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My Focus is a 2001 ZX3, Manual 5 speed. It has been having problems for a while now, but they have become worse lately. Heres the list:
  • Check Engine Light is on and last read p0171.
  • Car idles at 850, jumps to around 2000, and then bogs down before going back to 850, and then back to 2000. Ocassionally, itll stall out. Sometime it bogs down low enough that my battery light comes on, but jumps back up.
  • Car shakes and rattles.

Here are all the things I have done in the past to fix various problems:
  • Changed the Radiator Hoses. This isnt related but I fixed them New Years Day.
  • I changed the front O2 sensor back in Oct/Nov 2014.
  • The clutch is new as of 2 years ago
  • I've gotten the fuel injectors cleaned.

I dont know what to do. Its hard to drive, and slightly embarrassing. Im fairly car competent and do my own simple maintenance. I ran it and unplugged the MAF. There was no change, except it died when I plugged it back in. Im checking the IAC tomorrow when the sun is back out. I dont have much money or I would take it to my mechanic. Im hoping one of you guys might have some suggestions. I've read the other threads but wanted to get more specific. Thanks everyone. Let me know if you have had similar problems and what you did.
 

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Dat boxer rumble
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PO171 = system too lean

Usually, this is a result of a bad oxygen sensor, which is a relatively common issue with early model foci.

I'd pick up a new 02 sensor, and check your vacuum lines thoroughly for any leaks. Also, take a close look at your MAF, make sure it's clean.
 

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Why on earth would you change an O2 that's just been changed???

Vacuum or unmetered air leak, most likely the rear PCV hose.
 

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Vince your Moderator
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^Depends on the brand of o2 sensor. They are not all a like build quality wise and even the best can fail.
 

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Have heard that but never had a problem. I use Bosch and supposedly the worst. My view though, they INVENTED O2 technology, if they don't know who does? No issues and they last forever.

Not hard to tell the difference, whether sensor or not, most simply spend the money on them though. You can check the response with a propane torch and save the price of the sensor almost instantly. As well, an OBD scanner will show whether good or not too.

Every time I've gone looking I can find a vacuum leak to cure for virtually nothing.
 

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Given the age of your car, a vacuum leak is very likely. If you haven't replaced all your vacuum hoses, it's probably time--not expensive as there aren't many.
PCV hose is a definite suspect.

Shakes and rattles...if it's worse cold, it's likely your engine mounts, specifically the passenger side one, though the others are possible too.

Cleaning the MAF is always a good idea.
 

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Why on earth would you change an O2 that's just been changed???

Vacuum or unmetered air leak, most likely the rear PCV hose.
Have heard that but never had a problem. I use Bosch and supposedly the worst. My view though, they INVENTED O2 technology, if they don't know who does? No issues and they last forever.

Not hard to tell the difference, whether sensor or not, most simply spend the money on them though. You can check the response with a propane torch and save the price of the sensor almost instantly. As well, an OBD scanner will show whether good or not too.

Every time I've gone looking I can find a vacuum leak to cure for virtually nothing.

I went through Bosch 02 sensors like a fat kid in a candy store with my 2000 zx3. Back then at least, Bosch stuff was junk. I make a point of avoiding it.

Also, considering it is an electronic component, it's not entirely possible it arrived DOA, or went bad quickly.
 

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I would assume that a bad oxygen sensor would throw it's own code instead of a system too lean code. Sounds to me like you have a bad air/fuel mix ratio somewhere. But as stated, I would start by checking your vacuum lines and pcv. If that doesn't help, clean your mass air flow sensor.
 

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I got Bosch O2s in two Foci right now and been there forever.

If the bad O2 has simply begun to slow or no switch and stuck low it can register as a lean code.

If the engine is off you can poison the O2s left and right. Excess soot absolutely kills them. Fuel additives can too. As does misfiring. The wrong type of RTV used in repairs will too, but that should be pretty much solved by now.
 
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