LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

LT1 won't start, No injector pulse, spark ok, fuel ok

Old 03-16-2011, 01:30 PM
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LT1 won't start, No injector pulse, spark ok, fuel ok

I am trying to help a friend get his LT1 started. We swapped it into his 1966 Buick Skylark about a year ago, and when the swap was finished the car started on the first try, and ran fine. A couple weeks ago it died while driving and would not start back up.

I removed the harness and checked all the ground, power, ignition wires for continuity everything seemed okay. I also checked every pin on the PCM for continuity and the wiring harness seemed like it was working fine.
We put the harness back in the car and still no start.

I can see that it has fuel pressure because when I press on the schrader valve on the fuel line it spews fuel out. I didn't actually put a pressure gauge on it but I do have one.
Then we pulled the coil wire and checked for spark. Motor has spark.
We then tested the fuel injectors for power and ground, they have both.
When cranking the engine the ground on the fuel injectors stays solid and does not flash or pulse at all.

Next we pulled the opti wire and checked the harness it was getting 12 volts on one wire, 5 volts on 2 wires, and a ground. Which means the pcm is sending signals to the opti.

The PCM should be fine. I have another opti I can swap in about 45 minutes and an extra ignition module and coil. I'm not sure if any of these are the cause though because the motor has spark.

Is there something else that would cause no injector pulse?
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Old 03-16-2011, 04:10 PM
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Re: LT1 won't start, No injector pulse, spark ok, fuel ok

The PCM does not send any signal to the opti. It's strictly the other way round. While we understand it's getting voltage and ground, it's not actually signals. The opti sends 2 signals back to the PCM. Low and high res which you saw as 5 volts. They are actually square waves of different frequencies.

Next if the injectors were truly "grounded" solid, your engine would be full of fuel. Your cylinders would hydro lock and you(he) would bend rods.
So I highly doubt you actually saw them constantly grounded. Not to mention they would get so hot they would smoke. They can't take being on solid.

When the PCM fires the injectors, it grounds one side of the injector coil momentarily. The other side of the injectors, as you know, is powered all the time. If the PCM shuts down the injectors, as in the case of a problem, it throws a code. So if the PCM were shutting them down, there would be a code there. It would be best to scan it for codes and check the injector signals with a noid light.

As an alternative, you can squirt fuel in the intake by hand while someone cranks it over. If it truly has correctly timed spark, it would light off then. Albeit momentarily until you stop squirting fuel.

If you have spark and you squirt fuel in it and it does NOT fire up. Either it has bad compression which is unlikely or the spark is occurring at the wrong time.

Last edited by Guest47904; 03-16-2011 at 04:15 PM.
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Old 01-19-2019, 06:38 AM
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Re: LT1 won't start, No injector pulse, spark ok, fuel ok

Same problem Iím having I have spark spray gas in the intake still no start
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Old 01-19-2019, 11:50 AM
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Re: LT1 won't start, No injector pulse, spark ok, fuel ok

EXACT same problem????

Do you have spark at the coil wire and at the plugs?

Do you have 12 volts to the pink wire for each injector harness connector with the key on?

Have you checked for injector pulse (ground supplied by injector driver in PCM) with a 'noid light?

Have you scanned it for codes?

Is the "SECURITY" light on?

Have you verified ~40 PSI fuel pressure when the fuel pump primes?

Have you verified the PCM is getting the low resolution pulse signal from the Opti, and sending it on the white wire to the ignition control module (white wire)?

Courtesy of Shoebox:

4th Gen LT1 F-body Tech Articles

PS: What year is your engine/PCM - there are subtle differences from year-to-year?
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