P1401 cold start stumble diagnosis help

[rendoll911]

car: 1996 Legacy 130k

 

Symptoms: CEL P1401 on and off and a bit of rev jumping when on a cold start. Other than that everything seems fine. No problems during fueling or any vapors escaping when i open the gas cap.

 

I found this post copied below and used it to try to do some diagnosis. With the ECU in test mode the fuel tank pressure solenoid clicks and with a hose disconnected air blown in by mouth starts and stops with the clicks.  What should my next move be?

 

 

 

Fuel pressure regulator has nothing to do with this code. This code is related to a solenoid near the fuel tank which opens or closes at certain times to allow vapor pressure to build in the fuel tank. This is used to determine if the Evap system has any leaks. The tank is pressurized, then the Evap purge control solenoid is opened and the pressure drops as vacuum is applied to the tank. Pressure is monitored by the fuel tank pressure sensor.

The fuel tank pressure control solenoid is near the right front corner of the fuel tank. Use the green connectors under the dash to put the ECU in Test mode.
Check the valve to see if it clicks when in test mode.

Remove one of the hoses from the valve and see if you can blow air through as it clicks. Air should flow then stop in correlation with the clicking. If no air flows the valve is clogged or stuck closed. If air flows all the time the valve is stuck open or has some kind of obstruction keeping it from sealing.


If the valve doesn't click. Use a multimeter to check for 12v reaching the connector.
The ECU controls the ground side. Check for the ECU to switch the ground by probing both pins (red on one, black on the other) with the connector unplugged. Voltage will switch from 12 to 0.

If the valve clicks and air flows through it, there is probably a clog in the hoses somewhere between the valve and the tank, or between the valve and the evap canister.

Edited June 29, 2017 by rendoll911

[Fairtax4me]

If the pressure control solenoid checks out the problem might be caused by a failed fuel tank pressure sensor or wiring to the sensor.

The tank pressure sensor is mounted in the trunk/cargo area right side behind the trim panels kind of near the strut tower. Typically those sensors don't fail, a wire problem or dirty /corroded connection in the harness that runs along the right side floor pan is more likely.

Testing the sensor is simple with a voltmeter. It should have a 3 wire plug. Check one outer pin for 5v supply voltage from the ECU by back probing the pin with it still plugged in. Check the opposite pin for continuity to ground. Check the center pin for signal voltage back to the ECU.

High voltage (near 5v) on the center pin would indicate a poor ground (corroded/dirty connector) between the sensor and ECU. Low voltage ( near 0V) at the center pin could mean either the signal wire is shorted to ground or the sensor is bad.

Low voltage on the supply pin (less than 4v) is a dirty/corroded connector somewhere between the ECU and sensor. There are a few connectors in that harness that are typically accessible by removing the trim/kick panels that run along the floor on the right side.

IIRC there are another few connectors under the rear seat bottom cushion.

 

One other thing you can check which may cause this code is the purge control solenoid below the #3 intake manifold runner. I've had several purge solenoids clog or get partially stuck open by debris. Usually it causes an evap leak code to be set, but it could cause other codes as well. Test it the same way you tested the pressure solenoid.