Download In some areas of the u.S., certain basic chemical properties of gasoline (regardless of brand) may cause the intake manifold egr port to clog or the egr valve to fail. If this happens, the mil comes on with a dtc p0401 (insufficient egr flow) or p1491 (insufficient egr lift) stored. Repair procedure 1. Remove the throttle body cover, intake manifold covers, ignition wire holder, and ignition wire cover. Throttle body cover ignition wire cover intake manifold covers 2. Disconnect the brake booster vacuum hose and the vacuum hose from the intake manifold. 3. Remove the throttle body from the intake manifold (two nuts and two bolts). Leave all hoses, cables, and electrical connectors connected to the throttle body. Discard the throttle body gasket(s). Intake manifold manifold gasket (save.) pcv valve gasket (discard.) (discard – (’97 models ’97 models only) only) spacer 4. Remove the pcv valve from the cylinder head cover. Remove the intake manifold (three nuts and six bolts).