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Discussion Starter #1
Need some help tracing out a problem.

I have been having an issue with a miss/sputter above half throttle and above 3500 RPM. I seems to happen in at least 1st, 2nd, and 3rd although it is most pronounced in 2nd. It seems like you can feel the power drop off towards red line and then as soon as it engages the next gear it starts excellerating harder. Shifts are nice and firm and does not seem to be a tranny issue (tranny was just rebuilt March 2011).

So far, I have replaced the coils, wires, spark plugs, MAF, intake charge temperature sensor, EGR Valve, idle air control valve, and fuel filter. Some of these were replaced just for shits and giggles because the car has almost 150K miles. I did notice the car ran better after replacing the coils, however the miss was still there.

I haven't gotten any PCM codes until I replaced the EGR valve. I had the code read and it was a P0405 (Low EGR Sensor Circuit Voltage). I did a diagnostic test on the EGR valve, its wiring, and the PCM and was able to determine that the new EGR valve was bad. I checked the old EGR valve, it tested fine, as did the EGR wiring and PCM (at least as far as the part related to the EGR Valve). So, the old EGR Valve is back in and the P0405 code is gone. The miss however still persists.

The downpipe (and cat) was replaced about 2 years ago with a magnaflow unit (i hit a piece of pipe on the highway and ripped the flex connector at the top of the downpipe). Both Oxygen sensors are Denso units (front one is about 2 years old, back one is about 6 months old). Both O2 sensors were replaced due to failed O2 heater codes.

The only other anomolies I've noticed is that 1) the RPM spikes a bit when the car is first started (Maybe to 2K RPM) and then settles back to ~800RPM for idle (did this before and after the IAC replacement) and 2) every once in a while I'll hear a click and the idle will bump up to about 1200 RPM and then return to ~800RPM.

Beings I have replaced everything on the ignition side I am leaning towards fuel, but at this point, I've sunk a fair amount of money into the beast (I've also put on a new set of Nitto NT555 255/50's) and am looking for any ideas.
 

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Discussion Starter #2
Update....

Car finally started running bad enough that I got a code. P0102 (MAF Low Frequency). So I opted to replace the MAF again and disconnected the battery to reset the computer. Now, the car is throwing a code P0171 (Lean Condition). The car is running a whole lot better now, just concerned about running under a lean condition. It seems like under heavy acceleration you can feel a cylinder miss every once in a while. Its not rhythmic though to make me think its always the same cylinder. I'm going to install a new fuel pressure regulator to see if that helps at all. I checked for a manifold and vacuum leak using carb and choke cleaner yesterday and did not find any leaks.
 

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It may take a while for the PCM to relearn the fuel trims. Give it a few days and a few drive cycles for it to settle in, hopefully it'll run better.
 
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Mr. Handy
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Unhooking the battery cables and turning key to "on" for 10min or so should clear all codes. Therefor engine should run normally. A drive cycle shouldn't be necessary after this procedure.
 
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Discussion Starter #6
Well, it wasn't the fuel pressure regulator either. Left the battery disconnected over night (about 11 hours) and the P0171 code came back after a couple minutes of driving. When I get somd time off of work, I'm going to put a multimeter to the fuel pump wiring and make sure that's good. The only other thing that I can think of is the upstream oxygen sensor. About three years ago, I had a problem with the engine acting like it wanted to die when I was backing out of parking spots or when I started rolling from a dead stop. At that time, I ended up replacing the upstream oxygen sensor with a Denso unit to correct the problem. Intermittently, I am having that problem again. Its been about three years and 60k miles, so I'm thinking that the O2 sensor may be on its way out.
 
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