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2000 ZTS Zetec DOHC. Code P0171, STFT up to +30 at idle, LTFT at +20.

I've been over the vacuum system with a fine tooth comb. Vacuum gauge holds a steady 22psi at idle. All hoses and connecters tested, both with ether as well as the old "shove a hose in your ear" test. PCV valve is new, as is the entire hose assembly.

Electrical test of the MAF *seems* ok, voltage moves smoothly with throttle, idle voltage about .7, no crazy jumping around. (I say *seems* cuz I don't actually know what the idle voltage should be.) MAF is clean.

Scan tool shows both O2 sensors doing their roller coaster. (I don't have a high dollar scan setup, all I can see is the voltages and watch them toggle.) One interesting note - the downstream sensor is basically 0v at idle - it only starts to toggle at higher RPMS. (Is this normal?)

Pulling a vacuum on the EGR will kill it, as expected, so not an EGR leak.

TPS resistance scales smoothly from idle to WOT.

As you can see I think I've been pretty thorough, now at a loss for what could cause such a stupidly high STFT.

Next steps anyone? Thanks!
 

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O2 sensor one should most DEFINITELY be switching high/low at warm idle closed loop...............
 

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Discussion Starter #3
Clarification - it's only the downstream that seems quiet at idle. The pre-cat sensor jiggles around constantly, though the idle voltage and deltas are small. I did the test where you spray some ether in a vacuum line and watch the front sensor voltage spike, and it did spike as expected.
 

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I remember my downstream O2 voltage output to be lazy, slow or not very active at idle, so I would not be too concerned about what your seeing. AFAIK downstream O2's are not monitored for fuel-trim adjustments, and a downstream O2 would not set a P0171, says the web, makes sense, so I agree.

I would definitely replace the fuel filter as a next step. Fuel filter replacement has fixed a P0171 more than once.

Result of a quick search here->
http://forums.focaljet.com/mk-i-tec...oo-lean-code-p0171-could-you-please-help.html
 

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Will your scan tool give a fuel pressure reading?

If low it could relate to Marde's fuel filter suggestion.
 

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First things first, you have to understand how the P0171 code is triggered.
It's tripped when the calculated fuel mass is greater than measured by the O2 Sensor. Or in otherwords, the AFR the O2 reads is lean.
So that is where the low fuel volume delivery comes into play. Which can be low pressure or a leak (injector seals).

The bad part of the code is that it relies on the O2 Sensor being in proper working order. So there is the possibility of the O2 being bad and the code being a byproduct.
But it sounds like yours is probably working correctly. If it's old and hasn't been replaced in the last few years, you might want to keep it closer to the front of the list.



On the list of normal suspects based on age, Intake Seals (manifold to head and TB to manifold) and Injector Seals. These do go bad and can be very tough to diagnose. For how easy they are to replace and how cheap they are, just replace them so you know.
 
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