later, I'll look up the codes to try to help you out.

On my car, I got rid of the EGR and had all sorts of misfires and my short term fuel trims were pegged at .8 until I had the tune adjusted to compensate so I guess it could be an EGR issue. I don't know enough about the system to help you on that aspect though, just that it only is used at idle and cruising, not WOT
 

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I had the exact same problem with my carabout 2 years ago.....turned out to the my Patriot Heads....had a bad valve seal that was leaking oil into the chamber.....also could be a valve spring that is cracked or seat and is not opening the valve up all the way.....I always feared the worst and it was it....the car still ran damn good with a light misfire but at cruising speeds it was anoying!

"ED"
 

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Discussion Starter · #16 ·
Thanks for the input... my buddy ran some data logs with a nifty little gizmo hes got... Wierd stuff... When the misfires occur the ecu/pcm freezes and goes into something like a safe mode, and the data gets erratic ( eg 4,000+ psi fuel pressure ) drops off and comes back to pretty normal any ideas?
 

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Hurt internal PCM ground, or overall electrical grounding problem.
 

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Discussion Starter · #18 ·
Update: pretty sure the problem is excessive ac ripple/ ac leakage anybody know of a fix for that?
 

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Discussion Starter · #19 ·
Wmburns called i, bad alternator. It was leaking ac current which was causing the pcm to freeze, then shut down 2-4 cylinders. Replaced the alt sunday and have been code free so that was the fix fyi to anyone wit a similar issue the alt testted good on the car but once we got it apart and started poking around with a multi meter couldnt tell. Thanks all for steering me in the right direction.
 
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