So My buddy put the battery backwards in his FC.

He only got the positive terminal onto the negative battery post and it sparked.

Car still fires up but now runs very poorly, cant hold idle.

I checked the fuse panel in the engine bay, they all look ok.

My suspicion is ECU

Where else could this cause harm?

We found an ecu we can order but wanted to get some input here first.

 

Definitely diagnose the problem before assuming it's the ECU. I understand this isn't the best / most specific help but there are ways to determine exactly what the problem is.

 

Lol I know but how do I find it if it isn't obvious(ie a fuse)

 

You need to be a little more descriptive w/your problem. Exactly how does it not hold idle?