... on a '98 6 cyl? I'm trying to track down the cause for recurring P0171/4 codes. 'can't find any obvious vacuum leaks, manifold vacuum at idle is 19" (I'm at 4,000' elevation - this corrects to about 23" at sea level?). The vac gauge needle vibrates or jiggles just a little, maybe 1/2" either way but still moving.

Any ideas?

Oh, 98K on moter, replaced EGR and front O2 sensors, runs fine otherwise ... did seem a little peppier after the new O2 sensors.

Any help would be greatly appreciated.

98 Explorer
98 F150
91 F350
90 911 Cabriolet
95 Kioti 3054
95 Catalina 250WB

 

Not sre if this applies to your engine or not, but take a look at the "plumbing" for the PCV.

On some engines -as I recall- theres a 90 degree plastic tube at the back of the engine that can crack.

 

If it's the SOHC engine, I'd start looking at the intake manifold O-rings.