Yeah, takes a certain voltage to run the engine electronics, ~11.5 volts or more. If the alt is not putting out...
Just as a general battery recommendation, I've had good luck with plain old Costco (now Interstate labeled) batteries, lasting at least 10 years or more. My 91S just has always had the size 27, but while I do happen to have a size 31 in the LS, but I'm sure the car doesn't need that herky battery, it just came with it.
Keep in mind that when the ignition is turned off, the only thing draining the battery is the radio memory, which actually is more than you would think. I always pull the radio fuses in our 164s if we are away for a couple of weeks or more.
If you haven't gone through and cleaned/protected all electrical connections under the hood, including the ignition modules, do it just to be sure. These electrical problems can come and go, as I found out.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6
Last edited by Del; 08-27-2018 at 03:04 PM.