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1996 Z28 not priming, good relay

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Old 06-13-2018, 03:05 PM
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1996 Z28 not priming, good relay

Car started stalling while driving, then completely cut out. Fuel pump was changed about a month ago and the relay is good. On a side note, after I had it towed back to my house I tried it, she primed and ran, turned it off and tried it again and got nothing. Not sure why its not priming but it just cranks.
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Old 06-13-2018, 05:28 PM
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Re: 1996 Z28 not priming, good relay

Does the fuel pump turn on when you supply 12 volts to the prime connector? Bypasses relay and bypasses the security system interlock fuel pump permit signal from the PCM.

Have you scanned for codes? The PCM shuts down the fuel system and sets a code if it loses the low resolution pulse signal from the optical cam position sensor in the Optispark distributor. Code does not turn on SES light.

Courtesy of Shoebox

http://shbox.com/1/fuel_pump_prime.jpg
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Old 06-14-2018, 12:58 AM
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Re: 1996 Z28 not priming, good relay

Yeah, it primes when I jump the circuit to the prime connector. I haven't bypassed the security system yet, although, I've heard that it just disables the injectors and starter but I might be wrong. I't is possible that the opti is to blame since the car cut out on me when it was raining. I will check for codes but as of right now there is no security light or SES.
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Old 06-14-2018, 10:50 AM
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Re: 1996 Z28 not priming, good relay

The PCM needs to receive a 50 Hz square wave signal from the body control module (BCM) before the PCM activates the injectors. That's part of the PASS-Key (VATS) security system. Whether it also controls the fuel pump ground to the relay, supplied by the PCM, is unclear to me.

The PCM allows the pump to prime for ~two seconds when the key is turned to “run”. When the key is turned to “start”, the PCM looks for the low resolution cam position signal from the Opti. If it sees high resolution pulses, but no low resolution pulses, it sets the code, and as far as I know, does not send the ground to the fuel pump relay.

Problem with the Opti code for low resolution signal is that it needs the high resolution signal before it can set. If total failure of the Opti optical module, wouldn’t get the low resolution code, but should the high resolution code. And even if there is a high resolution signal, the engine needs to be cranking for an extended period to accumulate the number of high res pulses that sets the code for absence of low res pulses.

You mention that the engine has just cut off while driving. That focuses attention on the ignition switch. Does the dash go dead when the engine cuts out? There seems to be an uptick in problems with the ignition switch with the extended age of these vehicles.
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Old 06-14-2018, 07:27 PM
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Re: 1996 Z28 not priming, good relay

The Dash didn't die or dim at all when it started dying, although applying the gas peddle seemed to make it worse and stall it faster. I also re-checked the prime connector and got nothing, not even a spark. Could be from my almost dead battery, is there a minimum size wire needed to jump the prime from the car battery or is it just not arcing because its dead? I've got spark but obviously no fuel, it could be VATS related but I have no clue how to bypass it.
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Old 06-16-2018, 09:08 AM
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Re: 1996 Z28 not priming, good relay

Jumper from terminal box + to prime connector should be 12 ga. 14 ga would do in a pinch. A 20-amp in-line fuse is a good idea.

A failing/failed fuel pump seems like a good possibility. Applying throttle with inadequate fuel pressure would produce the results you describe.

Worth checking the fuel pump harness connector at the rear of the car.

Courtesy of Shoebox

http://shbox.com/1/fuel_pump_wiring.jpg

http://shbox.com/1/fuel_pump_connector.jpg
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