86TpiTransAm

About 3 months ago my 91 (3.1L V6) started stumbling and hesitating and eventually wouldn't start at all. I replaced the ignition module thinking that was the problem but it didn't fix it so I had it taken to the shop. The check engine light never came on so there was no codes. The shop tested the ECM and said the injector controller in the ECM was bad. They couldn't find anything else wrong so they replaced the ECM with a new one from GM. Everything seemed fine for the last 3 months or so and then about 2 weeks ago it did the exact same thing. I had it towed to the shop again and they said that one bank of injectors was getting a ground from the ECM. They said it was just like before but when they put the test light to the ground on the ECM, the engine fired right up and stayed running. So they shut it off and it kept restarting every time they tried. They went ahead and replaced the ECM again but the problem returned while they still had the car. They pulled the wiring harness apart and checked all of the wires for broken or loose wires but found nothing. They again put the test light to the ground on the ECM and got the engine to fire up. Then, for the next week and a half, they started and stopped the car about once an hour and it fired every time. 3 days ago they put the wiring harness back together and bolted the panels up and it still started. The next morning they started it again and called me to come pick the car up since they couldn't figure anything else out. (They didn't charge me for anything) So I picked up the car and drove it home and didn't have any problem. Didn't drive it for two days and then went out today and it wouldn't start again.

What would cause just one bank of injectors not to ground? I understand that the pickup coil/ignition module sends a signal to the ECM to ground the injectors right? If it were the ignition module (which has been replaced) or the pickup coil, I would assume that none of the injectors would be getting a ground from the ECM right and not just half of the injectors?

Anybody that has had this problem before or anyone that has any suggestions at all, any input would be greatly appreciated. This is really baffling myself and the guys at the shop.

Thanks in advance!