2003, 4.7 liter. Cranks fine, but won't start. Got it to start, but ran terrible. two days later it started fine and ran great. Ran great for 3 days, then back to cranking and no start. Pretty sure it has good spark and 3/4 tank of gas. ECM?

 

Check engine light on?

 

how are you sure it has spark? the 4.7 is a COP (coil on plug) setup there are no wires to check with a timing light, so you need a scope to check the coil wires for signal. it sounds like a bad cam or crank sensor not giving the PCM correct information to fire the plugs to me.

if you use starting fluid in it what happens? does it try to run or just crank? if it just cranks your likely not getting spark.