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Still having issues starting after a long sit and outside temperature change. When the day warms the car has a much higher chance of firing. Once it goes, it will go every time after that. I've replaced the distributor cap and rotor, ignition coil, and ignition control module. No change. Fuel flow confirmed. No spark.

I checked the voltage at the coil this morning on the positive and negative sides and got 2.8 volts. Figuring it should be 12. I've only cleaned one ground contact, the one underneath the vacuum lead on the manifold to the fuel pressure regulator. It appears clean. Where are the other grounds I should check? I'm figuring it has to be a loose/corroded connection somewhere. I'd love to know where to look.

The coil has no constant voltage on it.
Even though you can measure between + and ground (car body eg.), that will not give you a trustworthy result because there is no current draw in the coil while the car is not running (assuming you have electronic ignition). And when the car is running, there will be no constant load.

Check the spark. If weak, search further.
Check the resistor, this has a role on start, a cable runs from the starter to the resistor io to supply more current to the coil while starting.
https://www.bmwtechinfo.com/repair/main ... tm/133.htm
https://www.bmwtechinfo.com/repair/main ... tm/134.htm

last, according to this flow chart you might have a problem depending on how/where you measured:
https://www.bmwtechinfo.com/repair/main ... tm/135.htm