Ok, here's the situation. 1998 Olds Bravada 4.3L. Keeps blowing the ECM-B fuse.
1. It blows the fuse after about 10 min run time
1a. While actually driving, it drives pretty bad. Lack of power, stumbling, hesitating.
2. Even with key out, the fuse will keep popping once warmed up.
3. Changed Delphi Fuel pump. Turns out I probably didn't need to. The newer connector was added the first time.
4. Circuit 440 is for the fuel pump. Pulled the no.21 pin from the harness, as I had believed from limited info. that it provided constant power to the ECM for long term data storage. Apparently this also activates the relay and other stuff, because even if I bypass the relay and turn the fuel pump on the fuel gauge doesn't work and it still won't start. Plug #21 back in, starts immediately. So #21 does more than just provide power to the ECM. But I can't find out where it goes from there inside the ECM. Some system once it warms up blows the fuse and contributes to it running like crap.
Here's a link to diagrams. Index of /gearhead-efi/wiring/16250279
5. It's not the fuel pump or harness post relay. It blows fuses without the relay being present.

 

Hi, my factory manual for 1998 Bravada shows a slight difference in that there is also a circuit 440 branch going to the oil pressure sending unit. I have attached the schematics. If you have more than 1 wire going to your oil pressure sensor, then it would apply. Another place (back to underhood relay center) to look for a short. Good luck and best wishes for success. Also note that circuit 120 could also be a possibility when the oil pressure switch is closed.

 

So you have an intact fuse at the time that you remove the key from the ignition and it blows after removal?

George

 

Oh wow, this definitely gives me another area to look. Thanks you!!!

@GeorgeLG So the order of events was I have about 5 minutes, maybe more of run time. The Bravada was acting weird and I decided to head back home. Just after turning, it died, so I pulled over. Changed the fuse and thought maybe each fuse gave me 5 minutes. It died within 20 seconds and I luckily got it off the road again. Then I went to put in another fuse, and it blew the very instant it touched the holder. Key off.

 

I see a few different strategies to find the issue:

1. Have a butt load of ECM fuses on hand. Get the truck warmed up and blow the fuse. Turn of the ignition and verify that it will blow a replacement fuse then pull the fuel pressure relay and try again. If it still blows the fuse pull the VCM connector with 21/C3 and try again. Same with the oil switch
2. Another approach is to isolate these three wires (FP relay, VCM pin 21 and the oil pressure switch and put a ammeter clamp on them looking for something running too hot and near the ECM fuse limit
3. Follow Les' advice and unbolt and lift up the UHFB looking for corrosion or damage
4. Follow the cable bundles for all 3 circuits looking for corrosion or damage

George

 

UPDATE.
So I bought a 3 wire oil pressure sensor and started the journey to change it out. Wasn't too bad if you have some brains and patience.
Aaaaaaaaand the verdict...... that was the problem!!!!!!!! I just got back from a good test drive and it ran great, no hint of an issue or stumbling or anything.
Wow guys, thanks for all the help.

You can see the old one was corroded and broke in the sensor socket. So it must have shorted out once it got hot and at the same time gave the computer some sort of fits and why it stumbled and ran weird before it eventually fully shut down.,

 

Nice work, glad your sorted.

George

 

Thanks. Just wanted to make sure @LesMyer knows his reply is what fixed this issue. Thanks for taking the time to help!!!!!!!!!!

 

You are welcome. But all I did was provide a factory schematic after looking it up and noticing the circuit through the oil pressure sender.. I never trust all those generic ones you find on the Internet. They have a way of slightly changing.