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So my 04 F250 5.4l runs rough at start up tor like 1min and then runs fine. It has codes PO304, PO455 and PO315. Last fall had the same issue so I put a new MOTORCRAFT coil on #4. and seemed to go away. Now it been back since spring. Could it be #4 injector leaking while sitting? I did do a fuel pressure test and it goes down to zero over night. Any advice? Thanks
Fuel pressure dropping to zero overnight is perfectly normal in my experience. Otherwise the 2 second key on prime wouldn't be needed.
If it were to rapidly drop pressure once the pump is shut off, then you'd have something to chase.
Edit: sorry for the late reply, I've got cars from work on my mind.
I would probably check mode 6 for misfire data, just to see if #4 is acting alone or if it's just the first one to cross the code set threshold and probably scope crank sensor to see what's up with that P0315.
I would also want to compare temperature sensors before a cold start to make sure they all agree.
Then maybe clear codes and see which one comes back first when it acts up again. Older Fords do take a long time to set codes, so try to get whatever info you can before wiping it's memory.
I've seen cold start misfires caused by old spark plugs, intake manifold vacuum leaks and head gasket problems.
Over night is normal. Leak down tests are either so much loss per 5 minutes, or at all for 15. Depends on the engine. Next time you cold start it, prime it but do not crank. Then back off and prime again. If it primes fully twice, issue found. If it doesnt seem to prime twice, but starts normally, issue found.
Regarding THE po304 code. Make sure Heater hose is not leaking coolant on to #4 coil. Also that ign coil pigtail is secure, the lock on the ignition coil pigtail breaks and doesn’t stay for long. Either zip tie it in place or replace pigtail they are two wire and readily available
The po455 code. Take the purge valve and blow across it, it should not flow at all. It should be completely closed at all times It’s more Then likely stuck wide open dumping fumes into manifold and probably why it starts so poorly when cold. It’s easy to get to usually by the brake booster
