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Reply To: PO171 (too lean) case

vincenzo masiellovincenzo masiello

    This place looks like totally dead or some kind of deserted town in some future dystopia scifi movie or something, but I’ll add an update just in case someone may use the experience or someone may like the challenge.

    I did a water spray test in all the common places that I’d expect a vacuum leak but did not see a change.

    O2 sensor reading (both upstream and downstream) seem to be normal with a little bit of lean indication.

    Pinching the brake booster vacuum hose lowers the short term trim to normal (eliminates the lean condition) and causes the the long term to eventually go down. Keeping the brake pedal down to the floor has the same effect of fixing the lean condition on the fuel trims. Blocking the booster’s vacuum hose, on the other hand stalls the engine!!!

    Is that normal?
    Does it indicate a vacuum leak inside the booster or elsewhere?
    I took the check valve between the booster and its vacuum hose out and it feels like it works (sucking and/or blowing in one direction).
    I held the brake pedal down for about a minute while the engine was running and then shut it off and felt that the pedal was pushing my foot up with a moderate force.