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Changing A Fuel Pump

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On: Fri, Sep 16, 2011 at 2:02PM | By: Peter C Sessler


Changing A Fuel Pump

Before fuel injection came into vogue, mechanical fuel pumps mounted on the engine were used. Changing such pumps was just a matter of removing a couple of bolts and the fuel line. Now, practically all cars and trucks come with an electric fuel pump, which is typically located inside the gas tank.

They aren’t located in the gas tank just to make them difficult to replace, but because in-tank pumps are quieter, are protected from the elements, and fully pressurize the fuel along the entire system. Because fuel injection requires much higher fuel pressures (35-50 psi), vapor lock—a problem with old style mechanical fuel pumps—is a thing of the past.

Electric fuel pumps use a relay to turn the pump on and off. If you turn the key on before cranking the engine, you’ll often hear a slight whirring noise as the pump turns on to pressurize the system. If you don’t hear that whirring sound, then the pump isn’t turning on. Some cars, in addition to the relay, also use an oil-pressure switch to turn the pump on and off.

An interesting thing about electric fuel pumps is that they use fuel to cool the motor. That means there’s gas flowing around the motor’s brushes, where you’ll find electricity sparking. But you need three things to cause an explosion—fuel, a spark, and oxygen. Since the system is designed so there is never any oxygen present, there is no explosion.

Naturally, you’ll find that the pump will fail just after you’ve filled the tank. You’ll need a lot of gas cans, in that case. However, pumps can give you some warning that they are failing. If it takes longer for the engine to start (after it has been sitting a while, such as overnight), your pump might be on its way out. When the outside temperature gets hot and the engine sputters and dies but then restarts, your pump could be dying.

Like every other automotive repair, always start by checking the simplest and the most obvious. In the case of a fuel pump, when was the last time you changed the gas filter? If it has been some time, maybe the filter is clogged. Check the wiring. Perhaps there’s a bad ground or a poor connection so the pump isn’t getting enough voltage. A lot of late model cars and trucks now use wire connectors/plugs. Disconnecting the various plugs, cleaning the contact points, and using plenty of dielectric grease might do the trick. Probably not, though.

It might also be a bad relay. Relays are pretty cheap, so replacing one isn’t such a big deal. If that doesn’t do it, it might be a bad oil pressure switch or other sensor. Still, you’ll find that 99 percent of the time, it’s going to be a bad pump, which means down goes the tank—unless, of course, you own a particularly well-designed car that gives you access to the pump by just removing a panel in the trunk area.

Before you start removing the tank, disconnect the battery. If you know the pump is bad (but it hasn’t completely died yet), you can disconnect the fuel line at a convenient point and have the pump pump out whatever is left in the tank into a container or two. Otherwise, you’ll have to siphon the gas out. Never use a hose to suck the gas out—there are plenty of cheap siphon kits that’ll do the job. Some tanks even have a special hose already in place for that purpose.

Once the tank is empty, it’s just a matter of removing the straps that hold the tank in place and anything else that might be in the way. That usually includes the fuel filler tube. Most cars these days use plastic tanks, but older cars used steel tanks. Carefully inspect any steel tank for leaks before reinstallation and try not to get any dirt in the tank.

After the tank is down, disconnect any electrical connectors, fuel lines, and vent hoses. You might have to disconnect these before the tank is completely down. Some fuel pumps are held in place with a large locking ring while others are held in place by screws. Remove these and the pump will lift out. You’ll note that the fuel gauge sender is attached to the fuel pump, and there’s always a fuel sock or a filter attached to the pump. If your pump didn’t come with these or they are part of an additional kit, make sure your replace these as well.

Once you’ve replaced the pump, all you have to do is reattach all the connectors and hoses, lift the tank in place, and strap it in. After you’ve put a gallon or two of gas in the tank (don’t fill the tank, in case there’s something wrong), turn the car on and check to see there’re no leaks. If there aren’t and everything seems fine, you can then fill the tank up and be on your way.




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