M278 Mystery: Why a Smooth-Running Engine Won’t Start After ECU Swap

A frustrating situation for one M278 owner: after installing a new ECU, the engine cranks but refuses to fire, acting as if it’s not getting fuel from the tank, a problem that didn’t exist before. A friend ran a scope on the passenger-side cylinders and reported scoring worse than anything he’d seen, with dry compression readings between 120 and 150 psi on that bank.

Those numbers are low, but here’s the twist: the car was running smooth and strong before the ECU swap, with no misfires or hesitation. The only clue was a puff of smoke when lifting off the throttle after hard acceleration, something the owner says he rarely does. That suggests the engine was running fine, just with a little oil sneaking past the rings under heavy load.

So is it time to panic? Not yet. If the cylinders were truly scored badly enough to cause a no-start, compression would be near zero, not 120-150. The real issue likely lies elsewhere, probably in the fuel delivery system. Before writing off the engine and all the money spent on timing chains, cam adjusters, seals, turbos, and that oil pump mod, focus on why the pump isn’t sending fuel.

Check the fuel pump relay, wiring, and the pump itself. Something else is going on here, and it’s probably fixable without a teardown.