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I swapped in an LS3 into my 72 Vette a few years ago. It fired right up and has so for quite some time now. It did sit for a while, a few years due to life (divorce and so on). Now back to it and trying to finish it up. It started right up again and has on several occasions. Within the last month or two it has had intermittent start problems. Just cranks but no fire. Try this a few times and all of a sudden it fires and runs fine. Has done this on several occasions. Does not matter if it is hot or cold. Now I am getting no start at all. Just cranks. I have fuel pressure 58lbs, I have an rpm signal verified by code reader. The only thing I can think of is battery has gone south. I have 13.4 volts but when I crank it the voltage drops to 10.4 volts while cranking. Battery is a couple of years old but not many miles but it is fully charged. Will the ECU fire the coils and injectors with the voltage that low? I never measured it before because it always started.
Battery is fine. 10V at a load test (cranking) is OK. Below that is weak, but you seem OK there. Sorry about the divorce, it happens. :/ Yes, the ECU will fire down to much lower voltage than 10V, closer to 6V actually. Check for cam signal, and try a redundant ground to ECU? Check for animal chewed wires during storage. Hit it with a whiff of starting fluid to rule out ignition system, etc.....
+1 on the starting fluid / "Explosion In A Can" stuff. Very good diagnostic tool but I would not use it as a crutch to frequently start up a difficult engine. The stuff strips the oil film from cylinder walls.
How old is the dead dinosaur juice in the fuel tank? 58 PSI of old ethanol fuel won't do much. Do your injectors flow anything? They could also be badly clogged.
The fuel is actually fresh. It was running fine last time it was started which was a week ago. It is really weird when it wants to start it fires immediately. Then I shut it down and it just cranks next time firing it up. It has been intermittent for the last 2 months but now it is just cranking.
So I hit it with a shot of ether and it fires and runs for a few seconds until the ether is consumed. So no injector pulse to feed it fuel. I confirmed this with a noid light plugged into multiple injectors. I do have 12v at each injector just no pulse. Cam sensor? I'm not sure where it gets the signal for the injector pulse.
Have you cleaned the MAF?
Have you cleaned the TB (small) holes under the blade? -- Mine ran like crap until I cleaned those 2 small holes, and sprayed around the blade.
Did you try to start it while lightly pressing on the gas?
Replace your fuel relay.
I mounted my battery in the trunk and it would not start until I ran a separate feed from the battery to the CPU, injectors and fuel pump. Starter was pulling to many amps from the whole circuit not letting mine start.
I mounted my battery in the trunk and it would not start until I ran a separate feed from the battery to the CPU, injectors and fuel pump. Starter was pulling to many amps from the whole circuit not letting mine start.
This may be an old thread by it is still relevant.
My 71 Lemans with 5.3 Gen V sometimes won't fire when warm - it also has a trunk mounted battery. I will need to run a separate power and ground to hopefully solve this embarrassing issue.
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LS3 won't start now 
