Bleeding at the pump.

I recently purchased a 2000 Accord coupe v6 with 180k on the clock. I've noticed the gas mileage isn't what it should be. The check engine light was on when I bought it turns out it was a p0401 code. I was told at the dealership insufficient egr flow would cause the car to run rich. So I figured that's what is killing my gas mileage. Well after cleaning the egr port, replacing the egr valve, changing the spark plugs and changing the oil there is no improvement. I don't know what else to check has anyone else had this problem with this particular car? If so what was the problem and how did you fix it?

Hope this is helpful for you as it was for me. My situation was remedied by replacing the oxygen sensors. But not after running the gamut of tests and repairs described here. Some of which were needed. (The EGR valve itself isn't always the culprit but the system not allowing sufficient flow back to the cylinders) So here goes and I hope this helps your Honda 3.0:

P0401 - OBD-II Trouble Code (Honda)
Auto Systems and Repair Insufficient EGR Flow

Our emissions expert has put together the following information about the P0401 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

OBD II P0401
Fault Code Definition

Insufficient EGR Flow
Symptoms

Check Engine Light will illuminate
In most cases, there are no adverse conditions noticed by the driver
In some cases, there may be performance problems, such as pinging on acceleration, when the engine is under load or when driving the vehicle at higher speeds
Common Problems That Trigger the P0401 Code

Restriction in the EGR passages, usually caused by carbon buildup
Lack of a proper vacuum signal to the EGR Valve from EGR Vacuum Solenoid
EGR Vacuum Modulator is defective
EGR Temperature Sensor is defective
EGR Valve is defective




The Basics
NOx gases are formed when the combustion temperature is too high (2500° F). EGR systems are used to reduce the combustion temperature, thus reducing NOx formation.

The exhaust gas recirculation (EGR) system recycles a small amount of exhaust gas from the exhaust system (usually no more than 10 percent) and mixes it with the intake manifold air entering into the combustion chambers. The addition of this inert (or non-combustible) exhaust gas limits the peak combustion temperatures to a range that is below 2500° F, where the formation of nitrogen oxide (NOx) is know to occur. In some cases where the engine is pinging and/or knocking badly from a severe lack of EGR flow, misfires can take place which allow raw hydrocarbons (HC) to be released from the tailpipe.

P0401 Diagnostic Theory for Shops and Technicians
When the computer sets a code P0401, it means that the EGR flow monitoring criteria has not been met. The EGR monitoring criteria are a set of test values and are usually run during at least two different driving conditions—steady speed freeway driving and steady speed city driving.

The engine control module determines proper EGR flow in many ways:

Measurable amount of Manifold Pressure change when the EGR is supposed to be flowing
Proper amount of change in the EGR Temperature Sensor
Measurable change (usually a decrease) in the front Oxygen Sensor Signal
Amount of Spark Knock as measured by the Knock Sensor
The code P0401 is often not a problem with the EGR valve itself. Rather, the EGR system is not allowing enough EGR to flow back into the combustion process to sufficiently cool the peak firing temperatures. Once the code P401 has been retrieved with a scan tool, the freeze frame data should be documented and analyzed in order to determine what engine conditions were present when the code was triggered. It is recommended that the vehicle be driven in such a way as to duplicate the code setting conditions with a data streaming scan tool connected, so the behavior of the EGR actuating components and feedback sensors can be monitored. Pay close attention to the DPFE readings and make sure that they are changing from a .5 to 1 range to a 3 to 4 voltage range when the EGR System is activated. Also, pay close attention to the EGR duty cycle that typically grows percentage-wise as the EGR system is activated.

Common Tests to Determine Whether the Problem Is an EGR Control Problem, a Plugged or Restricted System, or a Defective Feedback Device

Does the engine die, not just stumble, when the EGR valve is manually raised to its maximum?
(Use either a vacuum pump or bi-directional scan tool if it’s a digital EGR valve.)
Is the EGR valve getting sufficient vacuum? (Use the manufacturer EGR vacuum spec.)
Is the EGR system restricted? (The engine stumbles, but does not die.)
Is the EGR system plugged? (Engine RPM does not change.)
Does the EGR valve work?
Raise the RPM to 3000 and check manifold vacuum. Then open the EGR valve to its maximum—the manifold vacuum should drop by at least 3" of mercury. If it does not, there is a flow and/or restriction problem.
Verify that the front Oxygen Sensor readings drop and the Short Term Fuel Trim increases when the EGR valve opens. (EGR leans out the mixture.)
Note
If the NOx goes down when the EGR valve is raised (this test is most commonly performed on a Dynamometer), it is likely that one or more EGR passages or cylinders are plugged or very restricted, making the EGR only go to one or two cylinders. When this occurs, you may notice misfires and even have misfire codes along with the P0401. This can occur on vehicles that use EGR "runners" for each cylinder.

Thanks for the info. The check engine light has not come back on since I changed the EGR valve and cleaned the port. But the gas mileage still hasn't improved. I'm just not sure what to look at next.

Coincidentally replacing the oxygen sensors did improve gas mileage.

Wouldn't the check engine light be on if there was a problem with the O2 sensors?

Not necessarily. The ECM does it's best to compensate the fuel trims. To a point. O2 sensors do wear out. I'm not saying this is YOUR problem. I'm just answering your last post.

Could a faulty fuel pressure regulator be the problem?

if you have a fuel pressure gauge you can hook it up, start the car and unplug the vaccuum hose to the regulator and see if pressure changes, if it does regulator is working