Have an '84 245 that refuses to start when the ambient temperature drops below 40F. We've had a few cold nights these last few weeks where the temperature at 5:00 AM when I left the house for work was in the low to mid 30's. The car cranked strong but would not start.

Went out later in the day when it had warmed up a good 10 degrees or so, and she started on the first turn of the key.

I've replaced the fuel pump relay and the main FI fuse. Also rotated/cleaned all of the ceramic fuses in the fuse box. Cap, rotor, wires and plugs are about a year old. Battery is an Interstate that was installed in late 2016.

Any and all ideas appreciated.

Hi,

My first guess from the read is this is a classic overnight loss of rest pressure equaling not having a full volume of gas in the fuel rail.
In most cases the Fuel Pressure Regulator as it returns gas directly back to the fuel tank. Leakage into the engine from injectors is rare and you are not claiming a rough running issue during warm up richness.
If you were, the FPR usually leaks back through a vacuum hose and back into he intake manifold.

Having the drop in temperature does shrink the overall volume a tiny bit. The size of the fuel filter/accumulator, next to the pump, I allows for this. So would chase another rabbit instead of concentrating on temperature being culprit.

I got your rabbit hunt starting right here!
This car uses two separate silver can relays. The relays can become lazy or flat out quit with age.

One of these is for the fuel pumps and one covers the injector power circuit.
On later years this got combined into one and is called a system relay. They are know to fail on one side or the other due to bad soldering or whatever.
There is no reason to think that these two cannot not suffer the same ills!

What is helpful is that they are interchangeable on their sockets.

On one you can hear the pumps run and shut off! This happens right after the engine cranking stops for about a second long and this is normal.
If the injector relay does not close regularly, this you will never know because what it controls is a totally silent during its operation and “So will” the engine stay, until it turns on!

Switch them around and keep listen for the fuel pumps to operate the same as they did before.
This verifies both relays to be operating normally.
If a relay now delays turning on the pumps, because you don’t hear the pumps shut down, you have found what stops the engine from hitting, because it did not turn on the injector circuits.
Maybe the cold could have something to do with?
Get use to listen for the pumps to hum!

I know it’s A Two rabbit possibility and maybe, just maybe, you can catch one out of its hole?

(:-)
Phil