A 2015 SAIC-GM Buick Grandview sedan with approximately 133,000 kilometers and an LLU 1.6T engine. The vehicle’s engine malfunction light illuminates and has high gas mileage, requiring a second pit stop for service. Let’s see how to handle it!
Buick Grand Prix fuel correction system depleted
Fault diagnosis
Start the engine, the engine fault light on the meter is lit. Check the maintenance records, the car was stationed the day before yesterday to repair the engine fault light is on, fault code P0171 (currently), maintenance measures to replace the EVAP solenoid valve, and delete the fault code.
Using GDS2 the fault code is P0171 Fuel Correction System Depletion (Pass and Fail), which is consistent with the previous day’s fault code. Checked engine data, long-term fuel correction is 0% to 2%, currently normal, for intermittent failure.
The engine control module (ECM) controls the closed-loop air/fuel metering system to provide the best possible combination of power, fuel economy, and emissions control. The engine control module monitors the heated oxygen sensor (H02S) signal voltage and adjusts fuel delivery based on the signal voltage in closed-loop mode. Changes in fuel delivery will change the long-term and short-term fuel adjustment values.
The short-term fuel adjustment will change rapidly in response to changes in the heated oxygen sensor signal voltage. These changes will fine-tune the engine fuel supply. Changing the long-term fuel adjustment value follows the trend of the short-term fuel adjustment.
The long and short-term adjustments are coarse adjustments to the injection to get back to the center and return the control to the short-term fuel adjustment. The ideal fuel adjustment value is approximately 0%. A positive fuel adjustment value indicates that the engine control module is increasing fuel to compensate for a lean fuel condition. A negative fuel adjustment value indicates that the engine control module is decreasing the amount of fuel to compensate for a dialed-in fuel lean condition.
Possible causes
- Sensor failures, e.g. MAF, MAPHO2S, etc.
- Missing, clogged, or leaking exhaust system components, etc.
- Cracked evaporative discharge charcoal canisters, clogged or leaking evaporative discharge tubes, etc.
- Vacuum hoses cracked, kinked, or improperly connected, etc.
- Fuel contamination, insufficient fuel in the tank, low fuel pressure, etc.
- In the intake manifold, throttle body, and injectors. There are vacuum leaks in the shape rings, air leaks in the intake system and intake tubes, etc.
- Crankcase ventilation system leaks, etc.
The service technician has deleted the fault code, now reproduced, indicating that there is still a problem, check the relevant data and components
- With the ignition switch in the ON position, the MAP pressure was 100kPa, which was in line with the specified range for the elevation at which it was located, and 33kPa at total speed, which was normal; the H02S fluctuated regularly from 0.1V to 0.9V, which was normal; and the MAF value rose as the engine RPMs increased, and was close to the calculated MAF value.
- The engine was inspected and the following was not observed: missing, clogged, or leaking exhaust system components; cracked evaporative emissions charcoal canister, clogged or leaking Cifa emissions hose; cracked, kinked, or improperly connected vacuum hose.
- Use chemical cleaner in the intake manifold, throttle body, and injectors. Shaped ring at the intake system components and piping at the spray test, the engine idle speed did not fluctuate, and there was no vacuum leakage or air leakage phenomenon.
- Check the remaining fuel for 33.5L, measured by the pressure value of 280kPa, normal.
- Park idle test is good, for road test half an hour, park and run at 3000-4000r/min high-speed min after idling, the fuel adjustment tends to be poor, as high as 35%, and the fault resurfaces.
- When the malfunction appears, check the relevant data again, in which the difference between the calculated airflow at idle and the measured airflow is about 0.7 q/s. Carry out the relevant tests, the engine’s high speed is 3000-4000r/min, and fuel adjustment tends to normalize. This indicates that the scope of normal failure in terms of NIAF and fuel is narrowed down to engine gas leakage.
- The fuel adjustment was normal under normal operating conditions, and the data were normal, and the intake and exhaust system leaks were checked and no leaks were found. The fault reappeared after 1 minute of high speed, indicating that there is a certain correlation with the crankcase pressure. Check the crankcase ventilation system, remove the ignition coil cover, found that the crankcase ventilation valve has a small air leak. Remove the valve cover to disassemble the PCV valve, find that the rubber diaphragm is broken, replace the valve cover assembly, and test drive troubleshooting.
Fault resolution
The valve covering the PCV valve rubber diaphragm is broken and leaking air, the fuel correction is depleted, and the engine malfunction light is illuminated.
Case Summary
As part of the mixture or exhaust gas leaks into the crankcase during engine operation, the gas inside the crankcase will continue to increase. This gas accelerates the deterioration of the oil in the crankcase and produces moisture, sludge, and acids that can damage the mechanical parts of the engine. In addition, exhaust gases can cause pressure buildup in the crankcase, which can lead to oil seal leakage and other malfunctions.
The crankcase forced ventilation system can draw exhaust gases from the crankcase into the engine for combustion during the combustion process, and at the same time effectively reduces the pressure of exhaust gases in the crankcase instead of exhausting them directly into the atmosphere. This effectively protects the engine and also reduces the pollution of the atmosphere.
This case is for the valve cover PCV valve rubber diaphragm broken leakage fuel correction depletion, and engine fault light. The difficulty lies in the reproduction of the fault phenomenon, often the customer into the station when the fault does not appear, the relevant data flow is normal, and routine test car fault is very accurate. But you can be very familiar with a system under the premise of effective testing, so that the failure to reproduce, improves the repair rate.
In this case for the valve cover PCV valve rubber diaphragm breakage leakage fuel correction depletion, the engine fault light lit. The difficulty lies in the reproduction of the fault phenomenon, often the customer into the station when the fault does not appear, the relevant data flow is normal, and routine test car fault is very accurate. However, effective testing can be carried out under the premise of being very familiar with a particular system, so that the fault is reproduced and the return rate is improved. The scope of the mixture failure is very large, and the possible causes are also very many, but through the relevant operation and view of the relevant important data, can distinguish and narrow down the scope of the failure.
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