Engine starts, then after a while it just dies, cranks but wont start. Its been doing this for a while (around 2 months) most recently when cranking, it back fires and throws a lot of smoke by the air filter housing. I have replaced coil, plug wires, distributor CAP and rotor. I have voltage from and to Crankshaft position sensor and camshaft position sensor, even when it stalls. I noticed spark is weak, both in distributor and right off the coil (in both, old and new coil). What should I check next? Could the ignition switch or shift lever position sensor be causing the stall?
All "crank, no start" conditions are approached in the same way. Every engine requires certain functions to be able to run. Some of these functions rely on specific components to work and some components are part of more than one function so it is important to see the whole picture to be able to conclude anything about what may have failed. Also, these functions can ONLY be tested during the failure. Any other time and they will simply test good because the problem isn't present at the moment.
If you approach this in any other way, you are merely guessing and that only serves to replace unnecessary parts and wastes money.
Every engine requires spark, fuel and compression to run. That's what we have to look for.
These are the basics that need to be tested and will give us the info required to isolate a cause.
1) Test for spark at the plug end of the wire using a spark tester. If none found, check for power supply on the + terminal of the coil with the key on.
2) Test for injector pulse using a small bulb called a noid light. If none found, check for power supply at one side of the injector with the key on.
3) Use a fuel pressure gauge to test for correct fuel pressure, also noticing if the pressure holds when key is shut off.
4) If all of these things check good, then you would need to do a complete compression test.
Once you have determined which of these functions has dropped out,
you will know which system is having the problem.