No, fully stock car.are ya running underdrive pullies?
Fuel pump, alternator are 30 years old. The filter was replaced last year. So I guess I should measure the alternator output?How old is pump and how long since fuel filter change? It kind of seems like the extra electrical load on the system (and hazard lamps are not that big of a load) is causing a higher than normal voltage drop and the pump is only barely keeping up with it. So it could actually be the alternator not supplying enough amperage to the entire system at idle. Stock alternators were never that good on 79-04 Mustangs to begin with. For example, you can roll up the windows for just a second when they are already fully up and you should see all lights get weak and dim. That's the voltage drop on the system.