I have a 350 in my 40. Has the Howell (early 1990's GM) TBI set up. Computer just stopped working the other day while driving. Fuses all ok. Power is going to the unit, but no output (check engine light, fuel pump, etc.) from the ECM. I swapped out the ECM a spare I carry and it started right up. I let it idle for a bit( < 5-6 min, it did not get hot). I turned it off. Again started right up, no check engine light came on. I backed into the driveway and on the way in it died. Same problem: NO output from the ECM. Fuses OK. I can't even get an error code w/ no power from the unit. Any thoughts on what is happening?

Check the grounds. Harness should be grounded to the frame and and the body. It must also get 12v at the injectors. Weak output from the alternator would do that. Just my immediate thoughts.

Thanks, alt is good, I checked I actually think a loose, goroud seems most likely. I just redid the wiring harness, but left the EFI harness alone, as it is not that old and looked ok.