Anytime new technologies come out, there are growing pains in the acceptance and operation of said novel systems. When GM released the Gen V LT-series engines, they were not the first to do direct injection, but for most gearheads, this was their initiation into this new world. Direct injection is a giant leap forward in EFI and engine management, but there are some things to consider when swapping into your vehicle, even if you previously had port injection. LTs are great engines, we do not want them to be cast aside due to some misunderstood problems. Our focus today is the fuel system as it pertains to Gen V direct injection swaps.

Gen V LT Fuel System Basics

Much like a diesel engine, the DI fuel system is split into two halves: the lower-pressure system from the tank to the High-Pressure Fuel Pump (HPFP), and the high-pressure engine side. The foundation of the direct injection fuel system is the mechanical high-pressure pump, which is driven by a lobe on the camshaft. The HPFP takes the low-pressure fuel and pumps it up to as high as 2,900psi (that number is engine specific), which is then blasted into the combustion chambers by the injectors.

The Gen V LT-series engines from GM are capable of LS-killing horsepower levels, but the direct-injection system can be an issue if you don’t feed it properly. Image courtesy of General Motors.

The low-side system is where the trouble can start. GM uses a  Pulse-width modulated (PWM)-controlled fuel pump to manage the fuel pressure to the HPFP, which varies greatly based on engine RPM and load. Because the system is returnless, there is no static fuel pressure, it is constantly varying. The low-pressure fuel pump (LPFP) is controlled by a remote fuel pump control module, which is managed by the ECM. Instead of a fat 10-gauge wire running to the pump, the pump is fed by two 14-gauge wires. This may seem way too small, but the pump doesn’t pull much amperage. Instead, the control module adjusts the voltage, amperage, and duty cycle to spin the pump to achieve the desired pressure. The PWM system fixes a common issue with two-stage fuel systems in diesel engines, but going to a static pressure lift pump can bring those issues back.

This is far more responsive than a static pressure system, as the PWM system can increase pressure nearly instantly without any drop in fuel delivery. Static pressure systems cannot react this fast. With a static system, when you drop the hammer to WOT, you can get a brief moment when the fuel pressure drops, and the pump has to rebuild the pressure. So why does this matter?

Damage as a result of cavitation can be seen in the B arrows, caused by thousands of micro-explosions. It doesn’t take much to reduce the efficiency of a fuel pump.

How Does Fuel Pressure Affect Flow?

As the HPFP spins, the fuel inlet opens and closes, filling the compression chamber. The speed at which the HPFP is operating determines the window for the pump to fill. Just like most electric fuel pumps, the HPFP needs to be full in order to generate the correct pressure. This is where the PWM system shines, at any given point, the low-pressure system is operating at the optimal pressure for the engine’s RPM and load. At idle and cruise, the LT may call for 58 psi of low-side pressure, as this is enough to fill the chamber, but at WOT and heavy load, the ECM will call for 72 psi, which is the pressure required to fill the HPFP in the reduced time of each rotation.

Now let’s take that same system and swap out the PWM pump for a typical return-style static pressure pump. In the LS swap world, 58 psi is the setting LS engines operate with. What has happened is that many swappers fear the PWM system (it’s actually easier than static!) and choose to stick with static pressure, set at 58 psi. This is where you get into trouble. With 58 psi, an LT direct injection engine will run, it will start, idle, and drive down the road. However, at WOT and heavy load, that 58 psi is just not quite enough to fill the HPFP. This means that the pump is partially filled, creating two problems. First, the HPFP can’t get the pressure high enough, so you lose a little power. Second, and more importantly, the HPFP will cavitate.

This little guy has been causing a ton of trouble, specifically the overseas version, which is leaky, often mislabeled, and has erratic pressure regulation. That said, even the high-dollar AC Delco units cause issues for LT swaps.

What Is Cavitation?

Two things kill fuel pumps — heat and cavitation. Cavitation occurs when vapor bubbles are induced in a liquid under vacuum. All electric fuel pumps are susceptible to this force. If the HPFP pump cavity is not filling all the way, a vacuum is generated on the inlet side of the HPFP, further lowering the pressure. This causes vapor bubbles to form and then split, imploding, causing a micro explosion as the fuel passes through the pump. This is extremely damaging; even a few minutes of cavitation can ruin a fuel pump. If the HPFP is not fully filled every time, that small amount of space allows the fuel to cavitate, eventually killing the pump.

In personal experience, we had a car running a static fuel system to a retrofit EFI system on a Buick 350. Because most of us gearheads think bigger is better, we strapped an Aeromotive A1000 inline pump to the chassis, with a half-inch fuel line from the sump feeding it. If you know much about fuel systems, you may have spotted the issue: the feed line was too small. That pump didn’t survive 500 miles. This is because the small fuel line could not supply the pump with enough fuel to properly fill it.

Then there is the heat issue. For PWM pumps, this is not much of a problem, as the pump runs slow enough to not get very hot, and the fuel in the tank keeps the pump cool (provided there is enough in the tank). Return-style systems are different, as they pump fuel to the engine, and then the regulator sends the rest back to the tank. That return fuel is significantly hotter than the fuel in the tank. If your return line is too close to the in-tank pump or outlet, you are sending the same hot fuel back to the engine in a brutal cycle. It is not uncommon to see a sumped tank have a feed line on one port, and the return line on the other, this is the worst kind of heat loop for a fuel system.

An aftermarket drop-in module like this one from Aeromotive makes an LT-swap with PWM fueling much easier, these are ready to go for LTs, you just have to drill a small relief hole (like seriously small, 1/32 inch).

So now, in our static fuel system, we have fuel that is heated by the engine as it is pumped through the engine bay, cycling back into the tank near the feed line, going right back to the HPFP, getting increasingly hotter. The hotter the fuel, the worse the cavitation gets, dropping pressure significantly. For a DI engine, fuel pressure is crucial for best performance at any power level.

Oh No, Not The Filter/Regulator!

Now that we have illustrated the issues with static pressure in a direct injection system, we can talk about the common causes and fixes. In the LS-swap world, the C5 Corvette filter/regulator is king. Unless you are running big power, this unit has been the go-to for many LS swaps. If you use the GM AC Delco model, you are good (with an LS). However, the overseas copies are largely terrible, providing unreliable pressure and lots of leaks.

You don’t need 10-gauge wire to power a PWM pump with fuses and a relay, just some 14-gauge wire in a standard braid with an isolating ground wire .

Unfortunately, the LT-swap realm has pulled in that same filter/regulator, which is not the best solution. First, you are only getting 58 psi, which reduces power and is “cavitation city,” but it also causes some other issues. In an LT swap, this regulator tends to have higher head pressures, putting strain on the pump and increasing the current draw (which turns into a fire risk), and wearing out the low-pressure pump prematurely. The LT-series engines need 45 gph (170 lph) at 72 psi, and the filter regulator is designed for a 190-lph pump. This is too close to the maximum flow rate; it just isn’t big enough to allow optimum performance in flow, not to mention the pressure is too low.

What Is The Solution To The Gen V Direct Injection Fuel System?

If you want to get the most out of your Gen V LT-series engine, you need to feed it correctly. The best option is the PWM system, as it is designed for these engines and is the most efficient. There is a myth in the swap world that a PWM fuel system is more complicated and expensive than a static return-style, but that just isn’t accurate. The pumps are the same price; you just need a PWM-capable pump, such as the Aeromotive 340. A stock GM control module can be found for $20 to $40 at any salvage yard, and the wiring is easier: you just have to braid three wires together, with the 3rd wire grounded at one end, or use a shielded sleeve. You have fewer fittings, fewer lines, and no regulator to mount. In our experience, the Aeromotive Phantom 340 works very well with the stock controller, and you can also use a stock Gen V pump mounted to the pick-up tube in the tank.

I already Have A Static Fuel System, Can I Use It With a Gen V LT Engine?

We get it, you don’t want to buy more stuff, and you already have a return-style system. No problem, it just needs to be set up correctly. The key is to increase the static fuel pressure to 72 psi and ensure that your existing pump is capable of producing 72 psi at 45 gph. Static systems also require an emergency valve set to 84 psi. Aeromotive makes a filter/regulator combo that is designed to replace a C5 filter/regulator, so you can mount it under the vehicle in the same position (it is a little larger than the C5 unit) and has an adjustable regulator.

It may not seem like much at first, but it doesn’t take much of a pressure difference to cause issues for your HPFP. Running a Gen V LT at 58 psi is just silly; you are starving the fuel pump at higher RPM, putting more strain on the pump, shortening its life. The choice is clear: PWM or 72 psi static.

This is a GM fuel pump control module; this and the sensor on the right is about all you need to control a PWM fuel pump with a stock ECM.