I've posted about my engine problem earlier this year with my Mercuiser 5L MPI. That turned out to be a bad fuel pressure regulator. On a test cruise after that was fixed, I ran into another problem with the engine stalling when I tried to start it after three days docked. It stalled while the rpm fluctuated up and down, on a down cycle. I ended up replacing the IAC (idle air controller) and that may have helped but I'm still hearing rpm fluctuation during idle. I also got my hands on an engine scanner, and recorded some some data while I started the engine. I'd like to get some opionions from you guys if it looks "normal"
On the attached plots, I'm showing battery voltage and engine speed. The engine rpm is oscillating up and down like it did when it stalled on me, but in this case (in my driveway) it didn't stall. Is this kind of rpm behavior normal for an engine at idle? Or should it be more constant at 600 rpm?
Also you'll see the battery voltage drop to near 8 volts during startup. That could be normal since the current draw goes up so high during cranking, but could it be an indicator of a weak battery?
Finally, you can see the spark advance in the second plot fluctuates alot during idle, from 2 to 15 degrees. Is this normal?
I welcome any opinions and insight, as I'm trying to learn so I can diagnose this engine myself as much as possible. Marine mechanics are not very accessible where I'm located.