Car occasionally loses all power when driving, misfiring, -25% and +25% fuel trim
 

Car is a 2000 Boxster S, with a 3.4 engine and ecu from a 2001 Carrera. Electronic throttle control.

I changed my o2 sensors a month ago, it solved the misfiring issues I had been having. Car has been driving perfectly until a couple days ago.

Out of nowhere, my car lost all power, shifted it to neutral, would not build any RPM with any throttle, and had noticeable vibrations. By shutting the car off and turning it back on, it returns to normal function.

The car has been stuttering, like low power or misfiring, as I build rpm through 2500. Its accompanied by occasional flashing CEL and codes for misfiring cylinders 4,5,6.

I have cleaned my MAF, swapped my o2 sensors around, reinstalled an old one, tried to find any obvious air leaks past the MAF. Nothing. I used an OBD tool and found my short term fuel trim for bank 1 is -25% at idle, and with rpm it goes to 0. Originally my short term fuel trim for bank 2 was looking normal around +/- 1%, but after messing with the o2 sensors it is at 25% following a short test drive.

I don't know if its related, but while cruising a week ago I heard a "POP" noise, didn't think it came my car but instead came from the road, but there are no such things as coincidences.

Any ideas?

Those numbers don't make any sense. Its not possible to consume 25% more fuel and still be at lambda on your nbo2 sensor. -25% is equally unlikely. To get the two together is physically impossible as far as I can reason. To that end, I would start with the nbo2. You should probably check your codes and see what the ECU is telling you first though.

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How many miles are the coils and plugs? If you don't know, it's time to replace those. The vibrations you're feeling are from cylinders not firing.

My Convertible Top Relay Module was recently causing a complete brown out of the electrical system with similiar fault symptoms as you describe. I would try removing that relay module, relay panel is above the driver's legs and see what happens.

I'd be looking at

• spark plugs and coil packs
• fuel pump and fuel sending unit

Either the misfires are caused by a lack of fuel or incorrect/poor ignition. Were the plugs/coils changed when the engine was swapped? How many miles on the swapped engine since last ignition refresh? Does the car sit a lot? Has fuel pump ever been done? The two times I've seen misfires on my '03 S were both of those things. A new fuel pump & sending unit & filter (plus a clean out of the fuel tank due to varnish buildup since it sat for too long with the PO) cured the first, fresh plugs and coils cured the second instance. Might want to check out and clean the throttle body too, just in case there's crud built up from a previous bad AOS.

flashing cel and miss-fires on one side lead me to recommend looking at a fuel injector