Just recently purchase a partially restored 71 Mustang with an aftermarket 460 crate engine And a C6 transmission. Most of the restore has been on the outside of the vehicle and its in good condition with very minimal to no rust. Some of the electrical work leaves something to be desired though.
So I noticed an issue that when I had any electronic device in the car turned on (headlights, A/C, radio) the cars temp would begin to climb and at idle the rpms would drop and you could hear the 2 electric radiator fans rpm drop as well. Figured out it was a 75amp alternator that was going bad. Replaced with a 1 wire 140 amp alt, and took it to a local shop to have them look the vehicle over.
They found exhaust leaks in the headers on both the driver and passenger side, turns out the gaskets were manifold gaskets cut to shape and installed upside down. Had a small oil leak due to a bad cap and vacuum hose. And had a coolant leak around the thermostat housing. They fixed all of those issues and installed an overflow tank for the radiator so I don't lose my fluids anymore and replaced my 192 degree thermostat with a 160 degree one.
Well after I picked it up from them I noticed that the car would start overheating after about 8-12 miles. Both the dash gauge and the radiator cap were telling me it was hot. So I was careful with it. I ran the car at temp with the radiator cap off so I could see if coolant was flowing in the radiator and it was. I took it back to them and they said they couldn't get it to overheat. I picked it up from them and drove it 24 miles without overheating. But then I filled it with gas, went home to pack for a wedding, and got on the highway. After about 12-20 miles the temp started to climb again, and then my fuel pump (or fuel filter in the gas tank or the fuel line) gave out on me and I was stranded and had to get towed home. I've heard that overheating can cause fuel pumps to fail.
I guess my questions would be what do you think the problem could be? I feel like the water pump is still working at least at low rpms. Is it possible the thermostat is bad and that it gets stuck sometimes but not all the time? Or am I just not getting rid of enough heat? I know its hard to diagnose from the internet but any ideas would be helpful.
Edit: The fans are always on btw, they come on with the acc power.