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Diagnosing diesel issues right the first time

Friday, March 1, 2019 - 09:00
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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 (5.25 volts).

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2014 F350 with 6.7 diesel power. The odometer shows 199,500 miles, and the vehicle uses an automatic transmission.

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 (Figure 4). 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. Basically 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; fuel quality, fuel volume and 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.

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