I have a 99 Mustang GT.
When I leave the car out overnight and the engine gets stone cold or over the 8 hours or so for work it will no longer start. All you hear is the solenid click in with a single click. But the strange part is if you plug the block heater in for 2 hours then it will start normal. It will then continue to start normal as long as the engine is warm.
The battery is fine, had it tested. I have cleaned the terminals on the battery. Cables don't look frayed or anything. And once warm the start is as quick and powerful as before. Never a non start either once warm, so it isn't intermittant in any way. Starter sounds fine also and if it was going would fail to start when the brushes are in the bad spot.
I checked the coolant. Good for -45 though it was down a little so I filled it back up with more about 3/4 of a liter.
Still, why would warm coolant affect the solenoid or starter?
It's got me baffled. Anyone got any ideas what it could be?