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Intermittent Misfire - Please Help!

Old 12-30-2018, 05:04 PM
  #1  
castnersr
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Default Intermittent Misfire - Please Help!

Hello, I am in need of some technical help. I have a 2002 Mustang GT, and it is misfiring intermittently. Originally, I had problems a while back with one of my O2 sensors, so I went ahead and replaced the remaining three. On to vacuum leaks. I could not find any vacuum leaks, so I moved on. I had a set of spark plugs I had been waiting to put in, so I went ahead and replaced all of those. The car only threw a code once, and it said that cylinder 5 was the culprit. So, coil was my next step. Still no change. Afterwards, injector. Still nothing. Checked the coil boots, they seemed to be getting a bit old so I went ahead and replaced those. At this point, I was fed up. I took it to one shop, they said it was the coil again, claimed to have fixed the issue, but they did not. Took it back to the same shop, and they told me the only ones who could help were Ford. Still fed up, I took the car to Ford in hopes of good news. What did Ford have to say? "We could not replicate the problem you say you are having." Hence the intermittent. The one thing Ford did take care of was a dye test to check for a blown head gasket, and that was also not the issue. Furthermore, they ran a smoke test to double check for vacuum leaks, and none were found, so that was officially ruled out. Tried the fuel filter, as well as cleaning the fuel rails. Still nothing. I read on the forums that a bad coolant temp sensor had been the problem for one guy, so I did a much needed coolant flush then replaced the sensor. Still no change. Just thirty minutes ago, I rented a fuel pressure checker, and all of my pressure seemed fine. 30 psi at idle, 40-45 WOT. (I read that that was the normal, please correct me if I'm wrong.) Next on my list is the TPS, EGR, and MAFS. I hadn't thought the MAF to be the issue seeing as I just cleaned it not too long ago, but will be double checking. Read somewhere that TPS and EGR can sometimes cause misfires.

The only reason I have not checked compression is because the original shop I took it to said that you would know if it was bad while idling (Rough idle, etc.), rather than while driving, seeing as pressure changes when under load and the low psi would be made up for. However, I am about to give in and run a compression check.

For ease of access, here is the full list of what I have checked/replaced:
-Coil Boots
-Spark Plugs
-O2 Sensors
-Injector
-Coil #5
-Dye test for head gasket
-Smoke test for vacuum leaks
-Fuel Filter
-Coolant temp sensor
-Checked fuel pressure

I am completely lost and genuinely irritated with this problem. Not to mention I have college starting up again soon and will need the car desperately. Any advice is welcome, and thank you in advance.
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Old 12-30-2018, 07:21 PM
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tadpole1969
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i had a simlar prob one time with my 01 gt , ended up being my wireing harness was shorting to ground
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Old 12-30-2018, 07:23 PM
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castnersr
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Originally Posted by tadpole1969 View Post
i had a simlar prob one time with my 01 gt , ended up being my wireing harness was shorting to ground
How did you go about diagnosing that?
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Old 12-31-2018, 02:58 PM
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tadpole1969
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well the way it went for me was one day i was driveing down the road and smelled burnt wireing ,which was my coil melting from the short ! so i just replaced the harness, but the guy who installed my headers for me had told me it needed replaceing right before that happened
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Old 12-31-2018, 03:50 PM
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just look your harness over very closely
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Old 12-31-2018, 09:13 PM
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dawson1112
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So the number 5 cylinder is on the drivers side front of the engine. Check your intake for coolant leaks around that area since that is where your thermostat and coolant xover are located.. You may have to remove intake and put in a new gasket and reseal the areas where coolant bypasses through. The 4.6l engines use a plastic intake and they are notorious for cracking right there at the thermostat housing and leaking into the #5 cylinder
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Old 01-07-2019, 10:40 AM
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castnersr
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Originally Posted by dawson1112 View Post
So the number 5 cylinder is on the drivers side front of the engine. Check your intake for coolant leaks around that area since that is where your thermostat and coolant xover are located.. You may have to remove intake and put in a new gasket and reseal the areas where coolant bypasses through. The 4.6l engines use a plastic intake and they are notorious for cracking right there at the thermostat housing and leaking into the #5 cylinder
Originally Posted by tadpole1969 View Post
well the way it went for me was one day i was driveing down the road and smelled burnt wireing ,which was my coil melting from the short ! so i just replaced the harness, but the guy who installed my headers for me had told me it needed replaceing right before that happened
So, it's been about a week, and I've checked all grounds, and replaced all the ones that appeared worn. After that, I checked the intake, no leaks, cleaned anyways. Ran the car, still no dice. Getting quite frustrated. Although I may have a lead, and want to know what you guys think. I read somewhere yesterday that a bad PCM can cause the car to have sporadic misfires/sputtering like I've been having. I made sure to check all grounds and connections anywhere near the PCM, so that part is out of the question. Is it possible that my PCM is just shot?
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Old 01-07-2019, 11:57 AM
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castnersr
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Also, if I were to replace my PCM, would that make my ignition keys useless and would I have to make a trip to the dealer? Sorry if this is a a dumb question, I just have no idea how this stuff works.
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Old 01-07-2019, 12:19 PM
  #9  
Derf00
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Did you check your plug gap before installing the new plugs? Also, cracked porcelain on a plug could cause a misfire. Just because they are new doesn't mean they weren't dropped once.
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Old 01-07-2019, 12:22 PM
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castnersr
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Originally Posted by Derf00 View Post
Did you check your plug gap before installing the new plugs? Also, cracked porcelain on a plug could cause a misfire. Just because they are new doesn't mean they weren't dropped once.
Plug gap was good. Didn't think about the cracked porcelain, will check that. Thank you
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