1999 Ford Ranger - P0455 & P0401

Abstract

A very good friend of mine called me and passingly stated that his 1999 Ford Ranger was idling a little higher than normal and his Check Engine Light was on. A couple of days later we got together and started the diagnosis by pulling the DTCs. We had to resolve a P0455, EVAP System Gross Leak Detected, and a P0401, EGR Insufficient Flow Detected.

Description

My friend's 1999 Ford Ranger had the following DTCs:

P0455 EVAP System Gross Leak Detected
P0401 EGR Insufficient Flow Detected.

P0455 - EVAP System Gross Leak Detected

We decided to attack the P0455 code first. This DTC indicates that the EVAP system has a very large leak.

In an effort to minimize gasoline vapors being released into the atmosphere, the Evaporative Emission Control (EVAP) system's responsibility is to capture all gasoline vapors and store them in a charcoal canister. When the engine operating conditions are correct, these stored vapors will be introduced back into the engine's intake system to be burnt with the normal air/fuel mixture.

For the EVAP system to be most effective, it needs to be a closed system, meaning it cannot contain any leaks. To detect an EVAP leak, the EVAP system sensors are sensitive enough to detect a leak where the hole diameter is 0.020". Wow! A gross leak represents a hole diameter of 0.090".

Since the most common EVAP large leak DTC is caused by a loosely installed gas cap, we decided to inspect the gas cap. It physically looked okay, but we decided to clean the gasket and mating filler pipe area with a break cleaner covered rag. We reinstalled the gas cap, cleared all DTCs, and executed Ford's KOER (Key On Engine Running) self-test. The P0455 DTC did not return, but the P0401 did return. Of course, the EVAP Readiness Monitor did not execute, so we knew we had to wait it out until his next decent drive cycle to ensure that this DTC did not return. By the way, after checking back with him several weeks later, this DTC did not return.

Given that cleaning the gas cap and reinstalling it property resolved the issue, it would appear that at a recent refueling that gas cap was not fully screwed in. This is the most common reason this specific DTC is set. But, if the the P0455 DTC returned, we would have then proceeded to test the EVAP system with a smoker and a bi-directional scan tool to manipulate both the canister vent and the purge solenoids.

P0401 - EGR Insufficient Flow Detected.

The EGR (Exhaust Gas Recirculation) system is used to literally recirculate exchange gas back into the combustion chamber to lower the combustion temperature and, as a result, the amount of harmful NOx gases are reduced. The is one of the few systems that actually enhance engine performance.

We started by using the scan tool to activate the EGR solenoid. While idling the engine, we slowly opened up the EGR solenoid. The engine responded by starting to stall, so we knew that the EGR solenoid, EGR valve, and passages were probably ok.

The scan tool utilized allows component information to be retrieved based on current make/model of vehicle. So, we looked up the DPFE (Differential Pressure Feedback EGR) component. We learned the following:

High level description, it is used to ensure that when the PCM opens up the EGR valve via the EGR solenoid, that flow occurs.
New Part (Black Plastic) vs Old Part (Metal Housing):
Has two ports named, HI, and REF.
The sensor produces a voltage by comparing the pressure between these two ports, HI and REF, separated by an orifice.
- HI - Closest to exhaust manifold
- REF - Closest to EGR valve
- Delta = HI - REF
Behaves like a potentiometer ranging from 0 to 5 volts
Has three pins
- 1 - VREF (5V)
- 2 - PCM(-) ground
- 3 - Signal
3 - Signal should be around 1v KOEO (Key On Engine Off)

With KOEO and DPFE connected, I back probed the DPFE and had the following results:

Scan tool displayed 4.93 volts for DPFE
1 - VREF (5V) : 5 volts : good
2 - PCM(-) ground : 23mv : good
3 - Signal : 4.93 volts : bad

With KOEO and DPFE disconnected, the following results were observed:

Scan tool displayed 4.93 volts for DPFE
1 - VREF (5V) : 5 volts : good
2 - PCM(-) ground : I didn't test
3 - Signal : 4.93 volts : bad

Since the disconnect connector had 4.93 volts on the signal wire, I knew that it was a pull-up resistor type circuit, biased, and the signal wire was good. But, because my friend thought that during his last clutch job that the wiring harness was pinched, he insisted that I test the signal wire again. Being that he could easily bench-press me, I fulfilled his wish. Connecting a test light to ground, I touched the test light to pin 3 - Signal, and the scan tool displayed 0 volts. This result confirmed that the wire between sensor and PCM was ok.

At this point, I knew the DPFE sensor was bad, and that we had signal integrity.

Several days later, we installed the new DPFE, being very careful to ensure we installed the hoses to the proper ports, HI and REF. Again, with KOEO and DPFE connected, the scan tool displayed the DPFE voltage as 1 volt. Perfect. We then executed a KOER self-test. All passed.

This is a fix.

What Went Well

With the large EVAP leak, we got lucky with such a simple fix.
We determined the faulty DPFE sensor quickly

What Could Have Gone Better

I should have leverage freeze frame data better to learn more about when the EVAP DTC was triggered