AT WITS END WITH P0171 (Check Engine Light System Too Lean)

[Daisywheels]

First, the two middle vents do not blow hot air as strong as the defrost vents do. Even the rear vents to the back seat blow strong and healthy.

This leads me to believe from what I read here that I must have a vacuum leak. I have checked every possible hose and replaced both upper and lower plenum gaskets, and tested for leaks. NONE.

1) Are the heater/A/C vacuum hoses connected to the main intake vacuum on my 1999 Ford Explorer XLT 4X4 or is it separate?

2) Could I have inadvertently broke a vacuum line when banging out the center console to finish replacing all 4 O2 sensors?

3) One vacuum hose that goes from underneath the glove box to that plastic ball in front bumper has a connector allowing one end to be pulled off about 1/2 way. There is no vacuum coming from that line at all. Again, not sure if the heater is in the whole vacuum loop?

4) I have O2 sensor voltage data and fuel trim data, but not smart enough to know what it means. Does anybody know that area well?

5) I do sense (hear) a slight exhaust leak from about where the manifold is on bank 1. But I do not think that would cause the P0171 code, or would it?

If I don’t get this fixed and smogged by Saturday, I am going to have to donate it. What a shame – Tranny is so smooth and strong.


HELP !!! -DW (And thank you all so much for any help)....

 

[koda2000]

All of your HVAC controls work off the vacuum reservoir ball. Vacuum is supplied to the ball by the engine when the engine is running and typically should measure around 19 HG at idle. One vac line enters the passenger compartment and supplies the HVAC controls. From there vacuum is directed to various vacuum motors, depending on setting, to direct air flow by opening various doors in the heater/AC box. A vac leak (even a tiny one) anywhere can cause the air flow to default to the defrost setting and is particularly noticeable when the engine is under a load (like when climbing a hill).

With a P0171 lean code, you most likely have a vacuum leak somewhere. I'd start with disconnecting and capping the vacuum feed line from the engine to the vacuum ball to see if that has any effect on the P0171 CEL. If it does, your leak is somewhere to/from the ball. If not, there are a bunch of places where engine vacuum can leak... a few examples below.

Brake booster
PCV hoses/elbow/grommet
Evap system/lines (front or rear of the vehicle)

 

[MAS Tequila]

A leak from the line going to the ball wasn't enough to throw a code in my case.

If you have the EATC there are 4 0-rings inside that dry out which need replacing that will cause the air to revert to the defrost default.

 

[koda2000]

A leak from the line going to the ball wasn't enough to throw a code in my case.

If you have the EATC there are 4 0-rings inside that dry out which need replacing that will cause the air to revert to the defrost default.

As the OP has an XLT, it most likely has the manual HVAC controls. It's got to be a pretty fair leak somewhere to throw a code. My XLT would divert to defroster vents when climbing a hill, but never set the CEL. Turned out it had a tiny leak from where a PO and installed a "T" fitting and plugged an extra hole with a sheet metal screw. It doesn't take much.