Welcome to the SeeMeCNC Community

I just ran into some PID quirkiness; G if you have any ideas let me know.

1) Ran a pid tune (so I would be tuned to a new material) and did an M500 to save the tune.
2) I went to print an stl and the extruder would not heat - (the print attempted to restart with a 0 temperature on the nozzle, rather than the set temperature) so I did an emergency stop
3) I rebooted the printer and then the stl printed fine, the temperatures were correct before printing.
4) Temperature is consistent +/-1.3 expect for the point in time where the layer cooling fan turned on (I saw a big move +-5 or more)

Also when PID tuning the hotend fan (not the part fan) was noisy suggesting it was at full speed, more noisy then it is when printing so I am assuming it is running at a low RPM.

Perhaps I should do a PID with the nozzle near the bed with the parts cooling fan on @50% which is where I have it set via the slicer profile.

Noticed that PID turning is better when the nozzle temperature is "cold" at room temperature.

After you run a PID tune, it shuts off the heater. You have to manually turn it back on (or restart the printer) before it'll send power to it again.

The Reprap firmware PID tune is mostly for the initial heatup and making sure it doesn't overshoot too far when it's coming up to temperature. Running it with the part cooling fans on won't really help. No matter how you tune it, the temperature will drop when the cooling fans kick on, but it'll come back up and adjust soon. If it's still got big oscillations in temperature, it might require further tweaking.

Thanks @pouncingiguana

Would you tweak then with M303 and then M301?

Also what do you think is the max retract distance?

You shouldn't really need to run PID tuning when you change the temperature, unless you experience problems.

It's better to tweak with M307 (see config-override.g for the M307 command that was saved when you ran M500):

Reducing D improves responsiveness, but the temperature will oscillate if D is too low.

Reduce A to correct undershoot, and increase A to correct overshoot.

DC42 covered your PID tuning ^

I run a retract length of 3.2mm on a stock SE300 hot end. I wouldn't go much over that. Maybe as high as 3.5, but even that's pushing it on a retract heavy print.