FSD Cooler
Improved 6.5 Reliability
June 2000

By Jim Bigley

The patented FSD Cooler (#D450,044) is a product whose time has come. The Stanadyne DS4 electronic fuel injection pump began as a marvelous concept in diesel fuel injection, but it has produced enough problems to warrant finding a way to improve reliability. The FSD Cooler featured here may offer a cure for many of these problems.

As referenced in GM Technical Service Bulletin #77-63-06A, stalling, hard starting, and hesitation are symptoms of a failing FSD (Fuel Solenoid Driver) module. These symptoms usually (but not always) set a code 35 (or sometimes 36) in the 1994-95 models, or a code 1216 in the 1996 and newer models. Once a pattern develops, the problem will only get worse over time. While a diesel tech (or the owner) shouldn't overlook the basics in diesel troubleshooting, these 6.5 electronic injection system failures can usually be traced to the FSD module. The FSD module is the black box, about the size of a deck of playing cards, screwed to the side of the DS4 fuel injection pump.

In the original configuration, the FSD module is mounted to the DS4 injection pump to allow heat dissipation. The injection pump, being a fairly large mass (about 22 pounds) and being cooled by fuel flow, acts like a large heat-sink for the FSD module. In theory and from an engineering perspective, the injection pump should provide ideal heat dissipation for the FSD. However, reality has a way of disputing theory.

The large finned surfaces of the new FSD Cooler provides better cooling, and offers a couple other benefits we'll discuss in a minute. But first, I should mention that I first heard of experimental remote mounted FSD modules in 1997. Some GM diesel technicians had begun experimenting with this concept in an effort to reduce the failures they were seeing. The problem with these early attempts was the engineering and implementation of the idea. A Swedish company called Beta Machine has done that engineering, and has developed a product that was designed to reduce the number of FSD failures.

To understand why the FSD fails, it helps to know what this device does and what factors contribute to these failures. The 1994 and newer 6.5 fuel injection systems are "drive by wire", meaning there is only an electronic connection between the driver's foot and the fuel solenoid in the injection pump. Pressing on the accelerator pedal causes an electronic signal to pass from the APP (Accelerator Pedal Position) module to the computer, then to the FSD module mounted to the injection pump. The computer signal is then amplified by the FSD module, which in-turn drives a mechanical fuel solenoid inside the injection pump.

The FSD module contains a pair of high-power driver transistors that generate heat during normal operation. Knowing the FSD module drives the fuel solenoid approximately 7,200 times a minute at just 1800 rpm brings the operational requirements into clearer focus. These driver modules are worked very hard and generate significant levels of heat. Use more throttle, and the fuel solenoid is driven harder and for a longer duration.

This means the FSD will generate even more heat under high load or high-speed conditions. Tests have shown that even a low fuel level in the tank or a non operating electric fuel-lift pump will cause the FSD to work harder and generate more heat (See chart below). Add a high ambient temperature (or a dry, thin fuel), such as that found in the Southwestern US during the summer, and you begin to see why these heat induced FSD failures occur more frequently in those areas of the country.

Like any engine, your 6.5 is subjected to countless heat/cool-down cycles. All this expansion and contraction of the FSD module can cause a reduction in clamping load applied by the four original Torx screws. The relatively small-headed Torx screws are used without washers (as originally installed), which allow the screws to sink deeper into the plastic of the FSD module over time. This reduces the clamping load, and reduces the ability of the FSD to transfer heat to the injection pump, eventually resulting in an overheated FSD module and the onset of drive-ability problems. Note: This may explain why some owners experience a repeat of the same drive-ability problems every 20-40,000 miles.

Mounting the FSD to the new FSD Cooler uses GM's recommended screw torque along with a new set of allen head screws and washers. The new washers reduce compression of the plastic, which helps maintain the clamping load over time.

Once mounted to the new cooler, moving air cools the FSD, where fuel circulating through the injection system provided the cooling before. Instrumented tests performed by the manufacturer have shown the FSD Cooler is more efficient at cooling the FSD than when it is mounted to the injection pump. Better cooling means better reliability. Even while idling after a long hard pull, the FSD Cooler continues to have the advantage in cooling the FSD module. Idling presents the least "load" on the FSD, and therefore doesn't require as much cooling capacity as when the vehicle is moving (more airflow). In my tests, I discovered the FSD module just gets slightly warm after an extended idling period when mounted to the new FSD Cooler.

In addition to improving FSD module cooling, removing the FSD from the injection pump removes a huge heat load the pump has to deal with. This may improve reliability of the Optical Encoder/Temperature Sensor in the pump, as well as reducing the temperature of the fuel passing through the injection pump. Cooler fuel improves fuel lubricity (enhancing pump life) and hot re-starts, as well as offering a slight power improvement (cooler fuel is denser, and has more BTU per unit volume).

The new FSD Cooler uses a new heat transfer pad manufactured by the very same company that produces the heat transfer pads for Stanadyne, being a silicone treated aluminum foil. Unlike the pad used by Stanadyne however, this pad has cutouts for the transistors. This allows more radiated heat to escape the module, further aiding in temperature reduction. Removing the plastic covers from the power transistors also helps dump the radiated heat. Every little bit helps.

To effectively transfer heat, both the FSD module and the original machined injection pump-mating surface must be of a certain "smoothness". This improves heat transfer efficiency. Beta Machine measured the relative smoothness of many DS4 pump-mounting surfaces, and discovered they fell within the range of 0.40-0.44 Ra.

The new FSD Cooler also uses a machined surface for the FSD, measuring in the same Ra range as the DS4 mounting surface. Don't accept a knock-off product that doesn't incorporate anodizing and a machined FSD mounting surface.

Mounting surface smoothness is just one criterion enabling good heat transfer. The relative flatness of each mating surface must also match, and must not warp out of contact (in even the slightest amount). Any portion of the mating surface not making contact will reduce the amount of heat transfer capability. The new FSD Cooler is as flat as a precision CNC milling machine can make it, and is at least as flat as the DS4 injection pump-mounting surface.



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