Bad battery causing wandering idle?

[ColeNautique]

Hey everyone,

Is it possible that a bad battery is causing my 2001 Nautique GT40 to bounce at idle between 600 and 1200 rpm and then die? It appears to be a lot worse on one battery than the other. I've replaced almost every sensor on the dang thing. IAC valve, TPS sensor, coolant temp sensor, MAP sensor, checked for vacuum leaks, charge air temp sensor etc etc.

Any insight is greatly appreciated!

[MLA]

If you have a 1/1+2/2 battery switch and issue is present in both 1 and 2 positions, then likely not a bad battery, as in those positions, the other battery is 100% isolated from the system.

[srock]

I would be checking my alternator and batteries. I have had issues with the failing of these components show up in the strangest of ways. Especially the alternator.

[shag]

I would bet it is your coil pack. I have seen a 'surging' at idle like you describe in a couple of those older GT40 boats and it ended up being the coils.

[Jonny Quest]

“Loping Idle”

Try doing a search for it on CorrectCraftFan.com. There are several good threads on how to diagnose and correct the Loping Idle issues.

JQ