Ok I just bought a 89 RS but it has a 90 tpi motor. When I bought it the guys was replacing he fuel pump since it didn't start. He came to find out that wasn't the issue. After some more troubleshooting it came down to it keeps blowing the 20 amp fuse on the passenger side by the battery. Changed the pump relay and it seemed to be working just fine,until the other day. Is it possible that the oil pressure switch could be shorting out? It seems to start and run fine cold only after it warms up does it pop and I noticed the oil pressure gauge bounces when it's a idle.

 

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Only one way to find out , test the pump circuit it with a DVM
Anything downstream of the fuse is a suspect