I have an 89 e30 touring that had an M50B25 NV previously and never had any issues like this. I swapped in a new engine and it's not willing to go past 4000rpm smoothly. Itll go past it but it stutters heavily and clearly does not want to. It does not happen when the car is in neutral or at a stand still. Only under load. I have replaced the following: spark plugs, coil packs, coil pack wiring, injectors, ecu, maf, fuel pump, fuel pump relay, main relay, o2 sensor, ICV. I have a custom exhaust with two o2's in the headers combined to one set of wires. It does not run better with them unplugged. The cat was clogged but I have gutted and cleaned it. It makes no difference either. It has no vacuum leaks either. I need some real help nothing is making this thing better. Yes I've checked the wiring and voltage everywhere. And done a compression test. This problem only started with the new engine.
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