1999 XK8, 80,000 miles

My xk8 exhibits all the classic symptoms of a throttle body with problems - 4000 rpms at start, system takes 1 minute to calibrate TB at start, hard acceleration triggers restricted mode, accelerates with foot off of gas, etc. All these symptoms are intermittent and occur randomly, hard to reproduce, scans reveal no codes.

Here's a bit of history:
1. March 2020, Covid arrives and car sits 6 months. Battery drains, car wont start, recharged to full, car then starts and runs normally for the next 2 months,
2. Car sits parked from October 2020 to Feb 2021. Battery drains again, recharges, go to start car an all the crazy behavior begins. Try to sort the problem for the next few months by replacing battery, doing hard resets, etc. Car acts up until shop clears codes even though none are shown. Car runs ok for the next 8 to 10 months.
3. Car parked from March through mid May 2022. Go to start is two weeks ago and all the wacky TB behavior returns. Bring it to shop. There is an ABS code present, but nothing related to the TB or engine management. Shop clears codes and the wacky behavior goes away.

Here's my question. Could it be that the long periods of sitting idle drains the battery where the ECU gets screwed up and whatever systems are associated with the TB don't work until the shop does a full reset. This makes sense that once the battery if fully back up to charge and the system gets cleared, the engine management system behaves as it should.

What to you think of my theory? Thanks

Thom

 

Thom, yes if there's a drain. Example: ignition key slot, gate stuck in up position = car thinks key in ignition will not sleep.

First thing I'd do is: all doors closed. Key in pocket. Disco + battery wire. Put ammeter between battery post and wire terminal. Measure current draw. I'd have to look up normal but pretty sure its less than 0.3amps (ttytt that seems too large. Come back with #)

John

 

Why dont you put a battery minder on it when its not in use ....

 

Make the time to drive the car on a regular basis. You risk a number of issues if you do not, including your fuel pump deciding to die on you....

 

Buy a brand new battery of the right size.

I know everyone on this site harps on that. I always figured a charged up battery was just fine, but recently found religion.

I had a marginal battery in my car for the last couple of years and as its marginal goodness waned, began to see more and more problems such as rough running, tachometer that didn't work, transmission faults, engine faults, etc. (I bought the old battery from an independent battery shop to help out the old fellow who ran the place and though he said it was the right size, it sadly, it didn't have enough oooommmph.)

Put a new a battery in the car from Advance Auto and miraculously the car runs great and the tachometer/transmission faults that popped up regularly seemingly have disappeared. It's not a panacea, as I'm still chasing some trouble light issues, but it really transformed the car. Incidentally, I've bought batteries for other cars from Advanced before. They don't always quite make it to the end of their end of expected life, but the store has always cheerfully pro-rated batteries that didn't quite go the distance - as sometimes is the case. So, if it is ok to make an endorsement, I like Advanced.

Now, it might be that that you have throttle body or other issues, but don't discount the importance of the correctly sized powerful battery in these cars.

My 2 cents.

 

`What many forget is that once the engine is running everything is powered by the alternator and the battery is just dead weight waiting for the next start.
Its letting the battery sit for long periods and the voltage drop is the killer.
Every 6 months you should charge the battery to 15v as the 14v given out by the alternator only gets you to 80% of its ultimate capacity. The green eye only changes to black at 50% charged so green means 50% +

 

Yesterday, I installed a new throttle body and gasket sourced from Jaguar. If the first 100 miles are indicative, problem solved.

BTW: battery was replaced when the problem occurred intermittently. Nothing changed.