My 03 XK8 with 79k miles on it had been in a garage for the winter since November. I take the plates off it in the winter and when the pandemic hit, my DMV wouldn’t let me renew the registration until a few weeks ago. Given the car sat for ~7 months, I’m wondering if that’s part of my problem. Anyway, about a month ago I got an engine coolant low error message. Refilled the coolant tank, and all seemed fine. Then again last week I got an engine coolant low error, but then it quickly went away and changed to restricted performance and the CEL light came on. The restricted performance light only stayed on for about a minute and then turned off, but the CEL remained. I haven’t seen the low engine coolant light since. I got the codes read and P0171 and P1111 were thrown. I drove the car again yesterday at highway speeds for about 15 miles and while the CEL was still on nothing else came on and the vehicle’s performance seemed normal.

My questions are:
1) I wouldn’t think the coolant issue and the 0171 codes are related. Is that correct?

2) Given I got the 1111 code, am I alright now, is whatever threw that 0171 code fixed?

3)Is old gas causing the 0171 code possible? I only put high test in it and I did use fuel stabilizer before storing it for the winter but there’s still about half a tank of old gas in it and I recently put half a tank of new high test in it.

4) When the problem is resolved will the CEL reset itself or does it need to be cleared with an OBDII reader?

Thanks for any help you guys have for me.

 

Well if you cleared the codes, and got 1111, that's okay, 1111 really isn't a fault code to worry about.
If the 0171 comes back then deal with it. Don't think fuel has anything to do with it.
A lot of problems can be battery related, but check for leaks, particularly the norma connections around the valley hoses at the back, the octopus hose, the base of the coolant tank, and of course the water pump for tell tale water mark behind the pulley. Whip the belt off and have a look.
These are the common places for coolant leak.

If the P0171 is combined with the P0174 code, it's very likely that the problem is caused by an intake leak. Very often the concertina section of the intake pipe can fracture underneath with the heat from the exhaust. Take if off and have a close look. If there are no intake leaks, the next step is to replace the air filter and clean the air flow meter. A faulty MAF sensor, can cause this code as it may just be outside the limit, and there can be variations between banks that leaves one within limits and one bank outside the limit. If the problem persists the front oxygen (O2) sensor may need to be replaced, but this usually throws another code.
Get a simple bluetooth code reader ELM for instance, and read codes on your phone. Then you can reset them if you're out and about, and see if they come back.
You can also monitor and record your fuel trims, your fuel pump pressure, and so forth so you know what's normal. That way you will recognise fuel and mixture issues.

 

I'd say don't throw money/effort blindly at the problem. P0171 is air leaks. In more details, the fuel system has to add 25% more fuel to what the air flow meter reports. Do a smoke test, i.e. inject low pressure smoke in the intake and see where it comes off. Likely intake tube, VVT seals (see oil?), dip stick o-ring, etc, etc. P0174 is for the other bank. As already pointed out, get a cheap ELM327 and read the long term fuel trims. Chances are they are high on both banks, but not quite high enough for a P0174. Ideally, trims should be zero, but single digit like 5% or less is great. And yes, the engine will feel tons better when these trims come down.

If you insist on doing something yourself, start by cleaning the AFM with a dedicated spray. The AFM is in the cavity of the sensor, the visible "pill" is the air temp sensor. Use a lot of spray, fill the cavity, let it sit. A casual spray will not do it. Double check the o-ring at the base. Keep your expectations low that it will fix this permanently. Fortunately the 03+ XK8s use a very common AFM, so they are pretty cheap.

Best of luck, keep us posted.