Ive got a 1990 B2600i. I bought it with a bad ecm, i replaced the ecm and it seemed to run fine for a couple days. then started dying after 30min of run time, the last time it died, no fire at all. Cant get it started. cranks but no fire. checked the ecm and it looks burnt in the same exact place the old one was burnt. What causes this? i cant afford to keep replacing ecms every two days. help please!!!

 

I don't know, so this is only a guess, but considering how sensitive electronics are to correct voltage perhaps the voltage regulator is allowing the system to overcharge? Or the alternator itself is puttiing out too much (higher than normal) voltage?
The only other thing I could guess at would be a bad ground somewhere causing resistance and the PCM to draw too much current to compensate?