Help me AEU86, you're my only hope

[Nakilad]

Morning Everyone,

This might be a bit of a rant but please bear with me.

First the question: Can an Engine internal mechanical issue (bottom end) lead to constantly lean fuel mixture?

The context:

The engine had a rebuild around three years ago. Since then it has lacked top end performance. It seems to all intent and purposes that it is starving of fuel above 4000 ish rpm.

I have recently installed an exhaust manifold with 02 sensor port, and installed a narrow band 02 sensor, which gives me around 0.14 volts, no matter where I position the mixture screw.

And what’s been checked / replaced:

Mechanical Parts that have been swapped out for at least one other include: Throttle body, Inlet Manifold, TVIS, Fuel Rail, Fuel Filter, Fuel Pump and even Cylinder Head.

Electrical Parts that have been swapped out for at least one other include: Engine Loom, ECU, TPS, Air Temp Sensor, Water Temp Sensor, Injectors.

Swapping these components has made no noticeable difference to the performance of the car.

Other Checks:

Electrical: voltages regarding temp sensors were checked at the ECU and were between 0 – 5 volts as expected. Disconnecting the TPS or MAP sensor causes engine stall. Disconnecting temp sensors (water or air) makes no discernible difference to O2 readings.

Air Leaks – Hand over throttle body causes immediate engine stall. Blocking of brake booster line has no effect on engine. Blowback from inlet cam cover does not seem excessive, and does not stall the engine when blocked, but does increase O2 readings for a short time if blocked for 30 seconds and then unblocked.

Fuel – pressure and leakdown tests all ok. Individual flow test not done, but injectors were cleaned before the engine rebuild and worked fine then. Second set of injectors (which actually do leak a bit) had no noticeable effect on engine performance or o2 readings.

Compression test was done last year, don’t have the figures to hand but they were all within spec and tolerance. Cam timings have been checked and are aligned correctly. Block was very lightly skimmed before rebuild if I remember correctly.

Engine starts pretty easily, revs ok when not being drive, but idle has been difficult to set (probably related to lean burning).

I feel like I’ve replaced pretty much everything except the bottom end. Crazy as it sounds I am wondering if there could be something there, even though that make no sense to me!

Any ideas, as random as they might be, would be appreciated!