Problems starting after 6+ hours of car standing...

run the fault codes
indicates a problem ambient temp sensor or a Baro sensor that is not report correctly to the ECM so the correct cold start air/fuel mixture is not being set

Hi Jacques, The idea of having On board diagnostics is to save you from guessing where the problem is. Have the diagnostic scan carried out before doing any other work, thereby avoiding un required work. As rats have been at the wiring, I'll take a guess and assume they have also been at the vacuum hoses and tubes. Check all for damage, breakage and loose fitting. Also check the ducting between the air flow sensor and the throttle body. If you find any damage replace them. Remove the air cold start bypass up idle valve and clean it. Make sure that the computer temperature sender unit is working correctly and then attend to the scan results. Regards John

Worn out battery--check that out

Did you not check for codes yet?

there is a problem with the rocker arms or the cam shaft. get it checked out before more problems acor

I just read that the fuel pressure regulator on the 2004 Sentra can bleed pressure overnight, resulting in long cranking times in the morning.

first remove your map sensor and clean it with carburetor cleaner let it soak for a few hours and then reinstall it sometimes they get dirt or carbon build up and the pcm cant tell how much fuel to send

you hit the nail on the head ,cold and silicone chips ,they do not like the cold weather they can be temperamental to say the least ,as for finding the faulty component you will need an identical car that works ok ,then when yours will not start try swopping bits till you hit the one causing the problem.or sell it for a nice manual diesel version,

If the engine has been flushed well including the radiator and you are still getting water in the oil. The culprit is the cylinder head or the block has a crack internally allowing water into the oil.

3 simple things to check:pressure check to see if head gasket is leaking,check thermostat(can be bad in the box), water punp bearing worn, not turning fast enough( and a stretched serp. belt)

ignition module.