Electrical Issue?
 

Hi all:

I've read several threads and think I know the answer, but I'd like your opinions on this specific set of facts.

Car: 1999 XK8, 101K miles, excellent condition overall

Background:

• Prior to Covid, car operated normally
• Stored for 6 months, battery connected, no maintenance charger
• After 6 months, battery discharged, voltage 4.7 volts
• Jumpstarted car, started normally, idled normally
• Began to recharge battery by taking an2 hour highway drive
• Continued recharge using a trickle charger for 72 hours
• Battery fully charged

One month later:

• Battery low, again voltage about 11 to 12 volts
• Turned on key
  • During system check, heard a loud rapid clicking sound from behind/under instrument panel
  • Starter car
  • For first minute, car idles at 3000 - 5000 rpms, clicking sound still present
  • After minute, idle drops to 1200 rpms, clicking less but still present
  • Idle jumps back up and loud clicking returns if I tap on the gas
  • After 5 minutes, idle return to about 800 rpms, clicking gone, cars runs normally

Next actions:

• Put battery back on smart charger for several days
• Battery fully recharged
• Try to start car and behavior described above return this time with many display warnings, for example
  • Reduced performance
  • Engine stability failure
  • Etc.
• Turn off car and restart car
• Warnings disappear but erratic idle and clicking still present
• After 5 minutes, all returns to normal and car drives perfectly for rest of the day
• Overnight, battery lose several one or two 10th of a volt, down to 12.5 or so
• Start car next morning and erratic behavior is back and last 5 minutes

Battery shop runs test and finds

• Battery voltage good
• CCA good
• Alternator output something like 14.5 volts (not surprised b/c it was replaced new about 6 months prior to covid.

Question: What is your best guess as to what is going on. Is this a battery that appears ok, but it is really on its last legs, perhaps it has hard sulphur crystals on the plates, won't hold or just drops two low on first start of the day and just need to be replaced. BTW: the battery is wet NAPA Legend H8 less than 2 years old.

Thanks

Thom

Buy a new battery. Every electrical problem thread will tell you that these cars need a battery in great condition and one that has been drained for 6 months will never fit that bill, regardless of how old it is.

^^ +1

Recipe for a slow battery death. New battery and CTEK is the most likely route back to normal.

I know we have some members who report they can get away with parking up for months on end with no battery maintainer but this is not the majority experience.

Graham

Even if not connected its 100 days to discharge and 100 days to turn to scrap ...so you did that.

Get a new battery and make a plan to routinely drive the vehicle at least 20 miles at highway speed once or twice a week. My wife has not driven her 2006 XK8 in more than a year, yet she wants to keep it primarily as a work of art (astute financial sense has never been her forte). So although I am far too tall to comfortably fit into it and thusly do not enjoy driving it, I make certain to take it out for some spirited exercise a couple of times per week throughout the year. Zero battery issues as a result (it has a three-year old Bosch AGM battery) and the various onboard fluids are kept pumping as well....

Thanks for confirming my guess. Lesson learned with a new battery installed in the car,. There is one remaining issue and I'd appreciate your thoughts. When starting cold after sitting overnight, the car jumps to 3000-4000 rpms for about 30 - 60 seconds. Then it settles down to normal operation. A scan showed no fault codes. I did a hard reset. Any ideas why this may be happening and what I might do to correct it? Thanks

Yeah, I think the replies here might be missing the forest for the trees, or maybe a single tree.

Sure, getting a new battery might be part of the troubleshooting, maybe even the first step in troubleshooting, but it's too dismissive to just leave it at that, especially since even if it's a battery problem, the battery problem might be a symptom of something else going on, like an unexpected drain while the car's turned off.

On the other hand, if the battery is less than two years old, maybe the replacement would be free or almost free?

One thing to remember is that there are different types of battery tester. A different shop's tester might confirm that the battery is dead.

I believe this is to some extent intentional to help the engine and catalytic converters get up to temperature faster, though it might be going too high for too long. Idle air control valve issue?

Sorry to accuse you of lying, but you started up an XK8 and there were no codes?
That's impossible.

Did you do a hard reset and 'teach it idle again'? If not, instructions attached...

This ^^^^^

When you go to start these cars they through a series of (although this might be different model year to model year) adjustments to prepare the TB for running.

Car works fine, then parked for 6 months and battery drains - I feel you are overthinking this. Leaving a car unattended for 6 months will always drain the battery - he's not talking about leaving it overnight and it draining!

Heck, as I read the post, it's not even left overnight! Toward the bottom it sounds like he charged overnight, and with a fully charged battery he still had problems. Yes, this could be due to an already shot battery, but the battery's age and the shop's testdoes cast some doubt on that, even though those tests aren't fully accurate.

So it absolutely could be that a battery replacement would clear it all up, but there's enough doubt in that narrative that I wouldn't be so quick to tell someone to pay $150+ to find out. That might not be a fortune, but it's still into the substantial amount of money area. And anyone with these cars needs to save up for bushings and octopus hoses :)

If they have the cash sitting around, sure, go for the replacement. It's a quick and easy thing to try. Personally, I'd do more troubleshooting, maybe seeing what happens when boosting from an additional battery or jumpstarter, for example, and doublechecking that battery/megafuse connections are tight.