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hesitates above 60% throttle and bad shifting

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Old 05-01-14, 11:01 AM   #1
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Default hesitates above 60% throttle and bad shifting

Hey everyone, I'm in need of some serious help with my 2003 IS300. It's an automatic with now 101k on the clock. I recently had a CEL on for about 6 months. My main problem is that the car drives fine at normal up to 60% throttle. Any more than that and the car misses a lot. It won't downshift and jump forward the way I'm used to. You can noticeably feel the car hesitating and the transmission not shifting right. My IS was definitely not running 100%, not even close. I didn't have the money to take it the dealer (we know how costly diagnostic time can get) so I at least pulled the codes and got: P0171 and P0174 pretty generic but at least it provided me a start on where to focus my attn to. My mechanic said that the usual culprits were:
1. bad O2 sensors (which i replaced 2 of the 3)
2. dirty or bad MAF sensor ( i cleaned it and even swapped it for another)
3. vacuum leak of some kind
4. dirty or bad injectors.

Before the problem started, I had replaced my MAF sensor a year prior. I also replaced 2 of the O2 sensors about 6 months prior to the CEL. The only aftermarket items I have on her are air intake and catback exhaust... pretty straightforward to in my opinion. Everything my mechanic tried never got rid of the CEL lights. He tried other MAFs, checking for leaks, rechecked all the O2 sensors, to no avail. Each time he would reset the codes, about 2 miles later they would reappear. Pretty damn annoying. The last thing I tried was to clean the injectors. I added some expensive fuel treatment and drove the car a bit more spirited than i usually do. I did this process twice over a month period. Finally!!! The CEL light turned off on its own!!! I couldn't really believe it, so I continued to monitor the car and gas pedal feel. It did indeed feel better, but only better, NOT the champ it used to be. I'm about to embark on a pretty costly journey it seems like. I hope you guys can help. Changed the plugs about a year ago (good DENSO IRIDIUMS) Plug wires were next on my list of maintenance items to replace, along with the water pump and timing belt due to mileage. I don't really hammer my car, but I definitely expect it to perform when I feel the need for some speed.

Thanks in advance for helping the FNG!!!
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Old 05-14-14, 08:38 PM   #2
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System too lean - how about checking for vacuum leaks?

If none, then next step would be to check fuel pressure - could have a bad pump / regulator...

how about the fuel filter? has it ever been replaced? it is in the tank with the pump, but is available separately...
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Old 05-22-14, 10:44 AM   #3
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I did check for vacuum leaks - none there. I agree that a fuel filter maybe the culprit, but didn't know if they sold them individually or only together with the fuel pump as most companys do these days. I will follow up on that lead. I never thought to check the fuel pump, good idea. Thanks!
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