Crankshaft position sensor help

[landon00]

I am looking for some help on crankshaft position sensors. I am getting some really wierd behavior out of my ignition system on a 2002 5.3L conversion into a 1986 chevy pickup. I hooked a timing light up to cylinder 1, and noticed that it was not firing every cycle consistently during starting. The truck doesn't run at the moment, so I was wondering if on the 4th cycle I don't get a spark, would it be due to a weak battery. The battery is still cranking the motor over a constant speed, so I would think it wouldn't be the battery. It is consistenly every 4th revolution of the motor. My next step is to check the square wave of the Crankshaft position sensor. And also the camshaft position sensor. The truck ran two weeks ago in open loop beautifully, and now I can't get it to start. If anyone has any suggestions please let me know.

Thank you,
Landon00

[mo gomez]

the first thing to check is plugs; they always tell a story. you said its been running fine in open loop; do you mean when the vehicle was cold or all the time you were running in open loop? if you are staying in open loop youre plugs will be black or fouled.you must pull at least 4 plugs on 1 bank if they are fouled,and clean or replace them if you want it to run. as far as the can and crank sensors if you are seeing a pattern youre ecu is seeing rpms they are doing their job. one sensor failure is possible but both failing at the same time is very unlikely.

[landon00]

When I say open loop, I mean the engine is waiting for the O2 sensors to heat up, and then when the ECT sensor is to the right value it will try to go to closed loop. And with my engine right now it doesn't have pre-cat O2 sensors, so it can't run in closed loop yet.

Thanks for the plug input. I am going to try replacing all the plugs. I replaced all 4 of the left bank, but still haven't replaced the right bank. It still will not start with the left bank all new.

I was monitoring the crankshaft position sensor tonight with a scope and I was getting square waves out 9 V out of the sensor. That to me means the crankshaft position sensor is working. I also got a square wave out of the camshaft position sensor.

I will try the plugs tomorrow. If that doesn't work, do you have any other input as to why it will not start? I will let you know.

Thanks a ton,
Landon00