2002 Sentra turns over but will not start.Turns over and almost fires.Ran rough when cold before. Check engine light would come on sometimes, but would not on other times.

Could be any of several things. If the crank sensor is marginal and not sending pulses, the fuel pump is shut off. This is for safety. The pump is energized for a few seconds as key is turned in preparation for starting then engine must rotate normally to keep the pump on. If it stops, in a few seconds the fuel is cut off.

check cylinder compresions,sounds like there low(worn rings)by the way the 5.7iroc is one of the worlds best engines.....

I don't think the rough running had much to do with oil level, since it started running rough a 2nd time. Check the fuel filter and ignition timing.

with th o2 sensor it was runnig in open loop for a few months, my guess is the spark plugs are fouled, inspect them

Make sure plugs and wires are good and coils.

sound like its missing
check sparkplugs wires and coil pack for fire if not that run a compression check

remove dog house locate test port and install guage should be 45-60 psi

have you checked the oil it may be low on oil or you may have a bad cps(cam posistion sensor) so it is not exactly firing correctly you can go to auto zone and they will check the codes for free even if you dont see the check engine light on there will always be some type of codes stored. even codes that say no codes found. this will atleast give you a starting point.

I hate to be the barer of bad news, but may be the head gasket.

My car started with rough idle. Then very hard to start in cold weather. Next started overheating, but no output from heater. Trickling sound from behind dash board. Overflow bottle overflowing.......

I replaced the water pump, radiator cap, thermostat, coolant, fan belts, power flushed the system. Nothing worked. Some research gave me the answer.

The head gasket leak was allowing exhaust gases to enter the cooling system, displacing the coolant. After use, as engine cools, coolant is sucked back into the system and coolant seeps into the cylinder. This gets worse over time.

First thing you'll notice is the rough idle as the spark plug burns off the coolant in the cylinder. (This is especially true at low temperatures.) If it is the head gasket, the other symptoms will follow as time goes by.

First change fuel filter and then change air filter. A turbo requires good air flow to spool-up correctly