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After servicing and answering questions about these engines for several years, it finally occurred to me that the owners of these engines didn’t understand how they work – and, frankly, neither did many repair techs. I remember when Ford introduced the Navistar engine in the middle of 1994.

The rumor on the block was that you could only use a certain 15W-40 diesel oil. At first we thought that this was a load of malarky and sounded like Ford was just trying to sell oil. But later, I learned that the rumor had a lot of merit and Ford wasn’t trying to increase oil sales – the type of oil actually mattered in this engine!

This all stems from a common problem that I see at the shop. Customers will come by and say that their Power Stroke is running rough and erratic. The first question that I ask is “When was the last time that you changed your oil?” I automatically get a chuckle and they think that I’m crazy. Well maybe I am, but after I change their oil and put the right oil in it seems to shape right up. This all comes from aeration in the oil, which makes the Power Stroke run erratically.

While these customers might know how  engines work, for the most part, Power Stroke owners do understand the service needs behind these diesel engines. Most owners tend to make sure that they perform the needed maintenance, regarding the fuel, oil and air filters. The fact these owners didn’t realize exactly how their diesel engine operated is understandable, since most previous Ford diesels were typical engines like those that had been in service for years.

The first 7.3L was produced from 1988 to 1993. In 1994, the engine underwent some changes, going from the original IDI (indirect injection) to DI (direct injection), getting an upgraded turbocharger and electronic fuel injection. This model produced 250 hp and 525ft.lbs. of torque. Navistar named this the Power Stroke. Ford and Navistar have had a working relationship for years, with Navistar producing engines for the Ford full-size trucks.

The invention of the Power Stroke was a great move, making it the most powerful diesel on the market at the time. Everyone knew the reliability of the previous diesel models, but of course they all wanted more power. So, let’s take a crash course in what makes these engines a work of art.       

The Power Stroke engines are sometimes referred to as HEUI or,  the slang term “oil fired,” engines. What in the world does “HEUI” or “oil fired” mean? As we discussed earlier, the first 7.3L was an indirect injection diesel with a rotary style injection pump with hard lines that came over to the injectors mounted in the cylinder heads.

With the Power Stroke, Navistar continued this block concept, but replaced the injection pump and lines with a hydraulic pump. This is commonly referred to as the high pressure oil pump (HPOP). Navistar moved the injectors from their original location and placed them under the valve covers, replacing them with an electronic style that gave them a better position in the combustion chamber.

The HPOP delivers high pressure oil that is used to activate the injectors. This is where the term HEUI (hydraulic electronic unit injectors) comes from. In the 7.3L Power Stroke, oil is delivered to the HPOP from the engine’s oil pump located in the front cover. The HPOP then takes the oil received from the crankcase and pumps it back out under extremely high pressures (600-3,000 psi) through hoses that go to top of the right and left side cylinder heads. Integrated into the cylinder heads is an oil passage or “barrel” that can be seen from the front or rear of the cylinder heads via a block-off plug.

The barrel in the cylinder heads is serviceable and can be accessed through these plugs. However, these plugs have O-rings that can leak over a period of time. Keep in mind that the O-ring is not sold separately, it is only available as a new plug assembly.

When the high-pressure oil enters the barrel of the cylinder heads there are passages that lead from the barrel to each injector. An oil chamber inside of each injector acts as an “amplifier piston.” Fuel also enters the cylinder head through a line at the front of each cylinder head (a fuel galley) from the fuel filter basket supplied by the pump at approximately 40 psi. The fuel then enters a chamber in each injector.

So now the stage is set for fuel to enter the combustion chamber. We have oil that has entered the cylinder head from the HPOP surrounding the injectors in each cylinder bank. We also have fuel that has been supplied to each injector through the fuel galley of the cylinder head via the fuel pump. Now we need to activate the injectors.

The electronic injectors are activated or “fired” by a driver module, which is activated by the engine’s powertrain control module (PCM). This information is provided to the PCM from a cam sensor located above the harmonic damper in the front timing cover. The cam sensor is a hall effect switch that tells the powertrain control module what cylinder is ready for injection.

When the electronic injector is energized from the injector driver module an electronic solenoid mounted on the injector opens a poppet valve. High-pressure oil from the HPOP flows into plunger, which moves downward, increasing fuel pressure and opening the nozzle. Fuel is then injected into the combustion chamber at pressures as high as 18,000 psi.

So, I said all of this to explain one thing. The oil does play a major role in the running of your Power Stroke engine. I have found that you don’t have to use the Motorcraft oil, but you should select and recommend one with adequate amounts of anti-foaming agents. I have found this seemingly simple explanation to be the main reason for the Power Stroke engine to run rough.

the 7.3l ford/navistar power stroke is often an elegant unknown.the oil barrel found in the cylinder head is used to help boost the oil pressure to activate the injectors.power stroke cam sensor.this is a photo of the aftermarket performance hpop mounted atop of the engine in place of the traditional injection pump.this pump is commonly referred to as the high pressure oil pump (hpop).Oil pump that is located in the front cover.

Robert McDonald is owner of Atlantic Engines in Granite Falls, NC, and specializes in high performance diesel and gasoline engines and cylinder heads for street, marine, dirt or drag racing.

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Bob McDonald

Bob McDonald

Owner at Atlantic Engines
Bob McDonald is owner of Atlantic Engines in Granite Falls, NC, and specializes in high performance diesel and gasoline engines and cylinder heads for street, marine, dirt and drag racing.