GM EcoTec3 Truck Engine GDI Concerns: New Technology Affects Service Practices

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GM EcoTec3 Truck Engine GDI Concerns: New Technology Affects Service Practices

General Motors’ EcoTec3 family of engines is a familiar sight under the hoods of many GMC and Chevrolet pickup trucks and SUVs. It is available in three versions: 4.3 V6, 5.3 V8 and a 6.2 V8. General Motors debuted this new engine design on the 2014 model year trucks and although the 6.2 V8 versions look comparable to the GEN 5 V8 LT of the Corvette and Camaro, they are uniquely individual engines.

The EcoTec3 was designed with towing, hauling and pickup truck grunt work in mind, not pulling 1G on the skid pad. The EcoTec3 engines introduced a tremendous amount of new technology for a truck engine and involved major internal architecture changes over its V6 and V8 predecessors.

These new technologies and design changes were needed to achieve increased fuel mileage and reduced emissions output while raising power output, torque and increasing engine longevity. Compression ratios increased, intake and exhaust valves swapped locations in the cylinder head, spark plug angles changed to provide better electrode placement in the combustion chamber, a special lubrication system was designed and the engines’ fuel delivery system changed.

The EcoTec3 engine was equipped with gasoline direct injection, or GDI, replacing the old port style fuel injection used in different versions for many years. GM had been using GDI in other vehicles for some time, but this was their first use of GDI in full-sized pickup and SUV applications.

Directly injecting the fuel into the cylinder optimizes combustion over a wide range of operating conditions, allows for increased compression ratios without knock and providing increased power output and fuel economy with lower emissions. The use of GDI helped reduce cold start hydrocarbon emissions by 25% on the EcoTec3 series of engines.

For all the positives that GDI achieves it has also created some diagnostic, service and safety challenges that today’s techs should be aware of before performing service on them. These trucks and SUVs are popular and many are starting to show up at our shops with fuel system-related issues.

No-starts, driveability glitches, illuminated malfunction indicator lamp (MIL) and other fuel system-related concerns are common and when that happens you are going to need a plan of attack for efficient diagnostics.

That plan starts with understanding how the EcoTec3’s GDI fuel system functions and breaking it down into three distinctive systems: low-pressure, high-pressure and injection. Each system has its own operational characteristics, and understanding those characteristics is important for effective diagnostics, repairs and personal safety.

The low-pressure system

The low-pressure system is responsible for moving the fuel from the fuel tank to the high-pressure fuel pump, and the EcoTec3’s low-pressure fuel system is electronically controlled and return-less. GM uses a modular reservoir assembly (MRA) that houses the fuel pump, fuel pressure regulator, filter assembly, sending unit and a jet pump that uses some of the fuel pump’s pressurized output to maintain the proper fuel level in the MRA. The low-pressure system utilizes a fuel pump control module (FPCM) to control the low-pressure fuel pump operation. The powertrain control module (PCM) monitors and requests low-pressure fuel pump operation using the GMLAN serial data line connected to the FPCM. The FPCM then controls the fuel pump speed and the desired fuel pressure requested by the PCM using a 25 kHz pulse width modulated (PWM) control signal. The maximum current that the can be supplied to the in-tank fuel pump from the FPCM is 22 A. A serviceable inline fuel pressure sensor (three wire, five volt sensor) mounted on the output fuel line from the fuel tank reports low-pressure fuel system information to the FPCM so that it can maintain the pressure desired PCM commanded pressure. The fuel pressure will vary with load, but typical readings at idle will be 43 psi to 45 psi and key on/engine off values range from 50 psi to 100 psi depending on operating conditions, temperature and other factors. The pressure will increase to almost 90 psi to 100 psi during initial cranking.

The FPCM is used to lower the electrical system load on the vehicles and increase the fuel pump longevity by reducing the fuel pump speed when the engine is under low load situations. During heavy load or hard acceleration, the FPCM will increase the fuel pump speed as required to make sure the engine gets the fuel needed. The FPCM can store DTCs and it will instruct the PCM to turn on the MIL due to issues that are under its control. Wiring to the low side fuel pressure sensor, circuit issues to the FPCM, the low-pressure fuel pump operation and fuel pressure concerns are just a few of the factors that can set DTCs. The FPCM can be easily overlooked and is a major cause of no-starts and stalling conditions, so it should always be checked for trouble codes when diagnosing a fuel system concern.

Low-pressure concerns

GM started using the FPCM back on some full-size pickups in 2008, as corrosion and water intrusion of both the connector and module have been an ongoing issue. The FPCM comes blank and needs to be programmed to function... it’s not plug and play. There were some factory manufacturing issues that installed the incorrect fuel pressure regulator inside the MRA resulting in hard starts, stalling and stumble concerns, and this repair requires a MRA replacement.

Some inferior quality aftermarket MRAs don’t have a jet pump and that can cause stalling and fuel starvation problems when the fuel level is low or the truck/SUV is on uneven ground. It’s important to make sure the replacement MRA is equipped with the jet pump.

The high-pressure system

The GDI system is injecting the fuel directly into the combustion chamber and has a small window of opportunity to deliver it (more on this later) so it requires a fuel pressure that is much higher than the old port fuel injection pressure values we are accustomed to dealing with. These high pressures are achieved using a PCM-controlled mechanically actuated high-pressure fuel pump assembly that is driven by a special three-lobed section on the camshaft. It is capable of a producing a maximum 3,118 psi before the mechanical internal relief valve opens. The normal operating pressure range is lower, typically in the 290 psi to 2,176 psi range depending on what the PCM is requesting to meet actual vehicle operating conditions. The PCM regulates the output pressure of the high-pressure fuel pump by controlling an integral electronic using a 12V PWM control signal.

The PCM regulates fuel pressure by opening and closing this electronic pressure control solenoid each time the piston is being stroked (three times per cam revolution) allowing for the fine-tuning of the fuel pressure in the fuel rail. The pressure control solenoid is normally closed or in the low-pressure mode and needs to have a 12V control signal to increase the fuel pressure (this is important to remember because a simple open circuit/damaged connection could cause fixed low pressure output). The high-pressure fuel pump is connected to the fuel rail assemblies using two intermediate stainless steel fuel feed pipes. The fuel line on the left bank contains the fuel injection fuel rail fuel pressure sensor (GM’s name for the fuel pressure sensor on the high-pressure side). This high-pressure sensor uses four wires and contains two analog pressure sensors and a 5V reference signal (this sensor is our window to the high-pressure side as there is not a test port). When the pressure is low, the output signal voltage is low and when the pressure is high the output signal voltage is high. The sensor is serviceable separately from the rail and the proper torque is required if replacement is needed.

Great care must be taken when dealing with the high-pressure side for safety reasons. Proper service information procedures must be followed carefully and all the proper safety precautions must be followed when any service is to be performed. Proper fuel depressurization procedures and personal safety equipment are essential as there is more than enough fuel pressure to cause serious harm.

High-pressure concerns

Proper depressurization is essential before any service can be performed on the high-pressure side and the service manual information will commonly tell you to remove the fuel pump fuse and start the engine. After it stalls work can be performed. But I have had these engines keep running! There can be enough pressure from the low-pressure side system to allow the engine to idle, and run like it is in a limp-in mode. I hook up a scanner and look at the high-pressure fuel sensor value; it should show 100 psi or less and I consider that a safe pressure to begin repairs. NOTE: If the high-pressure fuel pump has to be removed, the retaining bolts have to be replaced when reinstalling the pump as the bolts are torque to yield.

The cam needs to be set so that the pump is not in stroke (on the base circle, off the lobe) before installation. The pump flange wasn’t designed to pull the pump down over spring pressure and may bend. There is a factory tool available to verify this position but I use a dial indicator and slowly rotate the engine to locate the off-lobe position.

There is a pump roller bucket/follower underneath the pump’s actuator plunger that is indexed so make sure it is properly aligned before installation. Any time any of the high-pressure feed lines are removed or loosened, they need to be replaced. These feed pipes are single-use due to the slight distortion when tightened to give a proper seal, and during installation don’t use any sealer. Just a drop of clean engine oil is all that is needed.

There are TSBs for noises, buzzing and vibration concerns from bad check valves and high-pressure fuel pumps that may give off a fuel smell or actually leak. Both require replacement of the high-pressure fuel pump assembly, mounting bolts and fuel line. Another concern is in the engine harness connector X161 at the rear of the engine. This connector provides the high and low side circuits to pressure control solenoid and it may blacken or corrode causing intermittent opens in the circuit (and low fuel pressure).

The fuel pump diver lobe on the camshaft can suffer accelerated wear and scuffing from a lack of maintenance, improper oil (they recommend Dexos) or if the engine is continually low on oil. It’s also important to remember that all of the extra sound deadening material and covers must be replaced after service, or a ticking/ lifter type noise complaint may result.

The high-pressure fuel injectors:

The EcoTec3 engine’s fuel injectors fire directly into the combustion chambers and this presented some problems for the engine’s designers. The injector tip must deal with the cylinder’s compression pressures, temperatures over 1,000 degrees Fahrenheit, fuel pressures in excess of 2,000 psi, deposit resistance and limited space to be installed. Engine design changes were needed. Special pistons, combustion chamber shape and a GDI fuel injector had to be developed to meet these conditions.

A low impedance, peak and hold injector was developed that could spray atomized fuel through laser-drilled holes directionally into a specially designed piston dome relief or “fuel bowl” to maximize power and combustion efficiency. But getting these injectors to open under such extreme operating conditions requires more than 12V. The PCM was redesigned to incorporate an internal DC to DC converter boosting 12V to the needed 65V (stored in capacitors) to get these injectors to open. Once opened, 12V is all that is needed to hold it there. Providing the engine with the correct volume of needed fuel is another issue.

On a port fuel injector increasing the injectors’ on time worked, but that can’t happen with direct injection because it can only fire when the exhaust valve is closed.

Port style injectors had an on time of 1.5ms to 3.5ms and that is an eternity compared to the average GDI injector on time of only 0 .4ms. The volume of fuel delivered to each cylinder on these truck engines is now controlled by the fuel pressure. The higher the pressure, the more fuel that is injected. The PCM controls the high-pressure fuel pump’s electronic pressure control solenoid to maintain the proper fuel pressure available at the injectors and this provides the volume of fuel needed to meet the current operating conditions.

Again, similar to dealing with the high-pressure fuel pump and its components these injectors have enough fuel pressure and voltage to do serious harm, so be sure to follow all the proper safety and service precautions when working on or near them.

High-pressure injector concerns:

These injectors have a low resistance of 1.06 Ω at 68 degrees F, so any corrosion that increases the resistance anywhere in the circuit will likely trigger issues and trouble codes. Attention to the noise deadening materials and its proper placement is important to limit comeback complaints. Because the injector tips are in the combustion chambers they are susceptible to carbon build-up, and this can prevent removal issues with potential damage (think Ford Triton 3V spark plugs). Applying penetration oil at the sealing surface and carbon remover solution inside that cylinder via the spark plug hole is a recommended procedure.

There are special puller tools that apply even pulling pressure along the entire fuel rail during removal to prevent damage to the injectors and the cylinder head. Any time the injectors are removed, the retainer clips holding them to the fuel rail, the plastic spacers and dust and combustion chamber seals and fuel feed pipe must be replaced. The combustion chamber seals require specific tools for installation and no lubricants should be used to install them. After installation on the injector body, the seals need to be resized to provide a leak-proof seal.

Using GDI and firing the fuel directly into the combustion chamber certainly has its advantages, including better fuel economy, more torque and power from higher compression ratios and leaner cold start emissions. But it also has some disadvantages, including higher soot levels when cold, additional specialty tools, many single-use components and the issue of carbon on intake valves. Lack of proper maintenance, poor fuel quality and improper oils are all enemies of the GDI system. They can lead to accelerated wear and a host of other issues, but these systems are robust. Safety procedures and depressurization steps have to be followed and the use of 65V requires careful attention to detail before any back-probing of circuits is performed during testing.

The use of GDI in combination with other technologies like active cylinder management, variable valve timing and adaptive exhaust systems has made these engines and the trucks that use them smooth running, durable and reliable.    ■

Jeff Taylor boasts a 32-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.

To read more of Jeff Taylor's articles, see:

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