I have a P0170 and P0171 trouble codes. The fuel economy is poor (like 9L/100km instead of under 7 where it should be on summer gas), and during warm up, the engine sometimes sputters once or twice. If this happens while idling, the idle RPM suddenly drops very low and the engine runs rough for a few seconds, like it's about to stall (but never does actually stall). If this happens while driving, there is a loss of power: you can floor the pedal, but the car decelerates. Seconds later, there is a sudden surge of power, which literally feels like a kick. This may happen once or twice, and then driveability is just about normal except for a bit of a low idle.
Running Techron through (a bottle of concentrate into a two thirds full tank) has improved the driveability somewhat, and slightly seems to have mitigated the near-stalling problem. That indicates there maybe had been some buildup which contributes to problems like this.
Also, I tried disconnecting the fuel pressure regulator and pinched off the vaccuum hose. There is almost no difference, though I'd swear there is a bit more power at the lower RPMs. When accelerating normally in second gear, it seems more eager. Also, there is usually a certain little ``power notch'' somewhere above 2000 RPM which doesn't seem to be there when the FP is cut off from vaccuum. The near-stall on warmup still occurs, though.
I don't currently have a fuel pressure gauge to be able to investigate pressures.
But is there enough evidence that it may be purely a mechanical fuel pressure problem, (rather than, say, faulty O2 sensor readings?)
Is there anything I can diagnose with OBDII before I shop for a fuel pressure gauge?
I haven't found any loose vaccuum hoses around the intake manifold.
I don't think I have an exhaust manifold leak, either. I had one previously which was so bad, you could smell it from the ventillation; and there was no such code coming up. The driveability was way worse than now. I recently, about two months ago, changed the top O2 sensor (for a working used one, which came from my old broken manifold), at which time I inspected the exhaust for cracks; didn't see any at that time.