'85 GTS: Cranks, fires for a second, then dies. Doesn't respond to throttle or unplugging things. Previous owner went over all the usuals: changed fuel pump, fuel filter, pulled COR and jumped fuel pump with a switch, replaced AFM. PO thought it was maybe FPR? Strange thing to me is that there's no CEL activity. As in, no light at all. bad ECU? AFM circuit? Igniter?

If it runs for a second then the fuel and igintion systems are probably good. If your ECU does not see any signal from the AFM at start up it will shut down the engine. Check the AFM connection and wires. Manually move the flap and verify a resistance change. Davew7

Go to your diagnostic port and try jumping the fuel pump. I'd look into the fuel pump relay and the switch in the AFM.

any big vacuum leak would cause the exact same behaviour

jinx wrote:any big vacuum leak would cause the exact same behaviour

If everything else is good. I agree with jinx. Check your air duct from the AFM to the throttle body. cracked, broken, or maybe not even clamped correctly, who's knows. Worth a shot.

What was done to it since it was last running, immediately before the first time it didn't start?
Very similar symptom to the very usual problem of those unconnected grounds under the intake.

That's a very good question, I bought the car with these symptoms...

A further inspection shows that all fuses are good, grounds are intact, but the original COR was in the glovebox so I suspected the COR was jumped or something. I checked and there's a new aftermarket relay in place of the COR that doesn't look great. Like a few have mentioned, I'm leaning toward this being a grounding or connection issue, since even the 4AG diagnostic manual says there's an issue with +B flow if the ECU doesn't illuminate the CEL with the key on. My biggest problem is that lately I haven't had any time to really get into this thing, just a few minutes here and there to poke at different things.

Check for the grounding on your injector resistor box. Or the box itself. Sounds like your running off of you cold start only.

Check all grounds in the injector cuircuit. 4 brown wires that ground on the intake mani. + the others