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P2187 — System Too Lean at Idle (Bank 1)

P2187 is set when long-term fuel trim on Bank 1 goes excessively positive (typically beyond +20 %) specifically at idle, but stays closer to normal at part-throttle and cruise. The distinctive pattern — lean only at idle — almost always points at a vacuum leak rather than a fueling problem. At idle, intake manifold vacuum is at its highest, which amplifies the effect of any leak; off-idle, the leak path becomes a smaller fraction of total airflow and trims look fine.

P2187 means system too lean at idle (bank 1). A vehicle usually stays drivable short-term with this code, but it should be diagnosed promptly. The most common cause is vacuum leak (intake gasket, brake-booster hose, pcv, throttle-body gasket) (typically $80–$600). Causes and cost vary by make and model; confirm the root cause before replacing parts.

Severity: moderate powertrain Safe to drive (short term)

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What does P2187 mean?

P2187 is set when long-term fuel trim on Bank 1 goes excessively positive (typically beyond +20 %) specifically at idle, but stays closer to normal at part-throttle and cruise. The distinctive pattern — lean only at idle — almost always points at a vacuum leak rather than a fueling problem. At idle, intake manifold vacuum is at its highest, which amplifies the effect of any leak; off-idle, the leak path becomes a smaller fraction of total airflow and trims look fine.

What are the symptoms of P2187?

What causes P2187?

Cause Likelihood Estimated repair (USD)
Vacuum leak (intake gasket, brake-booster hose, PCV, throttle-body gasket) — P2187 is essentially "lean at idle = vacuum leak" until proven otherwise. Most common $80–$600
Stuck-open EVAP purge valve introducing fuel vapor unmanaged Common $80–$300
Cracked plastic intake manifold (Ford 4.6/5.4, Chrysler 4.0, GM 3.6) Common $250–$900
Failing PCV valve or hose Common $30–$200
Failed or biased Bank 1 upstream O2 sensor Occasional $150–$450
Throttle body gasket leak Occasional $30–$200
Carbon-coked throttle body letting blade hold slightly open Occasional $20–$200

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P2187?

In most cases a vehicle stays drivable for short trips with P2187 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P2187

  1. Compare idle vs cruise fuel trims

    Read Bank 1 LTFT at idle and at 2500 RPM cruise. P2187 with LTFT around +20 % at idle but near zero at cruise is the classic vacuum-leak signature. P2187 with high trim at all RPMs is a broader lean condition (likely P0171 alongside).

    Tools: Scan tool with live PIDs

  2. Smoke-test the intake comprehensively

    Pressurize the intake with smoke through the throttle body. Watch every gasket, hose, vacuum tee, the brake booster line, the PCV system, the EVAP purge line, and any unused vacuum ports. The leak path is somewhere in there.

    Tools: EVAP / intake smoke machine, Flashlight and mirror

  3. Inspect the PCV system

    The PCV system is a common P2187 cause — particularly the hose from the valve cover to the intake. Aged rubber cracks and PCV valves stick open. Pull the valve and shake — it should rattle. A non-rattling PCV is finished.

    Tools: Common hand tools

  4. Disconnect the EVAP purge to test

    With the engine warm and idling, disconnect the purge valve. If the rough idle smooths out and LTFT drops, the purge valve was stuck open. P2187 + idle improvement on purge disconnect = purge valve.

    Tools: Hose disconnect tools

  5. Inspect the brake booster hose

    A failing brake booster diaphragm or hose is the single most overlooked P2187 cause. Pull the brake-booster line off and cap the manifold port. If idle smooths out, the booster or its hose is the leak.

    Tools: Hose plug or vacuum cap

How do I fix P2187?

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P2187: frequently asked questions

What does diagnostic trouble code P2187 mean?

P2187 is set when long-term fuel trim on Bank 1 goes excessively positive (typically beyond +20 %) specifically at idle, but stays closer to normal at part-throttle and cruise. The distinctive pattern — lean only at idle — almost always points at a vacuum leak rather than a fueling problem. At idle, intake manifold vacuum is at its highest, which amplifies the effect of any leak; off-idle, the leak path becomes a smaller fraction of total airflow and trims look fine.

What are the symptoms of P2187?

Check Engine Light is illuminated. Rough or unstable idle, especially when warm. Possible hesitation or stumble immediately off-idle. Stalling at idle (worst cases). Slight reduction in fuel economy. May be paired with P0171 (general lean Bank 1) if the leak is large. No drivability issue at cruise

What causes P2187?

Vacuum leak (intake gasket, brake-booster hose, PCV, throttle-body gasket) (most-common). Stuck-open EVAP purge valve introducing fuel vapor unmanaged (common). Cracked plastic intake manifold (Ford 4.6/5.4, Chrysler 4.0, GM 3.6) (common). Failing PCV valve or hose (common). Failed or biased Bank 1 upstream O2 sensor (occasional). Throttle body gasket leak (occasional). Carbon-coked throttle body letting blade hold slightly open (occasional)

Is it safe to drive with P2187?

In most cases a vehicle stays drivable for short trips with P2187 active, but it should be diagnosed and repaired promptly — this is a moderate-severity code. Ignoring it can lead to further damage or a failed emissions test. Specific tolerance varies by make and model.