This is weird. My garaged XK8 had an intermittent "no start" issue. It's been without a battery for 5 months. Today I installed a new battery. It started right up. I pulled it out of the garage, washed it, and everything seemed to be okay. An hour later, no start. Update: Sunday 3-23-25, I spent more time in this. With the trunk open, and the interior trunk lights on, the car will start. If those lights are not on, it acts dumb and won't start. If I pull the battery cable and let this sit for 20 minutes and reconnect the battery, if those trunk lights stay on, it will start. If those trunk lights flash on for 1 second, then go off, this car will not start. Also, while it was running, the head lights were on, then at some point they went off, and later came back on all with me touching nothing. Like I said, this is a very weird fault. Any input will be appreciated. Cheers.

 

Deep in your trunk, underneath the massive fuse block is a module for the security system. I suspect this may have something to do with your issue, however I cannot confirm nor deny that.
vital info pertaining to your situation is not clearly stated. What year, model, region (us, uk, other), how old is the battery? Is it a proper group 49? Agm or lead acid? What is the voltage of battery once all connected before you try to start car? How long has it been sitting with no battery? Has anything been done to the ecu? As well as a hundred other other helpful questions. Be as thorough with your information as you can and you’ll be prone to more responses

 

First Law of Auto Electrical..

If operating one system (trunk lights) affects another its an earth fault.

 

Thanks for your help. It's a 1997. New battery. Been in the garage over the winter. Last driven in September 2024. USA model. Static battery voltage is 12.4v, running voltage in the car, is 14.3v. Nothing else done to any electronics other than a new alternator last June. The alarm system stopped working a few years back. Since this auto is so seldomly driven, I never pursued that issue. Thanks again.