Fuel Trim System Rich at Idle Bank 2 (GM)
What does P1187 mean?
The P1187 code means the vehicle's onboard diagnostic system has detected: Fuel Trim System Rich at Idle Bank 2 (GM). This is a moderate severity code.
Common Symptoms
- Check Engine Light on
- Rich exhaust smell at idle
- Rough idle
- Black soot on tailpipe
- Poor fuel economy
Probable Causes (Ranked by Likelihood)
- Leaking fuel injector on Bank 2 Very High
- High fuel pressure from stuck fuel pressure regulator High
- Faulty Bank 2 upstream O2 sensor Moderate
- EVAP purge valve stuck open Moderate
- PCM fuel control issue Low
Step-by-Step Diagnostic Procedure
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Monitor Bank 2 fuel trim values on a scan tool at idle
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Test fuel pressure and check for a stuck fuel pressure regulator
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Check fuel injectors for leaking using a balance test or rail pressure drop test
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Test the EVAP purge solenoid for proper operation
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Inspect the Bank 2 upstream O2 sensor response
Frequently Asked Questions
Why is the engine rich only at idle?
At idle, fuel flow is minimal and any extra fuel from a leaking injector, stuck purge valve, or high fuel pressure has a proportionally larger effect on the air-fuel ratio than it would at higher RPMs.
What are normal idle fuel trim values?
Normal idle fuel trim values should be within plus or minus 10%. P1187 sets when the negative fuel trim correction exceeds the threshold, meaning the PCM is trying to remove fuel to compensate for an overly rich condition.
Can the EVAP system cause a rich condition?
Yes. If the EVAP purge solenoid is stuck open, fuel vapors are continuously drawn into the intake manifold, enriching the air-fuel mixture especially at idle when airflow is low.