Not keeping stable temperature

riceturbo

I'm new to all this setting up of firmware and like a lot of others have been plagued  with heater decoupling whch I have cured with a TEST figure of 0.  I have done PID autotunune and entered the resulting figures,  When I start a print the calls for 230 deg the extruder will achieve the temperature but then when the print starts the temperature drops suddenly and never gets back to anywhere near 230 and I can't work out what's happening or why.  The heater is working as is the thermistor and temperature readings are reasonably accurate.

Repetier

First thing is always to run a temperature monitor like in repetier host/server and enable output view. See if it tries heating full time full power or if there is room for more heater power.

What often happens is that a fan starts cooling the heater and heater has not enough power to overcome this cooling. Or it cools but calibration was done with fan off not considering this important factor.

A good designed extruder should have a fan for cooling extruder if required and one for cooling the print and have not much effect on the heater block. I know that is easy to say and hard to accomplish so some mixing is always present and to some extend this can be compensated. But too much can cause that problem.