Service Repair | |||
You can also perform a Volumetric Efficiency (VE) test to judge not only the accuracy of the MAF sensor, but the ability of the engine itself to "breathe." VE is a measure of the ability of the engine to take in its theoretical air charge under maximum conditions. For example, a 1-liter (1000cc) 4-cylinder engine should be able to take in 250cc of air volume per cylinder if cylinder filling is 100 percent efficient. Miles Wada, an iATN member, created a VE calculator for the Standard Motor Products Training site, and you can find it at http://www.smp-training.com/.
MAF Systems and Unmetered Air Any air that enters without first going through the MAF sensor will not be reported to the ECM, and will cause lambda to go high (lean). To test for this condition, record Total Fuel Trim at idle and at 2,500 rpm. Because overall airflow is low at idle as compared to 2,500 rpm, unmetered air will have a greater impact on idle fuel trims.Look for LTFT corrections that are excessively high (positive) at idle, and more normal at speed. If so, inspect for obvious sources, like torn intake boots, cracked vacuum lines, etc. If no obvious faults are seen, you can use the same smoke machine you use for testing EVAP systems to smoke the intake and pinpoint where that unmetered air is getting in. Rich Corrections at Idle (Negative Fuel Trim) EVAP purge flow at idle supplies extra fuel that the ECM didn't send and will cause the exhaust gas oxygen content to be lower. So will Exhaust Gas Recirculation (EGR), but here it is the addition of inert gases passing through the exhaust to the O2 sensor and reducing the combustible gases present at its tip. Fuel pressure regulator failure will cause idle fuel pressure to be high, and this pressure differential will cause more fuel quantity to flow from the injectors than normal, resulting in an excess of fuel. A leaking vacuum diaphragm in the regulator itself will also allow fuel to be drawn in through the vacuum line — and lead to negative fuel trim corrections. Barometric Pressure PID This PID is more likely on speed-density systems. BARO tells the ECM how "heavy" a given volume of air is. If you live at sea level, the air is "heavier" than it would be at Pike's Peak, for example. If the ECM thinks otherwise, it will supply an incorrect amount of fuel with resulting fuel trim corrections. And After It's Fixed Once you have repaired the fault that caused the code, make sure you reset the fuel adaptives, either with a scan tool or by clearing the ECM memory by disconnecting the battery. While the ECM will eventually relearn LTFT, driveability issues will certainly be present for your customer until it does — and they may have your customer coming back with new complaints. Using Fuel Trims for Driveability Diagnostics Fuel trims are an observation. Like misfire codes, any factor that affects the proper mixture and combustion will cause a fuel trim adjustment. For driveability diagnosis, it is necessary to check the total fuel trim under a variety of conditions. A scan tool capable of recording this information is a valuable resource. Because of the refresh rate of most scan tools, it is also helpful to choose only those PIDs you want to look at to record — your information will be more accurate. |
