It may not be the pump: Problems can be caused by a variety of fuel system issues

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It may not be the pump: Problems can be caused by a variety of fuel system issues

Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto service technician, a shop manager and a regional manager for an automotive service franchise operation.

Unless you are into racing or collectable cars, you have probably not seen a mechanical fuel pump in many years. The same could be said for most external electric fuel pumps. With the death of the carburetor and the advent of fuel injection, electric, in-tank fuel pumps have become the norm.

In general, these fuel pumps are designed to last the life of the vehicle, or at least 200,000 miles. And they probably would if it were not for outside issues. And, quite often a problem that appears to be a fuel pump problem at first glance may be something entirely different.

No professional technician wants to have a comeback due to a blown diagnosis. A rough running, poor performing and even a non-running engine can be the result of a variety of fuel system issues.

Common contaminants

Before condemning a fuel pump, check for lack of fuel. Yes, it actually happens and techs have actually overlooked it. Check the fuel gauge, but don’t trust it 100%.

There may be contaminated fuel in the tank. Contamination is, in fact, the number one cause of fuel pump failure. Although the filter sock on the fuel pickup in the tank does a pretty good job, it cannot stop everything. If it filtered too well, fuel pressure and volume would be inadequate for the engine. If the debris is small enough, it will pass the sock and enter the pump where it will eventually cause internal wear and failure.

Dirt and other foreign matter can enter the tank during refueling. The gas station may not be changing its filters often enough or the tanker has just made a drop and stirred up the stuff in the underground storage tank.

Remind your customers to avoid buying gas when they see the tank truck on site. On fuel systems that feature a return loop, debris that is caught by the fuel injector screens may get washed back into the tank.

One of the most common contaminants is water. Not only will water cause a rough- running engine, it can wreak havoc inside the fuel tank.


Water can cause corrosion on the fuel pump electrical connectors. It can lead to corrosion of other materials and when that stuff breaks off and comes loose, trouble follows.

Water does not usually enter the tank from a loose or missing cap. After all, a loose or missing fill cap would trigger the MIL. More commonly, water accumulates as a result of condensation. Some drivers seem to maintain a low fuel level. Maybe they can’t always afford to fill the tank.

Whenever there is empty space in the fuel tank, it is filled with air. Air, especially in humid conditions, contains water vapor.

As the temperature drops, the water vapor condenses into droplets and eventually settles in the bottom of the tank.

A little bit of water, distributed throughout a full tank of gas, is not a major problem. But a puddle in the bottom of the tank is. This is a greater problem where ethanol is not blended into the fuel since the alcohol tends to surround the water molecules.

However, alcohol can be corrosive and any corrosion that flakes off may reach the pump, or at least clog the sock. Alcohol also may loosen deposits inside the tank.

If you suspect water in the tank, take a sample and put it in a glass container such as a Pyrex measuring cup. After a few minutes the water will separate and collect on the bottom.

In some cases, materials used on the fuel system are attacked by alcohols such as ethanol and methanol. Some materials, such as aluminum, may even dissolve into the fuel.

Granted, most gas stations sell oxygenated gasoline containing up to 10% ethanol. That is beginning to grow to 15% ethanol.

Alcohol also can damage the windings in the fuel injectors as well as affect the spray patterns. If the customer is unaccustomed to oxygenated gasoline with ethanol, a complaint of poor fuel economy may simply be due to alcohol’s lower energy content as compared to gasoline.

A simple test for measuring the amount of alcohol in the fuel is to use a science lab 10 ml (10 cc) glass graduated cylinder. Put 1 ml water in the graduate and then add about 7 ml of the gasoline to be tested. Cover and shake the graduate. Then let it rest for about an hour in a safe place. The water and any alcohol will settle to the bottom.

If that amount exceeds the original 1 ml of water, any additional indicates the amount of alcohol. Be sure to dispose of the test sample responsibly.

Bigger problems can come from bigger doses of alcohol. Unless the vehicle is designed to run on it, E85 can cause much more than driveability issues.

According to the U.S. Department of Energy: “Some materials become degraded by contact with fuel ethanol blends having high alcohol concentrations. Zinc, brass, lead and aluminum are sensitive metals. Terne (lead-tin-alloy)-plated steel, which is commonly used for metal gasoline fuel tanks, and lead-based solder are also incompatible with E85.

“Nonmetallic materials that degrade when in contact with fuel ethanol include natural rubber, polyurethane, cork gasket material, leather, polyvinyl chloride (PVC) polyamides, methacrylate plastics, and certain thermo and thermo-set plastics.”

Flexible fuel vehicles are designed to be compatible with E85. Other vehicles should never be fueled with E85.

Motorists also can inadvertently cause problems with the erroneous belief that they are doing something good. Some owners feel that, if a little fuel additive is good, then more must be even better. Like over-medicating someone, over-medicating a fuel system is a bad idea. Sometimes the solution to the driveability problem can be solved with an in-depth discussion with the customer.


Poor acceleration causes

If the complaint is poor acceleration, the pump may not be delivering enough fuel. However, the pump may be doing its job but is hampered by a defective fuel pressure regulator.

The time-honored quick check is to pull the vacuum hose from the fuel pressure regulator nipple and see if any fuel comes out. If so, replace the regulator before doing any further tests of the fuel delivery system. As more vehicles are being built with returnless fuel delivery systems, this trick, of course, won’t work.

Poor acceleration or lack of high speed may be due to any kind of restriction in the fuel system. Do not overlook pinched or deteriorated fuel hoses or crunched fuel pipes. A small perforation or crack in the fuel line may be too small to cause an external leak, yet large enough to pull air, causing a lean running condition. Visually inspect all lines carefully.

A common restriction occurs inside the fuel tank at the pickup screen (sock). However, the pickup sock is not the problem; the contaminants in the tank clogging the sock are the problem. Clogged socks seem to be more prevalent in rural areas where vehicles are operated in dusty conditions. Some of that dirt and dust can fall into the tank during refueling.

The sock can only trap contaminants down to a limited size. Below that, the contaminants can get into the fuel pump itself and wear away at the pump. So, replacing both the sock and pump together is a good idea. Many technicians opt to replace the entire fuel pump module rather than components. This makes sense since pulling the fuel tank alone is labor intensive not to mention the necessity of cleaning said tank before returning it to service. Also to be considered is the labor time to replace the pump in the module.

Although the sock may be serviced separately, most experts advise against this.

Electrical issues

Most replacement pumps that are returned to the manufacturer test out to be perfectly fine. In most cases the root cause is an external electrical problem.

An old service tech we knew used to admonish us to “leave the simple stuff for first.” Do not overlook something as simple as a blown fuse or bad fuel pump relay.

The second most common problem is electrical. Causes include poor conductivity or complete loss of continuity at electrical connections. One in particular, especially with some General Motors vehicles, is the external connection for the fuel pump module. Always inspect the connectors for evidence of arcing or burning.


Often the connectors are overlooked, the fuel pump is replaced and everything works fine. That is because during replacement electrical connections were disconnected and reconnected which temporarily cleans the contacts. Often, though, the corrosion and high resistance returns, the pump stops working and is replaced again. Clean, or better yet, replace the connectors. You may even want to put some dielectric grease in the housing to keep moisture out.

Before replacing any fuel pump, do a voltage drop test. Anything more that 0.2 volts is too much and will adversely affect the fuel pump operation. Although it won’t be easy, the best way is to connect the positive lead from your voltmeter directly to the battery (via a long length of wire) and the negative lead to the connector at the fuel pump.

To test the electrical system during operation, check the amperage draw of the pump. Put the clamp around the circuit wire and then activate the fuel pump using your scan tool. A high amp reading points to a fuel restriction while a low reading indicates free flow.

Replacing the fuel pump

Whether you choose to replace the fuel pump or the complete fuel pump/sender unit, the first step is to relieve pressure in the fuel lines. Start the engine and then remove the fuel pump fuse. When the engine stalls, it is safe to remove the fuel tank. Be sure to disconnect the battery negative. Drain the tank and store the fuel in a suitable container. With any luck, your customer has run the tank nearly dry. Another trick is to apply Stabilant 22 to the bare connector pins. It works, but it is not cheap.

Place a jack (a transmission jack usually works well) under the tank and loosen the retaining straps. Most technicians like to secure the tank to the jack, usually with ratcheting tie-down straps. Lower the tank only far enough to access the fuel lines and electrical connector and undo them. Then finish lowering the tank.

Before removing the retainer ring, make a note of the direction the pipes are pointing. Take a photo with your smartphone as a reference. Then, clean any dirt from around the flange area with an air gun and brush. Next, remove the old module, taking care not to spill the fuel from its reservoir, which you should pour into the container with the other extracted fuel.

Now is the time to clean and flush the tank. If it is dirty, steam cleaning is required.

After placing a new seal in the opening, carefully lower the module into the tank taking care not to bend the fuel level float arm. Then install the retainer ring. Although some technicians use a brass drift to seat the ring, the preferred method is to use the proper spanner tool to prevent bending the tabs.

Raise the tank into place and reconnect the fuel lines and electrical leads. Be sure to stuff a rag in the fill pipe to keep out dirt during this step.

You may now lower the vehicle, reconnect the battery and add fuel to the tank. Don’t forget to replace the fuel pump fuse. Start the engine and double check for leaks.   ●


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