The EGI fuse in the engine bay of my S5 TII blew yesterday on my way home from work. I replace it and the car started for a sec and blew the fuse again.

I have owned this car for over a year and taken it on various trips ranging from 2-4 hours and never had an issue. But today I was driving home from work and went to shift from 2nd to 3rd and the rpm’s went straight to 0 and would not start again. (It would crank but not start). So I opened the hood and started investigating. I checked the fuse box in the engine bay and saw that the EGI fuse was blown. Had my wife bring another one and it started again but only briefly before blowing the fuse again. I’m thinking maybe a short somewhere? By why now? Seems so random.

I did recently replace the radiator fan and shroud as well as upgrade the alternator to the 100 amp FD one. Would this have something to do with the fuse blowing?

 

You likely have a short somewhere. Shorts occur for all kinds of reasons, especially in cars as old as ours. The wires could have rubbed against the frame and shorted out, insulation could have cracked and fell off, a fastener may have fell out, etc... Disconnect the battery, start with a multimeter and check for shorts. You can find the factory service manual and wiring diagram on Foxed.ca. Go through those and see if you can track down the issue. Eliminate the simple causes first and then let us know what you find.

 

These cars are old.
Years ago I had a weird problem where if I went up a hill one of my important fuses would blow.
Turned out the chassis harness (car had 140k miles) just eroded from all the engine bay heat. While replacing the harness with a low mileage (70k) replacement, I did find the short since it was visible due to the harness wrap being eroded and exposed wires visibile. I ended up replacing all of the wiring while I was in there from the low mileage vehicle.

 

You definitely have a short somewhere. I'd start by looking at all the solenoids/output devices (e.g., fuel injectors, emissions & boost solenoid, etc.) that have a hot +12VDC supplied when you key on and get grounded by the ECU.

Years ago I had the same blown EGI fuse symptom on a drive a few days after I had done some work involving removing the IC & UIM. Turns out I forgot to reconnect that connector to the AWS (accelerated warmup solenoid); that's the one that causes the high idle on a cold start. Anyway, the loose cable/connector managed to dangle just enough to hit my downpipe on the drive, partially melted & shorted to ground, which blew the EGI fuse.

 

this ^

it's worth looking around your exhaust system for melted wires or connectors. here's a 2-day-old-thread that concluded with a similar short against a hot downpipe: https://www.rx7club.com/2nd-generati...-fuel-1166839/