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Fuel System

Fuel Pump Diagnostics:
Changes In System Technology Mirror State Of Industry

edited from an article by Gary Goms, ImportCar, June 2000

If nothing else, fuel pump diagnosis has become a "sign of the times," illustrating how technologically complex this industry has become. The contrast between fuel pump technology 30 years ago and fuel pump technology today is certainly an indicator of where the state of our industry is in the here and now.

Just today, for example, I had a vintage 1970 Volkswagen in the shop for a general check up and minor service. Since this particular VW toured Europe and other foreign countries, it may not have been purchased overseas and therefore may not meet federal emissions requirements. So it is one of the last classic "Beetles," equipped with a simple carburetor, contact point ignition, and no emissions equipment.

Fuel pump diagnosis on the Beetle is very straightforward, with a simple metal fuel line transferring gasoline from the forward tank to a clear plastic fuel filter mounted next to the ignition coil. Many of the VW fuel pumps also had a brass screen in the fuel pump inlet to retain debris from a ruptured or discarded paper fuel filter.

The diagnosis is quite simple. First, disconnect the fuel line at the fuel pump and allow the fuel to pump into a container located away from the ignition coil. If the pump works, it will squirt fuel into the container. If the pump doesn’t squirt fuel, A) the fuel filter is clogged, B) the fuel line is pinched, C) the fuel pump is defective, D) the fuel pump eccentric is worn off the camshaft, or, E) you’re out of gas!

The first electric pump I encountered on an Asian import was a pulse-type pump bolted onto a cross member of a new 1971 Chevrolet LUV pickup truck. The LUV was a cleverly built, very durable little truck, and the first one I encountered was while working as a line tech at a Chevrolet dealership. Clearly, the pump wasn’t running, but why? Although I worked at a Chevy dealership, I didn’t have electrical schematics on the then-new LUV truck. After much tribulation, I found that a faulty electrical connection at the alternator caused the fuel pump failure. In this instance, the alternator was used as a safety disconnect for the electric fuel pump. This was assuming, of course, that the carbureted engine would stall during a collision or rollover!

But as electronic fuel injection became more common, the safety disconnect was built into the air flow meter. If air wasn’t flowing through the meter, the meter disconnected current flow to the fuel pump. Of course, an override was built into the ignition switch to prime the fuel injectors as the engine was cranked during starting. But we also had to keep in mind that early Bosch systems often used a primary low-pressure or priming pump mounted in the fuel tank to deliver fuel to a secondary high-pressure pump mounted on the frame. If the priming pump failed, a high-pressure pump failure would almost certainly follow.

Many years ago, I diagnosed an intermittent stalling complaint on an early Volvo system by measuring the amperage draw on the secondary pump. A number of other shops had attempted to diagnose the problem by measuring fuel pressure. The fuel pressure was just fine, which didn’t explain the intermittent stall. But, as I recall, the amperage draw was exceeding the specification by 25% or two amps. I replaced the pump and cured the complaint. After I published a story about this diagnostic method in an early edition of ImportCar, I received some negative commentary about the reliability of the amperage draw method. Today, however, the amperage test is standard procedure. But more about that later!

VARIETY OF CAUSES
Nevertheless, the safety issues concerning electric fuel pumps has made modern fuel delivery system diagnosis extremely complicated. The fuel pump relay, for example, can be activated by an oil pressure switch, an inertia safety switch, a reference signal from the distributor, an interpretation of a crank reference signal from the PCM, or our old stand-by, an air-flow meter connection, among other things.

I’ve had electric fuel pump failures caused by faulty ignition modules, intermittent "fail-safe" inertia safety switches, faulty factory splices in the fuel pump power wire, bad ground connections, worn fuel pump relays, and engines being run without oil. Some early domestic versions incorporated plastic fuel tank liners to dampen fuel pump noise. Unfortunately, fuel additives disintegrated these liners and the particulate matter would clog the fuel pump inlet filter. Similarly, some of these early pumps would develop low fuel pressure because the connecting hose in the fuel tank ruptured.

Although fuel pressure regulator failures are relatively rare, I’ve found regulators that cause intermittent driveability complaints by sticking at high- or low-pressure levels. In other, more increasingly rare cases, the regulator return line outlet is located too close to the fuel pump inlet screen in the fuel tank. During hot spring days, especially if the vehicle is fueled with winter-blended gasoline, the fuel vapor pouring out of the return line will actually be ingested into the pump, causing a vapor-lock condition.

The best tool to use for testing intermittent fuel pressure problems is a graphing multimeter coupled to an electronic pressure transducer. Some graphing multimeters will record pressure variations for hours of driving time, so don’t overlook this particular diagnostic option!

Premature fuel pump failure can be caused by abrasive particles accumulated in the fuel tank. Clogged fuel filters may also aggravate premature fuel pump failure because they increase the amperage draw at the pump. Unfortunately, many motorists continue to ignore fuel filter maintenance at their own peril by avoiding bringing their vehicles in contact with qualified technicians. Given the "Wal-Mart attitude" so prevalent nowadays, this type of failure is inevitable.

WAVEFORM TECHNOLOGY
Today, electric fuel pump diagnostics relies heavily on waveform technology derived by attaching a low-amp probe to a digital storage oscilloscope (DSO). These waveforms may tend to be very vehicle-specific, but a good tech can measure fuel pump speed, amperage draw, commutator condition, and brush wear by doing a simple inductive connection at the fuel pump. I might caution, however, that waveform diagnosis takes some practice. It also requires the ability to download waveform patterns into a PC computer. These patterns can then be compiled into a reference library that illustrates good and bad waveforms for specific vehicle applications.

We’ve come a long way from the simple fuel delivery system of a VW Beetle to the more complex systems being built today. As a technical yardstick, the complexity of today’s fuel pump diagnostics tells us how sophisticated our business has become.

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