Check the small hoses that go from the EGR tube to the DPFE. Is your DPFE metal of plastic?
Possible causes:
-- Fault in vacuum supply to EGR vacuum regulator solenoid.
-- EGR valve stuck closed or iced.
-- EGR valve diaphragm leaks.
-- EGR valve or flow path restricted.
-- EGR vacuum hose off, plugged or leaks.
-- VPWR circuit open to EGR vacuum regulator solenoid.
-- EGRVR circuit to PCM open.
-- EGRVR circuit to PCM shorted to PWR.
-- Differential pressure feedback EGR (D.P.F. EGR) sensor pressure hoses both connected improperly.
-- D.P.F. EGR sensor pressure hoses reversed.
-- D.P.F. EGR sensor VREF circuit open.
-- Downstream pressure hose off.
-- Downstream pressure hose plugged.
-- Damaged orifice tube assembly.
-- Damaged EGR vacuum regulator solenoid.
-- Damaged D.P.F. EGR sensor.
-- Damaged PCM.
l Run KOER Self-Test.
l Key off.
l Was KOER DTC P1408 output?
Yes
The KOER Self-Test has detected an EGR fault that is currently present. GO to «HE71».
No
REMOVE and INSPECT the EGR valve and intake manifold EGR port for restriction. If OK, the fault that produced DTC P0401 is intermittent. GO to «HE90».
ALL this just from code 0401 insuffiecent and 0402 excessive flow.
1 The EGR vacuum regulator solenoid trace back the vacuum line from the EGR that is the solenoid and check the filter to see if it's plugged.
2 DPFE bad or hose issues.
3 Carbon plugged passages behind the manifold gasket on the Throttle body.
my 98 built (07/97) had a aluminium DPFE F48E-9J460-BB and was replaced with a plastic body F77Z-9J460-AB no problems for over a year and a half.
If you do pull the throttle body get a new gasket first.
Paul
If the hoses between the DPFE and the EGR tube are off possible Cats are starting to clog.
