This is an odd one. I'll explain it as best I can, but the whole situation doesn't make a lot of sense to me so it might not to you either.
Car sat for a year or so. Currently it misfires on cold idle, particularly moving at low speed when engaging the clutch. Once it warms up it's fine other than a very occasional small pop. Nothing regular enough that you could troubleshoot by pulling plugs or injectors. CEL is lit and I have a variety of P030X codes.
No vacuum leaks, plugs are all light gray/brown, no cracks in the plug leads. Valve clearance is good. I tried cleaning the minimal carbon off the cap and rotor but that didn't change anything. Fuel pump and filter have both been changed in the last few years, and this car sees only a few thousand miles a year anyway.
That being said, there is a high pitched sound coming from the fuel pump. Just barely audible at idle and not during normal driving. When I depressurized the system to check the pump I noticed the car wouldn't run at all with the pump disconnected when normally you'll have 10-15 seconds with the remaining pressure. Additionally the feed line backfed a lot of gas when I disconnected it from the pump. I've been told a bad pump will do this, where it will not maintain pressure and allow fuel to leak back on the feed line, and that makes sense, except for one thing: the car runs fine when I drive it hard. It accelerates normally and gets right to redline with no issue. Is it possible the pump has bad low speed pressure but it's OK at high speed? Does the pump pump faster relative to engine speed or is it constant?
