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A guide to Ford diesel diagnostics

Wednesday, March 1, 2017 - 09:00
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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.

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