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89 Buick Century 3.3L only runs sometimes


Buick Newbie
Buick Ownership
Buick Century
Hi. I bought a 89 Buick Century which had a broken starter from people trying to diagnose something beforehand and killing the starter in the process.
After i replaced it the car wouldnt wanna start. It only fired now and then one cylinder if you gave it full throttle, backfired, and it completely flooded out the spark plugs and the oil with gasoline (Also from people trying before).
Here is what we did so far:
-Checked / changed spark plugs and spark plug wires with OEM parts
-Switched out ignition module and coils with proven working ones
-Checked fuel pressure (Spot on where it should be)
-Confirmed a working fuel pressure regulator
-Checked idle air valve
-Checked resistance on injectors
-Checked fuses
-Switched out ECM's.

And now it gets interessting. After we switched out the ECM's (Fuel pressure regulator was stil bypassed by that time), and disconnected the coolant temp sensor, randomly the fan kicked on while the ignition was turned ON, you could also hear the relais clicking on, off on off on off, like if there was a short. At some point the fans stayed on completely. Then we tried to start again and it suddenly wanted to run, and after blowing out all that crap it ran smooth and perfectly fine. We shut it off again and restarted it without any problems. (Key on off, fan turns off, key on ON position, fan turns on).
Then we switched out the ECM's again and it stopped running again, and the fan wasnt kicking on anymore with ignition in ON position either. So again, we switched back to the ECM it was working it before, but the fan didnt turn on anymore, and the car didnt start anymore.
After a few tries the fan kicked on randomly again, and now the car started normally again and ran fine.
We reassembled the fuel pressure regulator, and it ran perfectly with it as well. Then after shutting it off, the fan stayed off, and the car wouldnt start again.

What could this be? We also noticed the coolant warning light on the dashboard is not illuminating at all.

I hope someone could give me some informations and help to diagnose this.
Pedal Commander