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Hey all, so the issue that I'm having with my car has started since back when I bought the car about 5 years ago at 100xxx miles. The car is a 2004 v6 mustang with the 3.9-liter engine. The car also has a manual transmission. The problem that I'm having started fairly small and insignificant and has started to grow into a really big problem that is consistently getting worse. Five years ago every now and then when I would restart the car after hot city driving it would have a slight misfire that might only shake the engine slightly, but would go away within 1 min or so, or as soon I would start driving the car. Over the years the problem has gotten worse but has always been triggered by the same event - hot driving and heat soaking the vehicle while turned off for at least 10 mins. It never misfires when cold!! If I let the car cool down mostly it will not have the issue when I restart it. Now the issue has gotten to the point that when I restart the car after it has heat soaked for about 10 mins or so I may start up with a complete cylinder misfire or at least a partial misfire. Finally, the car threw a code for cylinder 4 misfire detected on startup.... but here's the problem--literally everything on the engine has been replaced at this point and I have no idea what could be causing this issue. I have tried and tried to find the source of this issue with no luck. I have figured out that the hotter it is outside the more likely the car is to start with a misfire, and when it is colder out the car might not even have the issue at all. The misfire has NEVER started while the car is running, only when the car is started up and it will always go away after I drive it for a bit or rev the engine. So, if anyone thinks they are good enough for this one go ahead and try, but I think the car might be cursed at this point. There is one code stored in the computer that just says P10 but nothing else. There are no other pending codes in the system other than the misfire detected on startup.

Also, car spits and sputters when it is misfiring and seems to be getting fuel to all cylinders spitting out the exhaust. I have also taken the plug wires off while it is misfiring to see if I could narrow it down to one cylinder and had the crap shocked out of me by all 6 outlets on the coil pack.

Here is everything that has been replaced on the engine so far
Both Upstream 02 sensors
Camshaft position sensor
Crankshaft position sensor
Coil pack
Alternator
Plugs and wires
Mass airflow sensor
No vacuum leaks
bank 2 fuel injectors replaced
New BBK shorty headers
Waterpump
Fuel pressure is good
NOT A HEAD GASKET SO DONT EVEN SAY IT ( car has gone through a very small amount of coolant in 5 years (less than a half-gallon) and I have done a coolant pressure test )
 

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Answered in your other post. You cross posted in multiple sections.
Three identical postes aren’t needed, and are a No No...
 

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Discussion Starter #4
Answered in your other post. You cross posted in multiple sections.
Three identical postes aren’t needed, and are a No No...
Also a stuck valve would cause a host of other problems and doesn’t really fit the bill at all. For one I would have a valve and piston pancake in my oil pan. If the valve was only holding open slightly I would hear a ton of ticking from the pushrod being lose and jumping up to the rocker arm.
 

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Interesting, i have the same issue. 99, 3.8 v6 auto tho (140xxx mi). Before me, the car was incorrectly maintained, they decided to put 15w-40 instead of 5w-20 because, well, they are stupid. I am from the idea that a thicker oil just wont flow correctly, and if it calls for 5w-20 synthetic blend, that's what it will have till the day it dies. It also didn't have any oil dipstick, and supposedly had a valve problem on the passenger side of the motor, a half asses fuel pump, Bleh the list goes on and on. I hope it is a valve, and when i do my oil change i will be using a flush to hopefully clean up some valves. I hope this fixes my missing on heatsoaked start up.
Did you happen to find a solution?
 
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