96 windstar 3.8 Intermitemt P0402

[gasmith46]

I have a 1996 Windstar with a 3.8 Ltr.eng. DTC code P0402 is intermitment. Check eng. lite come on with code in computer. Does not run rough at any time when light is on. I cancel code and vehicle will go days without problem. Than code will come on at idle or slow driving speed.

[wiswind]

P0402 is indicating that the PCM is getting an indication that there is more EGR flow through the EGR pipe than what is expected.

The EGR pipe is in front of the motor, it goes from the front exhaust manifold to the EGR valve, which is located on the driver's side of the lower intake manifold.....somewhat below where the upper radiator hose connects to the lower intake manifold.

The EGR flow is sensed by the Differential Pressure FEedback sensor (DPFE).

There are 2 hoses that go from the EGR pipe, to the DPFE.
In between where the 2 hoses come off the EGR pipe is a narrow point in the pipe, causing a slight restriction to the flow of exhaust through the pipe.
This restriction will cause the pressure to be higher in the bottom hose, closer to the exhaust manifold as relative to the top hose.....when there is flow through the pipe.
There will be no pressure difference with no flow through the EGR pipe.
The pressure drop will increase as flow through the EGR pipe increases.

The voltage reading from the DPFE is an indication of how much pressure drop there is between the 2 hoses, which is proportional to the flow through the EGR pipe.

The flow through the EGR pipe is controlled by the EGR valve
The EGR valve is controlled by vaccum applied to it from the vaccum line that connects to the top....Closed with no vaccum, open with vaccum applied.
The EGR vaccum control solenoid controls the EGR valve......
An electrical signal from the PCM opens the EGR vaccum solenoid to provide vaccum to the EGR valve, to open it.
There is normally a small amount of vaccum in the vaccum line to the EGR valve....but not enough to open it.

The PCM uses the voltage feedback from the DPFE to modulate the EGR valve to control the EGR flow.

The MOST common failure in this loop is the DPFE.
The EGR valve rarely fails (not that it cannot fail).
You can also inspect the EGR pipe for any cracks, rust, or other cause of leakage.
Also inspect the hoses that go from the EGR pipe to the DPFE.

You can remove the vaccum line from the top of the EGR valve......and with another vaccum line (clean) you can connect it to the vaccum line connection on the EGR valve....and suck on the line to manually work the EGR valve......in case, perhaps there is some small grain of carbon on the EGR valve seat.......
You should hear the valve click as it moves.

Then, if that does not solve the problem....you can do another test.....but it will light the CEL (if things are working).
You WILL want to clear the codes before doing this test.
The test is, to disconnect the vaccum line that connects to the top of the EGR valve.
Plug the end of the vaccum line with a small screw and some tape.....
No need to worry about plugging the vaccum line connection on the EGR valve.
Take the vehicle for a drive.
Have the codes read / or read the codes.
Your code reader should indicate P0401 for insufficient EGR flow....as you have disabled the ability for the EGR valve to open.
IF you have the P0402 code back.....chances are that you have a defective DPFE, as there should be NO EGR flow during this test.