beating my head against a wall here

[jon]

ok im at a loss here working on a 1996 gmc yukon with a vortec 350 (not the ls center bolt valve cover 350) that just randomly died and will not restart i have replaced the ignition module as well as the ecu once i discovered a burnt resistor in the old one had the new ecu flashed to the truck at the chevy dealership and nothing fuel pressure is at 60 psi its acting like the timing is off it will crank fast then slow to almost a stop before going fast again old ecu had no codes before replacing it not sure where else to go from here

Topic

[Mike]

There’s the possibility of corrosion on battery terminals and a battery at less than full charge causing the odd cranking.
Clean the terminals and charge the battery fully.
On side terminal batteries, the plastic terminal covers can conceal considerable corrosion.
Remove the bolt, pry out the round contact and the cover will then slide off.

Next suspect is the distributor pickup coil.
Over time, the vacuum advance can damage the PU coil wires.
PU coil gets replaced, rather than attempting to splice the wires.

Page 7 tells how to test the pickup coil, if the wires are intact.
You should wiggle the wires while testing to see if there’s a break.
http://www.gomog.com/allmorgan/IgnitionSystemDiagnosis%26Service.pdf

Also possible that the distributor shaft magnet is cracked or the bushing is worn out, so the shaft turns elliptically, not just spinning.
To eliminate the distributor as an issue, disconnect the wires and see if cranking becomes regular.

This link provides a systematic way to troubleshoot
http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-troubleshoot-a-no-start-1

[Zac Aleksovski]

1996 engine I think was 1st year for SCPI. Are you getting fuel? 412 kPa pressure should be enough to open the poppet valves. Apparently no spark if you changed the ignition module. No mechanical or vacuum advance on this, spark-timing controlled by ECU. Not to say the pick-up coil in the distributor isn’t bad. I’ve seen dist. bushings wear out & wipe out the pick-up coil. Maybe timing chain if a lot of miles on engine. Check cap & rotor & coil wire?

[Author]

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