A guide to Ford diesel diagnostics

March 1, 2017
When a Ford diesel pickup with a drivability problem rolls into your shop, do you have reservations about tackling the job?

Light trucks have been using diesel power for quite some time. Back in the early 80’s when diesel power started becoming popular; the engines were noisy, produced a lot of smoke and were lacking for power for the most part. The mid 90’s saw electronic fuel injection systems coming in popularity and along with the electronic fuel injection systems, turbochargers were also the name of the game. The light duty diesel engines were now operating without much smoke, were becoming quieter and would produce loads of power.

This was all well and good until the clean, powerful diesel didn’t run quite right or didn’t produce the power that it should. The days of hooking up a fuel pressure gauge or cracking an injection line or two to find that weak cylinder were gone. Now in the electronic age, we cannot see, feel or hear what is controlling the fuel injection system.

This is a 2001 F250 powered with the 7.3 powerstroke engine. This vehicle is using an automatic transmission and there are 156,000 miles on the odometer.

Before we get too far into this discussion, I would like to mention two very important things about a diesel engine, or any engine for that matter. The basis for an engine making an engine run and the engine making power is the need for; air being pumped through the cylinder, proper compression, the proper amount of heat in the combustion chamber and the proper amount of fuel being injected at the proper time.  In my diagnostic world, any engine power complaint, no start or misfire always comes back to these four things.

Ford came out with their first 7.3 powerstroke in late 1994, this run of engines lasted until early 2003. In late 2003 the 6.0 was the engine of choice. The 6.0 lasted until late 2007, then it was replaced by the 6.4, which lasted a few years, then was replaced by the 6.7.  In my little shop, I still see many 7.3 and a lot of 6.0 engines. I will limit the discussions in this article to these two engines. Both of these engines use the HEUI (hydraulic electronic unit injection) fuel injection system. One of the nice things about this fuel injection system is the ease of using a scan tool to analyze fuel injection problems. By using a scan tool to test the injection system, you have the best fuel injector test bench known to man. The fuel injectors can be tested dynamically under real world working conditions, which cannot be done in a fuel injection shop.

Proper combustion chamber heat needed for starting

To get a diesel to start, there are three basic needs; proper amount of fuel injected at the proper time, the proper amount of heat in the combustion chamber and the ability of the cylinder to pump air. To meet these needs, glow plugs are used in the combustion chamber to achieve the needed heat. The glow plug alone will not produce enough heat; a proper cranking speed and proper compression are also needed to get the combustion chambers hot enough to get the engine to start.

Most times, a cranking speed of 160 rpm is ideal. To get this speed, the batteries need to be up to the task, along with the starter motor. If either of these are not correct, you will have starting problems.

Proper fuel pressure

In the HEUI engine, fuel is delivered to the injectors by either a mechanical or an electric fuel pump. This pump is referred to as a transfer pump, or horizontal fuel conditioning module. The pump will either be mounted in the engine valley, in the case of the early version 7.3 (1994 - 1997) or mounted to the left frame rail directly under the driver’s feet. The fuel pressure should be around 60 psi, or a little higher on the 6.0 if the updated fuel pressure regulator kit has been installed. Without proper fuel pressure and volume, the engine will lack power.

ICP and the HEUI engine

ICP (Injector control pressure) is a very important thing to understand when analyzing starting and running problems on these engines. The ICP sensor is screwed into the high pressure oil rail and is a feedback signal to the PCM (powertrain control module) The high pressure oil system operates in a closed loop system, where the PCM commands the IPR (injector pressure regulator) which controls the high pressure oil, which operates the hydraulic fuel injectors. Without proper feedback from the ICP sensor, it is possible to get; a no start, a rough running engine, or a poor power complaint. Here again, keep in mind the three basic needs of the diesel engine; fuel, compression and air. These three things need to be in the correct order and quantity.

IPR, the PCM command of high-pressure oil

The last thing on the list is the IPR (Injector Pressure regulator). This pressure regulator is a pulse width modulated device, which is operated by the PCM. This regulator is screwed into the high pressure oil pump and controls the pressure in the fuel injector high pressure oil rail. This oil pressure is monitored by the ICP sensor, which sends the pressure feedback to the PCM. If the ICP sensor reports the wrong pressure, the symptoms can be; a no start, poor power or a rough running engine. This system works in closed loop (PCM commands the IPR, which is monitored by the ICP). This IPR regulator is normally open, so, if there is no command from the PCM, or there is no power to the regulator, the engine will not start.

2001 F250 cranks/no start

The vehicle is a 2001 F250 powered with the 7.4 direct injected powerstroke diesel engine. The odometer shows 156,000 miles and uses an automatic transmission. This vehicle is at a shop that has installed a set of new glow plugs and valve cover gaskets in an attempt to fix the no start when the engine is cold problem. The engine will start fine if the electric engine heater is plugged in for an hour or so.

Knowing this information narrows the starting problem down to only a few things. When I arrived at the vehicle, it was sitting outside in the snow. The temperature was about 15ºF and has been sitting for about 4 hours. On any problem like this on HUEI engines, I start with a scan tool and do the KOEO (key on engine off) injector electrical buzz test. This test is testing the upper portion of the fuel injector, the electric solenoid that sits on top of the fuel injector and the wiring between the injector control module. There are two parts to this test, the electrical part and the audio part. If there are any problems with the electrical circuits to the injectors, the scan tool will display a DTC. If there is anything stuck in the mechanical hydraulic control, you will be able to hear it as the scan tool activates each fuel injector. In the case of this vehicle, there were no DTC’s stored and I was able to hear each of the fuel injectors buzz with the same sound.

With an electrical or hydraulic problem ruled out, I will use the scan tool to record four data PIDS from the PCM. These pids are; RPM, fuel injector pulse, ICP voltage and IPR control. The reason I use ICP voltage instead of ICP pressure is, the ICP voltage is always correct. The ICP pressure is an inferred value, and during a cranks/no start problem, many times the PCM will substitute a value, something like 2250 psi and this can easily lead you down a rabbit hole. The PCM will not pulse the fuel injectors until it the ICP voltage reaches .8 volts. In the case of an ICP sensor that does not report correctly, this can easily cause a no start if the ICP voltage does not get above .8 volts. The PCM will only react to the voltage sent by this sensor. Always be sure to take a look at the ICP voltage before you start cranking the engine. The KOEO voltage should be .2 - .25 volts. If this KOEO voltage is higher or lower than these values, then it is time to replace the sensor. In the meantime, unplug the sensor and give the engine a crank.

Figure 1 - Scan data from the Crank/no start. This scan data gives me a direction to my next test. I know the hydraulic side of the fuel injectors is working properly; the engine will fire but will not start.

This is one of the few times you will hear me say to unplug something. When the ICP sensor is unplugged, the PCM will default to an ICP pressure of 700 psi, and it will pulse the injectors. Figure 1 shows the scan data captured on the first attempt to start the engine. With this scan data, I can verify proper cranking speed, proper ICP pressure and proper IPR command. If the PCM did not see a proper cranking speed (caused by either low cranking speed or no input from the CMP (camshaft position sensor), there would be no fuel injector pulse displayed. The scan data shows proper fuel injector commands and by watching the graph of RPM, it is easy to see the engine trying to start.

With this information and the history of the vehicle, I am assured there is some fuel supply to the fuel injectors, at least enough to get the engine to start. I can now narrow the problem down to either low compression, stuck fuel injector poppet valves, or no glow plug operation. Since the easiest test is the glow plugs, I will go there first, but wait, this engine has a new set of glow plugs and valve cover gaskets. That leaves only a glow plug relay not working or the PCM is not grounding the glow plug relay.

Figure 2 - Scope capture of the glow plug circuit. Red trace is a volt drop across the glow plug relay terminals. Blue trace is the glow plug current. The waveform tells me there is power to the relay; the relay has tried to work, but will not carry the needed current to the glow plugs.

The next test is to use a labscope and current probe to test for proper glow plug operation. I will use channel A (blue trace) to test the current draw from the glow plugs, and channel B (red trace) to volt drop across the glow plug relay terminals. Figure 2 shows the results of the test. The labscope trace shows something happened when the ignition switch was turn to the on position. There was some current flow, and some volt drop across the relay terminals, what happened is not proper glow plug operation.

Wiring diagram of the glow plug circuit. The high current side of the relay is powered from the battery. The relay coil is also powered directly from the battery. The relay coil is grounded through pin 101 of the PCM.
Figure 3 - Scope capture of the glow plug current. The glow plugs on a 7.3 powerstroke engine should draw 192 amps when first turned on. As the glow plugs warm up, the current flow drops. This waveform proves out the circuits supplying power to the glow plugs and the glow plugs are working correctly.

By using a wiring diagram of the glow plug circuit, we find the glow plug relay is powered directly from the battery; the relay coil is also powered directly from the battery and is grounded by the PCM at terminal 101. Knowing this should make the pinpoint testing of the relay pretty easy. The pinpoint test found system voltage at both positive terminals of the glow plug relay. The negative terminal of the relay coil showed system voltage and when the key was turned to the on position, the voltage was pulled to zero volts (the relay coil should be grounded at this point) but the relay contacts were never closed. This test has proved the PCM is capable of grounding the relay coil and the relay is defective. To test for proper glow plug circuit operation, Figure 3 shows where I have used a screwdriver to short the two relay terminals across to test for proper glow plug current. The waveform shows an initial current of 192 amps. This value is correct, so I know there is nothing wrong with either the glow plugs, or the wiring. It is time to install a new glow plug relay.

2005 F350 poor power complaint

The next vehicle is a 2005 F350 powered by the 6.0 direct injected diesel engine. The vehicle is using the Ford Torqueshift transmission and the odometer shows the vehicle has traveled 297,500 miles. The complaint is the engine lacks power especially when starting a heavy load. Once the engine gets up to speed, it seems to have good power, although the transmission will downshift when climbing even small hills. There is also no check engine light on with the engine running.

Figure 4 - DTC P0472 gives a good direction on where to start on this poor power complaint.

The first step in the diagnostic process is to hook up a scan tool and see if there is any diagnostic information stored in any module. The scan tool found one DTC hiding in the PCM. Figure 4 shows a DTC P0472 with a scan tool description of “exhaust pressure sensor low”; my question is low what? Is it low voltage or low pressure? Well, since any sensor like these only inputs a voltage to the PCM, then the problem is a low voltage. Since this is an electrical circuit DTC, we must treat it as such.

The next step in the diagnostic process is to stroll over to our service information system and see what the manufacturer has to say about this DTC. Service information (Figure 5) gives a code description of “exhaust pressure sensor low input”. Over to the right side of the DTC description is a little blue “X”; by clicking on that X, you jump to the pinpoint test (Figure 6). Here is a lot of information about the DTC; I have highlighted the important part in red. The 6.0 engine uses this EBP (exhaust back pressure) input for several things and when the PCM doesn’t like the voltage sent from the EBP sensor, it locks down the EGR valve and does some funny stuff with the VGT (variable geometry turbocharger).

Figure 5 - Service information is always a good thing to check when wanting to know the correct information on a DTC. In this case, the information also includes a hyper link to the next step in the diagnostic process, the pinpoint testing.
Figure 6 - The pinpoint testing starts out with the description of the circuit and the theory designed into the system. What more information do you need? Knowing how the system works is half the battle of finding out what is wrong with the system.

Please keep in mind for any engine to produce power they need to have fuel and air to make that power. In this case, the PCM will limit the turbocharger operation, which will also limit the amount of fuel the PCM can inject into the combustion chamber. This all results in low power output from the engine. Before embarking on any electrical testing I like to take a look at scan data to determine the next step.

The scan data shown in Figure 7 was taken during a test drive. Notice in the upper left corner the two EBP pids, EBP voltage and pressure. The EBP voltage is showing zero volts. This information is telling me there is an open circuit in the EBP sensor circuit. I took the vehicle on a WOT (wide open throttle) test drive to see how the ICP, IPR and engine load pids looked. I have circled the important PID informant in red. To me, this data doesn’t look right. The data shows the engine is lacking power. Now if you are not familiar with this data, be patient and I will put up the data captured after the engine is fixed. By comparing the two sets of data, it will be easy to see the difference.

Figure 7 - Scan data from the engine when the EBP (exhaust back pressure) sensor was not reporting properly. Notice in the pids circled in red, how the data is pointing to the poor power problem. Please compare this data with the known good data captured in figure 9.

Since the EBP voltage shows zero volts, we need to go to the sensor and take a look to see if the plug is fastened in the sensor properly, (do a visual) or see if the wiring has not been installed properly in its correct place. These engines have a lot of electrical harnesses lying on the top of the engine and if they are not put back in their correct places, bad things happen. In the case of this vehicle, everything looked good. The harness was nice and clean, installed in its proper place and the plug was installed in the sensor properly.

Since the scan tool displays the sensor voltage, there are two things we can do to use the system to test its self, one would be to gently wiggle the plug while watching the scan data, or the other would be to unplug the scan tool and use a small jumper wire to jump the reference voltage to the signal line and see if the scan tool voltage responded. In this case, all I had to do was to gently press on the EBP plug and the scan tool voltage changed. Now I am on to something, the problem is in the EBP plug. With the plug removed, it was easy to see the problem; the plug is broken and does not make good contact when plugged in (figure 8).

Figure 8 - This is a photo taken of the EBP plug. The problem with the vehicle is the plug is broken and will not properly contact with the terminals in the sensor.
Figure 9 - Scan data of the fixed vehicle. By comparing this data with figure 7, it is easy to see how the misreporting EBP sensor has caused the lack of power in the engine.

With a new plug installed on the EBP harness the vehicle was taken on a test drive while capturing data (figure 9). This scan data shows the engine running as it should. Again I have circled the important PID information is red. This problem was quite simple, just an EBP sensor that was not reporting correctly. By using the power of the scan tool and the power of service information, the problem was easily found, easily fixed and it was fixed right the first time.

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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