I actually shot this video earlier to shame my neighbor who insists he needs that silly block heater, it's 8 f out here today.
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I agree, Battery, clean cables, (big 3 upgrade if possible), should get you back where you need to be.
I shot a video of mine giving all it has to start at -17C... but at least it did start on it's own and without a block heater.
The cables are new (1.5y), they were replaced at the same time is the starter. The battery poles and connectors on the cables are clean too. I suspect the PO put in some cheap battery, he didn't drive it much during the 7 years he had it...
What's a good CCA? I looked at one battery and it said 710 but I don't know if that's good or not?