Misfires, combustion quality low, replaced many parts, many tests

[Empireauto]

We are a BMW independent shop. Engine will run 5-10 seconds with scanner set to test mode to run REX. Codes are 125001, 125002, 12500: Combustion misfire & Combustion quality too low cyl 1, 2. Engine will run if Throttle body (TB) unplugged. You will see TB not opened at all, you can hold it open and hear improvement. With engine off and viewing scanner can see throttle pedal position change from 0-100%, Throttle valve sensor 1 (0.84v) & sensor 2 (4.13v) voltages and position % stuck at 16.8% with no change when changing gas pedal. If TB plate never opens it seems logical that "Combustion quality" would be bad. Of course this is a strange code, why not "rich" codes?
Also replaced upstream O2 sensor, smoke tested intake (no leaks present), inspected both turbulence pipes to intake, replaced fuel pump relay. As stated I am running the rex via service mode start but it will not stay running more than 10 seconds unless I have unplugged the TB. I have scanned all systems with an Autel Ultra scanner. Others online have had same problems.
Compression - 240 psi
ignition coils good - visibly compared spark to new coils.
REPLACED: spark plugs, fuel injectors, upper O2 sensor.
Fuel replaced in entire system including fuel rail. Pulled fuel tank down to test fuel pressure inline, got 75psi,
throttle body seems questionable - replaced with new throttle body, no change. Any ideas?

 

[Stevick]

Just brainstorming here, hoping to help. My rough understanding of the REX is that its speed is controlled electronically, so pedal position would have no direct effect on throttle position. I believe I read that the REX only runs at 3 speeds, depending upon the power requirements of the HV system (electric motor draw?). Just guessing, but the 16.8% throttle position may just be the default setting for initial startup of the REX, and as I just noted, it would not be surprising that it did not change with pedal position, as the pedal only controls electric motor speed.

It seems you have covered most issues that would cause misfires, except for timing issues. Hard to explain why you're not getting sensor-specific codes and why unplugging the the throttle body helps though. I had misfires/shutdowns on my REX and my scanner gave a crankshaft position sensor implausibility code. Replaced it and all was good. Were I in your position, I might be looking into the crankshaft and/or camshaft position sensors. The former is a bear to replace, but the latter seems to be easy, and both are inexpensive. If you do have to replace the crankshaft position sensor, take a look at my comments on this post for an avant-garde solution that won't require REX removal and thereby make your customer much happier: https://www.mybmwi3.com/threads/hello-new-i3-owner-with-broken-rex.18768/

Best of luck!