Point of no return

Motor, Jul 1998 by Warren, Mark

There's a lot more to returnless fuel systems than just moving the pressure regulator from the rail to the tank. Here's what you'll need to know to service these systems as they become more and more prominent.

Tightening federal evaporative emissions requirements are driving dramatic changes in fuel delivery systems. One of the main problems is the soaring temperature of the fuel in the rails due to high underhood temperatures. Most of this fuel makes its way back to the tank. In-tank fuel temperatures in summer can be greater than 160oF today; and are a major contributor to excess evaporative emissions. As a result, Chrysler has gone to retumless fuel systems on all its 1998 vehicles, with most manufacturers introducing similar setups on at least some of their models this year. Estimates are that bv the year 2000, all cars and lightduty tracks will be using these so-called returnless, or in-tank return, systems. To help you understand the changes that these new fuel systems bring, let's compare them to the conventional return-style fuel delivery system.

Standard passenger-car fuel systems pump about 30 gallons per hour (gph), or a half-gallon per minute. A car traveling 60 mph that gets 30 miles to the gallon, therefore, consumes about 2 gph. That means 28 gph are filtered and returned to the tank. On conventional systems, the fidel is pulled through a screen sock, pressurized in the pump and then routed through a filter. From there it goes into the fuel rail, past the injectors and through the pressure regulator, then is returned to the tank. The screen sock is designed to filter out large contaminants that would quickly damage the pump assembly. The screen holes are large enough to keep the screen from getting plugged easily. The downside is that some smaller particles have to travel through the pump. In designing returnless systems, engineers have three filtration options: They can filter the fuel before the pump (in-tank), filter it after the pump at the pressure regulator return (intank) or filter the fuel outside the tank. Let's look at the last option first. With the filter outside the tank, service obviously is much easier. However, using this method means that the fuel returning to the tank is never filtered, except by the screen sock. Fig. 1 shows this option. Note the in-tank pressure regulator. With the filter outside the tank, the pump can suck in rust from bad tanks at filling stations. Although some of this rust goes to the filter, most is returned to the tank (the ratio of returned vs. consumed fuel is about 15:1 under normal operating conditions). This rust is then ground up by the pump and returned to the tank again, along with ground up parts from the pump itself. This cycle repeats itself, getting worse each time. I call it the fuel pump grinder effect. In areas with poor-quality fuel (fuel that contains particulate matter), some of these pumps aren't lasting 8000 miles. Factory repair procedures call for removing and disposing of the fuel (in an appropriate manner) and steamcleaning the tank. Many dealer techs aren't following the procedures, causing rapid new-pump failure and increasing warranty costs. As a result, more than one manufacturer is considering offering pumps only as a complete tank assembly. Every fuel pump replacement, returnless system or not, should include a tank inspection, cleaning and new filter sock. The first option mentioned-filtering the fuel before the pump-looks like the best once you're past the hassle of in-tank filter set,ice. However, when this filter gets plugged, the pump has to work hard*to suck through it. This creates low pressure (suction) that causes the fuel to boil and the pump to cavitate. The pump is also stan,ed for its coolant and lubricant (fuel). Many manufacturers that picked this option are using a large "lifetime" filter to combat the problem.

The second option-filtering at the pressure regulator return-is probably the best. The fuel is picked up through a standard screen sock, with a replaceable filter built into the regulator assembly on top of the pump. Using this method, all the fuel in the tank is continually filtered. Many manufacturers are installing fuel pump access panels to make service faster and easier.

Prior to returnless systems, OEMs went to great lengths to keep the fuel in the tank cool. Fig. 2 is a typical fuel pump "module" off a conventional return-style system. Note its complexity and the extent to which the manufacturers went to keep hot returning fuel from coming into contact with the bulk fuel in the tank. The fuel pump is inside a sealed housing. In addition to isolating the hot fuel, this works as a noise barrier and keeps the car from misfiring under low-fizel/sloshing conditions.

Look at points 5 and 6 in Fig. 2. This is the real key to the operation of these systems. High-pressure/volume fuel is taken from the pump at point 5 and directed right at point 6, a small venturi. Remember, when fuel nishes over the venturi, a local low-pressure area is created. This draws fuel from under the float (point 1) when there's adequate fuel in the tank or from the secondary umbrella valve (point 2) when low-fuel conditions close the float valve. This is where the term floated jet valve assembly comes from. Point 3 is called the primary umbrella valve here. This valve is stronger and is used as backup in the event the secondary umbrella valve fails closed. Also notice the extra sock filter at the base of the pump, which serves to filter and exclude, or break up, bubbles in the fuel. Vaporization & Cavitation Fuel vaporization and cavitation are major problems in fuel delivery systems. Never actually seen them? Me neither, until I got the Fuel System Analvzer from Emi-Tech, a 199 MOTOn Top 20 Tools award winner. This tool measures fizel pressure, vacuum and, most importanti; fiel flow and quality I define quali,its; as the absence of water or vapor (bubbles) in the fuel. This can be easily observed in the tubed sight glass.