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Discussion Starter · #1 ·
I have a 2002 Legacy 2.5 that has an idle surging issue. Here is the breakdown:

Engine was running rough and surging at idle and would eventually stall. It needed head gaskets, timing belt etc. so I pulled the engine and did all the work. All went well and after putting it back together it still has the same surging issue which is not surprising. Before the tear down I got P0171 - Fuel Trim; Bank 1: System Too Lean, I now get B1S1 oxygen sensor circuit malfunction.

I replaced all gaskets (including exhaust and intake gaskets)
Upstream o2 sensor (New Bosch)
Spark Plugs

No timing issues
AICV seems to work correctly and is in decent shape
Throttle position sensor functions
Throttle body and butterfly valve and clean
PCV valve functions and is relatively clean
Idle speed is correct
Aux air injection unit works (I bypassed it and no changes in idle issues)
Air vacuum solenoid under the passenger side intake bank seems to work
Replaced Fuel filter
Fuel system pressure within spec

Basically it seems like a vacuum issue. I tried the usual carb cleaner tactic with no change. Next I pressurized the intake manifold to about 15psi and the only leak I could find was through the seal on the throttle body that exposes the pickup for the TPS. The shaft has a little play and the seal defiantly leaks. Thinking this may be the problem, I put a large amount of grease over the seal to stop any vacuum leaks to no avail. The manual states the intake manifold should be at about 60kpa. I am getting a reading of ~20kpa at idle and at a steady hold at any RPM. Also just for kicks, at idle the upstream o2 sensor gives me 1.4ish lambda and 2.2ish volts.

Any ideas?
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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16,315 Posts
Its running rich. Check the MAP sensor connections. Check the IAT sensor. The 20 kPa is telling the computer that the engine is pulling vacuum at a rate equal to the current Barometric pressure minus (-) 20 kPa. That's a lot of air, even at sea level, so the ECM increases the fuel as if under load and you have your foot on the floor. By the time the AF sensor sees what's coming out of the engine, The ECM sees rich but keeps fuel flowing based on a calculated air flow. So the ECM determines a failure in the AF sensor. Too much fuel and not enough air means flooded cylinder and incomplete burn. Surging.

One or both of those sensors is failing. Or, you didn't get all the ground lugs attached from the engine harness to block and firewall.
 

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Discussion Starter · #3 ·
I unplugged the intake manifold pressure/temp gauge while the car was running and on the colder side. It brought the revs up a bit but seemed to smooth out temporarily, then back to rough idle. After plugging back in the whole idle problem seems to be marginally better. I checked the data and now I am getting more like mid 20's KPA and the air temp seems to be correct at about 33 degrees C at full engine temperature. I suppose it could be a bad sensor, but connections seems good. I did happen to take it out earlier and clean it, but generally looked good...
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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I don't often look at kPa for pressure. The scan tools I use default to psi.

With 20 kPa at idle, that relates to approximately 3 psi. Given the engine should be pulling about 18 inHg at idle, that transfers to about 8-9 psi or 55-60 kPa. The ECM is fueling the car for dense air or high volume how ever you want to look at it and its flooding the combustion with fuel.

Replace the MAP sensor.
 

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Discussion Starter · #5 ·
Well you guessed it! Faulty MAP sensor. Tested with vacuum gauge and read very different than OBDII readout. Replaced today and smooth idle. Thanks for your help!
 

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2005 Legacy 2.5i Wagon 4EAT; 2005 Forester XS 4EAT
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Mine threw a P0171 and something else (P03-something, I think) shortly after an oil change and tire rotation the other day. Everything seems normal. My buddy suggested clearing the codes to see how long it takes for them to come back. After 400 miles +, still no light.
 
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