Diagnosing diesel issues right the first time

March 1, 2019
The vehicle at the shop is a 2014 Ford F350. The truck is powered with the 6.7 Powerstroke diesel engine and has an automatic transmission. In the last 250 miles, the engine has run poorly and lacked power.

The vehicle at the shop is a 2014 Ford F350. The truck is powered with the 6.7 Powerstroke diesel engine and has an automatic transmission. In the last 250 miles, the engine has run poorly and lacked power. It was taken to a diesel shop, where the shop put an EGR delete kit on and removed the exhaust after treatment. This did not fix the problem.

The fuel filters were replaced then replaced again, which did not fix the problem either. With the second set of filters installed, the engine would not start; at that point, it was towed to my shop. The next morning, I went to check for any stored DTCs that might be of interest and found that both of the batteries were dead (5.25V).

With the batteries recharged, and the scan tool hooked up I found four diagnostic trouble codes that would give a diagnostic direction. All four pointed to either a lack of fuel being supplied to the engine or a leak in the high pressure fuel system. The four DTCs were P0087 (fuel rail pressure too low), P008A (low side fuel pressure too low), P0093 (fuel system large leak), and P2291 (low fuel pressure during cranking).

A scan tool can be used to monitor the fuel rail pressure PID, along with the fuel delivery pressure switch PID (pressure switch in the low fuel supply) to get a direction before the hood is even opened. I selected these two PIDs because they will tell me if the proper amount of fuel is being supplied to the engine, and if the CP4 pump is trying to build pressure in the fuel rails.

Please keep in mind, on this engine everything is hard to access except the secondary fuel filter and the air filter. The engine is not mechanic friendly, so plan out your next move carefully. The IDS scan tool was used to monitor the fuel rail pressure, engine RPM and fuel delivery pressure switch status, and the engine was cranked for a few seconds while recording the data. The only change in data was the engine RPM. At this point, I need to start back at the beginning, the P008A DTC, which is for the low side fuel pressure being too low.

Before we move on, the P008A DTC is stored when the fuel delivery pressure switch doesn’t change from its normally closed state to open when the engine is cranked. The fuel delivery pressure switch opens when the fuel system pressure reaches 365 kPa (53 psi) or above. If the fuel delivery system pressure drops below 365 kPa (53 psi) the switch closes, and if the fuel delivery pressure switch remains closed for more than 60 seconds, the PCM notifies the driver by displaying a low fuel pressure warning in the message center, and an engine derate occurs.

The fuel delivery pressure switch is located at the top left of the engine in the fuel injection pump supply tube, forward of the secondary fuel filter. Before you grab your favorite fuel pressure gauge to test the fuel pressure, stop and consider where you are going to hook it up. There is no pressure test port. If you really want to test the pressure, you can remove the fuel delivery pressure switch and screw a gauge into the fitting. This takes time to do, so I will be satisfied with using the scan tool and let it tell me if the pressure is high enough or not.

Before we delve into any testing, stop and consider the hydraulic principle. We are working on a hydraulic system, and if a hydraulic system is going to produce any pressure, it first must have a pump that is capable of pumping volume and pressure; it also needs a restriction to push against. I opted to start with the electric pump (fuel conditioning module) and work from there.

I unhooked the fuel discharge line from the secondary fuel filter and attached a hose, so I could take a fuel sample. I also used my scan tool to turn the fuel pump on and off, although this can be done by just cycling the key. The electric fuel pump will run for 30 seconds each time the key is turned to the "on" position.

On to the first test

In taking the fuel sample, I want to test three things:

  1. Fuel quality
  2. Fuel volume
  3. Check for any air in the fuel system

In this case, the fuel had lots and lots of air in it, and it took about one minute to pump a quart jar full with fuel. The fuel smelled ok and was nice and clean. At this point, the primary fuel system has two strikes against it, low fuel volume and air. Will a new fuel pump get the engine to start? The only way I know to find this out is to install a new frame mounted “fuel conditioning module” and see what happens.

With the new fuel conditioning module mounted to the vehicle, the fuel system was primed and tested. The new pump will pump the quart jar full in about 15 seconds, and the fuel has no air in it. With the new electric pump hooked up and cranking the engine, the engine did not start. in fact, the recorded scan data did not change at all from the initial test. At this point, we need to move on to the other three DTCs.

The three DTCs in question are the P0087, P2291 and P0093. Before I start taking parts off this engine, I want to do a little research to see what causes these codes to set.

The P0087 DTC, (Fuel rail pressure too low) sets when “the PCM regulates the fuel rail pressure by controlling the fuel volume control valve and fuel pressure control valve. This DTC sets when the PCM is no longer capable of maintaining the fuel pressure”. In a nutshell, it sets when the fuel volume control valve has been opened as far as it can open and the required system pressure cannot be maintained.

DTC P2291 will set when the PCM monitors the fuel rail pressure (FRP) during the engine cranking. This DTC sets when the FRP does not increase to the calibrated threshold while the engine is cranked”. This is a symptom DTC, which is caused by something else.

DTC P0093 (Fuel system large leak) is the place to start. “This DTC sets when the requested fuel volume control value exceeds a calibrated threshold indicating a large fuel system leak.”

With this information, I can see the low fuel pressure problem is not caused by the electric pump not being up to its task, but because there is an internal leak in the high pressure system. With this leak, the electric pump has nothing to push against, thus no low fuel pressure in the CP4 pump can be built.

Where is the fuel pressure regulator?

The question at hand is “where is the restriction to the fuel flow located in this injection system?” In this case, this restriction is missing. In all fuel systems I have ever worked on, there is some sort of a fuel pressure regulator or fuel restrictor in all fuel injection systems.

Let me start out with a schematic of the fuel system pressure and return circuits. Figure 1 shows the low pressure fuel in red and the return fuel in green. The schematic shows there are only two places for the low pressure fuel to return to the fuel tank - either through the CP4 pump or through the fuel pressure regulator valve that is housed in the rear of the right hand fuel rail.

By doing a little research about the CP4 pump, I found the restriction. The fuel pressure regulator is called an “overflow valve”. This valve is housed in the back side of the CP4 pump, as seen in Figure 2. I have circled the overflow valve in red. Knowing this, the diagnostic process can be simplified.

Figure 3 shows the valve with it removed from its bore. The valve is nothing more than a spring loaded pressure relief valve. This valve regulates the low pressure fuel to 55 PSI.

On this engine, there is no access to the CP4 pump without removing most of the air intake system. The only easy access to the fuel return is on the left side of the engine (Figure 4) where the fuel return line is attached. The fuel return hose can be removed, a short piece of hose attached to the exposed nipple, which will allow you to catch the fuel in a container. Now, turn on the electric fuel pump and see how much fuel flows from the CP4 pump.

In this case, a large amount of fuel flowed from the CP4 pump. I found if I put a restriction in my fuel hose, I could raise the low side fuel pressure to the 55 PSI that was required to open the contacts on the fuel delivery pressure switch, but the engine would not start.

Getting closer!

At this point, the fuel pressure issue has been found, the system has a huge internal leak, and the next step is to verify the integrity of the CP4 pump. To gain access to the CP4 pump, several of the air intake pieces must be removed. Once they are removed you will see the FVCV (fuel volume control valve) mounted on top of the CP4 pump.

This valve is held to the pump by two screws. Remove those screws and wiggle the FVCV out of its bore and take a look at the screen. If perchance you see something like is seen in Figure 5, your problem analysis is finished since the whole fuel system will be infiltrated with metal trash, and the complete fuel injection system will need to be replaced.

In the case of this F350, the FVCV screen was clean. You can also remove the overflow valve from its bore and inspect it. Inspect the screen that is on the end of the valve for trash and anything that might be out of order. There are two more places to look for trash: The primary and secondary fuel filters.

This fuel system returns its fuel to the fuel conditioning module (frame mounted fuel pump) so most of the metal trash will be collected there, but like I have already mentioned, once metal trash gets into this system, it goes everywhere, including the fuel tank, filters, fuel rails and the fuel injectors. Metal trash is the death wish on the fuel system. In the case of this truck, the screen on the FVCV was clean and both filters had been replaced in an attempt to get the engine to run again, so cutting them apart did not find any trash. I do not see any evidence of metal trash in this system, so I am good with not having to replace any of the injectors.

With the problems I have found so far, I came to the conclusion this truck needed to have the CP4 pump replaced. A rebuilt CP4 pump was fit on the engine, and the engine started on the first try. Test driving the vehicle, I found it had good power and ran as it should.

The problem is fixed, and its time to collect my money and move on to the next Ford Powerstroke problem. I did not take this CP4 pump apart, but I would imagine there has been a piece of steel break loose inside the pump and get lodged in the overflow valve, which is holding the valve open and letting the fuel bypass back to the fuel tank.

How important is fuel quality

Thinking back many years to the mid 50s, the first diesel powered machine I can remember being around was a Caterpillar D6. The fuel cap was a large aluminum screw on cap and molded into the cap were the words, "Buy clean fuel, keep it clean.” When working with diesel fuel systems, these words are words to live by. Any dirt or water getting into the fuel can and will work its way into the pumps and injectors and will just plain ruin the fuel system.

Diesel fuel had four jobs in the engine:

  1. Lubricate the internals of the fuel injection system
  2. Seal the inside of the injection system
  3. Cool the injection system
  4. Power the engine

Don’t diminish the importance of the diesel fuel that is running through the fuel injection system.

About the Author

Albin Moore

Albin Moore spent the first 21 years of his working life in the logging industry. In 1992 he made the transition to shop ownership and opened Big Wrench Repair in Dryden Washington. Since opening the shop he has moved the business to specialize in driveability problem analysis, both with gasoline and diesel vehicles. Albin is an ASE CMAT L1 technician, and brings with him 40 years of analyzing and fixing mechanical and electrical problems. Albin enjoys sharing his many years to experience and training with the younger generation as a way of improving the quality of the automotive repair industry.

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