Cause of P0171 & P0174 Codes 1996 Ford Windstar

I have a 1996 Ford Windstar with a 3.8 litre engine. The OBD2 meter is showing trouble codes P0171 (System to Lean Bank 1) and P0174 (System to Lean Bank 2). This is after I replaced the gaskets (6 red rubber gaskets) between the Plenum and the Intake Manifold. I replaced the gaskets after having a diagnostics work up done at a local shop for $98 at which time I was told there was a vacuum leak between the Plenum and the Intake Manifold and that the gasket between the Plenum and Intake Manifold would have to be replaced. The mechanic called it the Upper Intake Manifold gasket. What else could cause these trouble codes (i.e. P0171 & P0174)? Thanks as any help would be much appreciated.

I'm having the same problem, I plan to test my whole vacuum system this weekend.

From what I've read here, there's lots of things that can cause this code but it seems to usually be a vacuum leak.

when you replaced the gaskets, did you also do the isolator bolts and the gasket between the the top and bottom sections of the intake manifold? Did you clean the EGR valves as well? Just replacing the 6 EGR gaskets doesn't stop the 171/174 codes from coming back.

To properly address these 2 codes you should fully follow this procedure. http://leckemby.net/windstar/windstar01.html

-p

when you replaced the gaskets, did you also do the isolator bolts and the gasket between the the top and bottom sections of the intake manifold? Did you clean the EGR valves as well? Just replacing the 6 EGR gaskets doesn't stop the 171/174 codes from coming back.

To properly address these 2 codes you should fully follow this procedure. http://leckemby.net/windstar/windstar01.html

-p
The TSB 031-6-1 states it applies to 1999-2003 Ford Windstars. I looked at the lickemby website and the picture shown there of the plenum and description of the plenum (i.e., two parts, long gasket around plenum) is very different from my 1996. My plenum is in one piece and has six red rubber gaskets between it and the intake manifold. Are you sure the TSB addressed on the leckemby website applies to my 96 3.8 engine?

I'm pretty sure it doesn't apply to my '98, but I still had leaky gaskets between the lower intake manifold and cylinder heads.