Alrighty, as some are aware (and some are experiencing), the car will sometimes seemingly misfire/stumble under low-load conditions, causing violent lurching. It will go away when you get off the gas, or give it lots more. It WAS totally intermittent and somewhat rare.
Well, this morning was the first morning I had to scrape ice off of the car this season. It started right up and idled fine while I was scraping the windows. After idling for probably a good 2 minutes I hop in and get ready to head to work. Just backing out of the driveway and trying to get to the end of the street was an ordeal.
The car would BARELY idle, fluctuating from 800 to near-stalling. The RPM got so low, the dash lights were coming on like they do in accessory mode. The powerband was virtually nonexistant after 2500RPM. Anyway, the car was still warming up. After about a minute, the car would idle fine again, and the powerband seemed to be back.
Now, the thing would seemingly misfire (seemed more like not at all...no bad noises, just an engine stumble). It would occur while being at just like 1400RPM momentarily to move the car in the metering light line. After it warmed up, it got better.
Now, on the highway...ANY time I was at low-load partial throttle the car would start feeling powerless and lurch violently from the "silent" misfires lol. If I got off the gas, all was well. If I gave it MORE gas, probably a throttle position of 50% or more it was fine...no misfiring. But man, at low-load part throttle it was a real bit*h.
I have been unable to diagnose the problem when it was not as bad, due to its rarity and it hitting me unexpectedly. It has never triggered a Check Engine light. This morning I FINALLY got a little evidence. When it began lurching, I just kept the gas pedal in place and observed my gauges during the problem. The MPG indicator was fluctuating (and seemed to be at a VERY low position considering my foot's position) with my unmoving foot. Soo....that says to me the problem lies in the fuel system. The MPG meter is basically a direct teller of injector duty cycle. If it was saying I was getting really bad mileage and moving around...that says to me that the problem is fuel pressure.
The hypothesis:
Since it was saying that my mileage was very low, that means the injector duty cycle was set very high. Considering the vehicle speed and throttle position, it wuold only do that if the fuel pressure were dropping and it was trying to compensate by opening the injectors longer. So, that would lead me to believe that the fuel pressure regulator is bad. If it was the pump, the problem should get worse as I open the throttle as it puts more load on the pump. The regulator (if it is the case) seems only to be working under high- or no-vacuum conditions.
Anyway, help me out here. Do the FPR's go bad on these cars? Is my theory crap? Shouldn't this be triggering a check-engine light???!!
