OK, so I thought I was going out of my mind but after a couple weeks of experimentation when my gas level falls below somewhere between 3/4 of a tank and half full I start getting the check engine light and restricted performance. As long as I top the tank off every morning and keep it 3/4 or above I never get the error.
The code is P0121 but as I've explained elsewhere, I'm not sure that's the "actual" code because I lost two ignition coils and several days before each one failed I started getting the P0121 error, then when it finally failed I got the cylinder X misfire and cylinder X ignition coil fault codes.
So my question is, what related to fuel tank level could be causing a fault code? Any help is appreciated. Thanks.

 

OK so I'd like some feed back as to if you think I'm on the right track or not.

The reason I think the I got the TPS (P0121) error with the ignition coils is that the TPS switch is voltage sensitive, more so than other items, which is probably why they are being changed out (throttle bodies) to the tune of $2000 + labor everywhere. According to the wiring diagram I have for a 2000 XJ8 the ignition coils wire into the same terminal on the ECM, EM83. I think that as the coil started to fail it just draws the voltage down on the whole connector and the more sensitive thing (TPS) gives the fault message. Once the injector coil finally failed then it no longer adversly effected the circuit and I got the proper error.

Looking at the drawing again, other items on the same terminal are FTPS (Fuel tank pressure switch?), fuel pump relay, MAFS, Engine Oil & coolnt temperature switches, knock witches, cam and crank shaft position switches.

Based on the problem appearing to be fuel related I'm thinking fuel pump relay or fuel tank pressure switch. Anyone know how to access and test either of those two items?

Thanks,

 

OK Johnny, I'll bite. Yep, I was thinking you were grasping at straws, but you may be on to something. When you have these kind of electrical problems, the challenge can be distinguishing between the cause and the effect. In your case, you have the three seemingly non related things appearing to having some relationship. In addition to your observation, I would suggest that the coil/ TPS relationship could be that a failed or failing coil would cause lower idle, which COULD cause the TPS fault.
I can believe the TPS is voltage sensitive, since it is a voltage divider sensor, but have you any indicatioon that that is a known issue? When you say circuits wire to the same terminal EM-83, do you mean the same multi-pin connector? I do not see a terminal 83 on the ECU. I will keep looking at that.
Now, as to the fuel level; It is possible that your restricted performance is not actually related to the P0121 and the coils. It might be a separate phenomenon. I don't believe the fuel pump current or other operating variable is related much to fuel level. The things I would think relate to fuel level are the sender for fuel tank level and the control of the evap canister valves. I THINK I remember reading that the the evap system operates differently for fuel level over 2/3 full. I am sure that I read that on some car I worked on. So, I will look at that circuit too and get back to you.

 

I'm not sure the TPS thing is a published issue or a known issue, I'm just stating what I've observed, and I've mostly observed it because I continue to drive the car even though I get the errors. I think most people would have given up and replaced the TPS by now, but the $100 code reader works fine for me.

Anyway, I'd say I've finally progressed because now the TPS error is stopped and I'm getting the "real" error codes, P0172 and P0174, banks 1 & 2 lean. It's still happening somewhere between full and half and typically only happens when I'm accelerating. So now I just need to figure out what that means.

Again, any thoughts are appreciated.

 

Yea Man!
I got a solution for that one. That is fuel starvation. (Although nothing is absolute when working on a car!) In my case, it was a plugged fuel pump inlet screen. To diagnose it accurately, get a fuel pressure test gauge and connect it to the fuel prssure test port on the right fuel rail, then position it through the hood and lay it on the windshield so you can see it while driving. The fuel pressure should be about 38 psi and should not vary much as you accelerate. If it does, them you have a fuel pump, inlet screen, or fuel filter problem. Just a wild guess is that the inlet screen would SEEM to be more likely to be related to inlet suction pressure.

Remember, some of what I told you might be partially true.