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1999 Ford Explorer XLT P1151 and P0171: How to Diagnose Vacuum Leak or Fuel System Issues

Model: 1999 Ford Explorer XLT Fault Code: P0171 Posted: 2011-07-17 09:12

My 1999 Ford Explorer XLT with a 4.0L engine has had the Check Engine light on for some time. After investigating, we found fault codes P1151 and P0171. We've tried checking for vacuum leaks using carb cleaner, but no obvious source was identified. The intake gaskets have never been replaced. Should we replace the O2 sensors next, or are there other underlying issues to consider? Any accurate diagnosis or troubleshooting steps would be greatly appreciated.

Related fault codes
P0171P1151
Comments (8)
Anonymous 2011-07-17 18:32

Since we can't physically inspect your vehicle, any helpful advice is valuable. How exactly did you test for vacuum leaks? Has the intake gasket been replaced before?

Anonymous 2011-07-18 21:16

We used carb cleaner sprayed on suspected hoses and connections to check for vacuum leaks. The intake gaskets have never been replaced, so that's a possibility we're considering.

Anonymous 2011-07-19 05:14

Using carb cleaner to detect vacuum leaks can sometimes miss actual issues—fuel trim readings from an OBD2 scanner are more reliable for diagnosing problems. Have the intake gaskets been replaced? In my experience, these trucks often have leaking intake manifolds as a root cause.

Anonymous 2011-07-21 03:16

I've seen this issue before—check the EGR vacuum sealants (sealants located near the intake manifold). A faulty or missing seal can cause engine misfires and rough idling, triggering both P1151 and P0171. These codes suggest insufficient fuel delivery, possibly due to a clogged fuel filter. Verify your fuel pump pressure—below 60 psi indicates a need for filter replacement. The fuel filter is accessible under the driver's side, and the EGR vacuum sealants are located near the intake manifold. Let me know if you have questions!

Anonymous 2011-07-29 01:08

Intake manifolds on this model often leak only when cold and seal as they warm up. Additionally, for P1151, consider replacing the B2S1 oxygen sensor (upstream) and inspect its connector. It's likely that intake gasket replacement is needed—does your vehicle run roughly during cold starts?

Anonymous 2011-09-15 10:53

Confirmed: Intake manifolds on 1999 Ford Explorers typically leak only when cold and seal as they warm. For P1151, the B2S1 O2 sensor should be replaced (it's recommended to replace upstream sensors in pairs for balanced switching). Also inspect the sensor connector. Intake gasket replacement is likely needed—does your vehicle exhibit rough idling or performance during cold starts?

Anonymous 2011-10-26 18:51

I'm experiencing the same issue. I've already replaced both Bank 2 O2 sensors and cleaned the MAF sensor. Can someone clarify how to properly inspect the EGR vacuum sealants?

Anonymous 2011-10-27 04:47

The P1151 and P0171 codes are typically caused by a vacuum leak—focus on identifying and repairing the source of the leak before replacing sensors or other components.