1996 Mercury Marquis Codes P0401 and P1504

1996 MERCURY MARQUIS
61,593 MILES • V8 • 2WD • AUTOMATIC
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ANNETTEUZZELL
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My 1996 Mercury Marquis V8 starts fine...if you give it more gas than an idle. It accelerates fine, but when you let off the gas it dies instantly.

OBD II Code reader yields the two following codes:

1. P0401 - Exhaust Gas Recirculation Flow Insufficient Detected

2. P1504 - Intake Air Control Circuit Malfunction

The weather here is bitterly cold...so I am waiting a few days for warmer weather when I can work on the car. I suspect something simple like a leaking or broken vacuum hose or perhaps the PCV is clogged.

Any ideas from those who have had or experienced this problem before.

Your experience/thoughts on this matter are greatly appreciated.

Thanking you in advance for your time and effort.
Jan 7, 2010 at 5:59 PM
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RASMATAZ
  • ✓ASE Certified Mechanic
  • 75,992 POSTS
  • ASE Certified Mechanic
DTC P0401 most likely means one or more of the following has happened:

The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced
There is a blockage in the EGR (tube), most likely carbon buildup
The EGR valve is faulty
The EGR valve may not be opening due to a lack of vaccuum
Possible Solutions
In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit.

Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open
Clean out the EGR valve and/or tubing to remove deposits
Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model)
Replace the DPFE sensor (with a good quality / OEM one)

Check and test the IACV and MAF sensor
Jan 8, 2010 at 1:02 AM
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