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Failed Smog Test

Ford F150 has insufficent EGR flow. Why?

My 1998 Ford F150 5.4L has a EGR problem. My EGR flow is insufficent. I've replaced the EGR valve, EGR vacuum regulator, and differential feedback pressure sensor.

The vacuum is low off the regulator when the RPMs are increaced. The short term trim is on 3 percent, and the long term is on 4 percent. The EGR monitor on the scanner shows that it is complete. Where are the long and short term fuel trims of 3 and 4 coming from?


Fuel trim of 3 or 4 percent is not unusual. This means the ECU is trying to correct your Ford's air/fuel ratio by leaning out the mixture. The ECU is receiving information from the oxygen sensors indicating the system is running slightly rich. Now this might be due to something as simple as a dirty air filter to a defective (lazy) oxygen sensor(s). We don't feel this issue is related to the low EGR flow problem. The Differential Pressure Feedback EGR (DPFE) sensor is a pressure transducer that monitors the pressure difference across the EGR tube assembly. The DPFE sensor receives signals through 2 hoses (downstream pressure hose which is the "REF SIGNAL" and upstream pressure hose "HI SIGNAL"). The DPFE sensor outputs a voltage proportional to the pressure drop across the metering orifice and supplies it to the PCM as EGR flow rate feedback. The PCM controls the EGR vacuum regulator solenoid. The EGR vacuum regulator solenoid controls the vacuum to the EGR valve. When the EGR valve opens, exhaust gas flows to the intake manifold to be returned to the combustion cycle. The differential pressure feedback EGR system monitors the flow and returns a signal to the PCM. The amount of recirculated exhaust gas depends upon your engine's RPM's, intake manifold vacuum, and most importantly, exhaust backpressure. Since you've replaced all the major EGR components, we suggest focusing on carbon buildup within the EGR flow tube (connecting between the exhaust manifold and EGR valve), as well as ensuring your Ford's exhaust system has adequate backpressure. The use of aftermarket exhaust components (high performance CAT and/or muffler) often cause EGR system problems.

posted by SmogTips Support 12-03-2011 09:36 AM

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