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Re: 1992 astro starting problem
In my experience, ECMs typically do not experience intermittent problems. They either work, or they fail and remain that way. They sometimes experience poor circuit board connections due to thermal creep or external stresses, but usually don't simply fail on their own. ECM hardware failures are almost always due to external influences such as shorted outputs or system overvoltage. Intermittent problems are usually related to the connections at the ECM, the MEMCAL or PROM, or the ECM input or output devices. ECM connections can usually be improved by simply unplugging and reconnecting/reseating the connnectors several times to wipe the contacts clean and establish a new connections surface.
In basic design, the two ECMs are identical. The difference will be in the PROM. The '88-91 TBIs used a $4E hex ID sequence while '92-93 used the $A0. Both are for the LB4 engine. I wasn't able to find the BCC for either program AULN OR AUMH), so I cannot state whether they are compatible.
If you still suspect the ECM itself after diagnosis of the other parts of the system, you should be able to exchange ECMs, but use the PROM designed for the vehicle in either case. Pay particular attention to the orientation of the PROM and the plastic skirt/carrier before performing the swap.
The fact that the ECM will operate the injectors so long as you provide some startup fuel leads me to think that the ECM is doing its job. One way to determine this is to check the injector(s) for operating pulses during the no-start episode, and while the engine is cranking. If there are no injector pulses, either the ECM is not acting upon the input of the distributor reference signal, or the distributor reference signal is missing or weak. Since the ECM does operate the injectors after you have manually administered fuel, I would suspect the latter.
If injector pulses are present but the injectors do not spray fuel, you need to look at fuel pressure.
Also, there is a G.M. TSB regarding cold startup knock:
COLD ENGINE KNOCK - #37-61-05A - (Oct 1, 1995)
This is NOT a recall, and if it applies to your vehicle, is not done as a free service.
This TSB may apply to your vehicle, or may not. The "fix" is to retard ignition timing when cold to minimize cylinder pressure until the engine is warm and parts have grown to their design clearances, thereby minimizing cold knock. The later PROM you have may have this feature, but it also may not be appropriate for the axle ratio, injector size, and may be incompatible due to other variables.
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