Hey fellas,
I have a 2002 ba xr6 turbo that’s having intermittent problems.
Firstly I have done valve springs,1000cc injectors,460 fuel pump,billet oil pump gears and backing plate,battery relocation,bigger intercooler and piping, cold air intake,increased line pressure in the BTR.
Car is running a custom tune 330rwkw.
First issue is the car will randomly go into limp mode you’ll be driving and it’s like the brakes have slammed on and D symbol will start flashing and will be stuck in 3rd gear the car will then carry on and jump around and rev it’s like if you went down a gear and high speeds. The car will then stall once I disconnect battery it’s fine for sometimes a day or might happen 10 times a day.

The other issue is the car will randomly die as if the fuel pump is s**t itself but it done it even before I put the new pump in, the car will the stall I’ll go to start it and it will stall it can do this 3-4 now this is where it gets weird when it first stalls you go to start it and it won’t even turn over the reds come on but it’s like start won’t engage I have to take the relay out and put it back in(I have changed the relays)then it will turn over I have to wait a few mins or trying to start it until it’s right.
This car is turning into a lemon I’ve changed so much I done a full service including trans.
I’m thinking now that the ecu is the problem? Or the tune? Any thoughts on what could be the issue

Sounds like a tricky situation, but here’s what I’d check:

For the limp mode and flashing "D," it’s likely something with the transmission. The increased line pressure could be stressing the solenoids or causing electrical issues. Check the solenoid block, transmission wiring, and grounds, especially after the battery relocation. A bad speed sensor or neutral safety switch might also confuse the ECU.

For the stalling and relay issues, it could be the crankshaft position sensor (CPS), as they often cause random stalls and no-start conditions. Even though the fuel pump is new, double-check the wiring, regulator, and pressure sensor. The starter circuit might also have an issue if removing the relay helps.

Lastly, the ECU or tune could be a problem. If the tune isn’t perfect, it might throw the car into limp mode or cause other weird behavior. I’d have the tune and ECU checked by a trusted tuner.

Use a step-by-step approach: verify symptoms, check diagnostic codes, and consult your owner's manual or a professional for guidance.