Testing a Fuel Pump

Fuel pumps in fuel injected vehicles are mounted inside the gas tank. It's a major job to replace it and these pumps are expensive. So before you replace one, you better do your homework. I've seen too many DIY mechanics make the assumption that their engine running problems are due to a bad pump. Several hundred dollars later, they still have the same problem.

If your diagnostics lead you to believe you have a fuel pump problem, here's the proper way to check out the pump. First, perform a fuel pressure test. You'll need a fuel pressure gauge and the adapters to fit your vehicle. Some auto parts stores rent this test gear. You'll also need the specs for your vehicle. Check out a printed or online shop manual for those.

Perform the fuel pressure check first thing in the morning, with the engine cool. A cool engine provides two things: better working conditions with less chance of a fuel-spill related fire, and a more accurate reading of residual fuel pressure after sitting for a long time. Fuel pumps have a check valve that prevents the fuel from draining back into the tank when the engine is shut off. If the valve isn't working properly, you'll have fuel drain back and that causes hard starting first thing in the morning. Shop manuals have a specific procedure for performing this "leakdown" test if the vehicle is brought in warm with high fuel pressure. Usually, the tech looks for pressure to stay within 5 lbs of the running pressure.

If your fuel pressure and leakdown readings are within spec, you'll need to check fuel delivery volume. Yup, that's an important test especially if your performance problems occur at high speeds or heavy throttle. Some manufacturers list a volume of fuel for a specified period of time. If you can't find a spec, use this rule of thumb-a fuel pump should deliver 1 quart of fuel in 30 seconds. If yours passes that test, move on to the electrical tests.

The electrical test can spot a fuel pump problem that evades all other diagnostics. Remember, the pump is always pumping against resistance (the pressure regulator and he fuel injectors). Just like any other electric motor that meets resistance, it uses more power. So like an EKG at your doctor's office, measuring current draw at different throttle openings tells you how the pump is performing. A properly operating fuel pump should draw from 3 to 6.5 amps.

If you own a scope, you can get even better readings. A scope will tell you if you have a bad winding in the pump since it detects every lobe on the armature.

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Replacing the Fuel Pump Fuse on Your Car

Do you think your fuel pump may be shot? Before you spend time replacing it, check the fuse. It may be a simple matter of changing that. Here's how you can change the fuse for fuel pumps.

You must go through the car's manual. To replace the fuse is an easy repair you can do with your vehicle. You should know where the fuse box is located in your car. The places to look for this are in the glove box and/or just underneath the steering wheel. Look for a hidden fuse box with the help of owner's manual before tearing away at the interior of your car. Then you will have no problem figuring out where the fuse box is in your car.

The fuse should be disconnected. Before you can work on the fuse box, you need to remove the cover from it. The manual will have a diagram that lays out what each fuse powers. Another way to discover this is to look on the cover of the fuse box. There will be a diagram showing the location of each fuse. Next, locate the correct fuse for your fuel pump, remove and discard it.

Put in a fuse which is new. Now all you need to do is replace the burnt out fuel pump fuse with a new one. Head down to the auto store and purchase a new fuse if you don't already have on in your collection. Be sure to replace it with exactly the same type of fuse that you removed. Again, your owner's manual will tell which is the exact fuse needed for your fuel pump. Feel around for the proper location until you feel a firm click. This means the fuse is properly connected and will run the fuel pump.

Check it works. Simply replace the cover on your fuse box and try to start the car. If it starts, then you're done. Feel free to bask in your accomplishments as a seasoned mechanic! However if you have issues with your new fuel pump fuse, it could be caused by many factors.

If you put a new fuse in and it quickly dies, it may be that your fuel pump itself is dying.

You may need to do a thorough cleaning as it could also be clogged. It could be a completely different problem, such as the electrical system of the car. It could be that your it is already blown - if it is you will need to replace it. A simple solution that should not be overlooked is the possibility that the fuse might just be a dud. Check another new fuse before running out to replace your whole fuel pump.

Car Engine Cranks Over But Doesn't Start

Many times in the past as an automotive technician, I would be given a car to diagnose that either had a problem that prevented the car's engine from turning over or that prevented the car's engine from starting. These are two completely different scenarios and these problems are approached differently. Sometimes when the service writer would create a work order, he wouldn't convey the REAL problem that the customer was experiencing. If the customer or the service writer would make sure the correct explanation of the problem is written, some unnecessary trips out to the car with the wrong tools would be avoided. For instance if the car doesn't crank over at all, I would need to take my jump box with me. And if the car cranked over but didn't start, I would want my scan tool and some starting fluid.

A car that cranks over but doesn't start, in many cases will be due to a bad fuel pump, a broken timing belt, an inoperative cam sensor or crank sensor. The first step in diagnosing a no start is to scan for codes with a code reader or scan tool. Approximately 40% of the cars towed in that I worked on in the last 25 years, either had a bad fuel pump or the fuel gage read incorrectly and the car was just out of fuel. Yes, some people actually have their cars towed in when all they need is some gas! If the fuel gage indicates that there is fuel in the tank, most people will assume naturally that the car does have fuel. But if the fuel sending unit or gage is off slightly, the car's gas tank could actually be empty. One of the easiest ways to check for a fuel pump related problem is to spray a little starting fluid into the air intake while trying to start the car. If the car starts momentarily, then there's a good chance the fuel pump is not working or the car is out of gas. You can also listen for the fuel pump's humming sound when the key is first turned on. Also if a inoperative fuel pump is suspected, a good technician will always check to verify there is power and ground being supplied to the pump, to eliminate the possibility of a bad relay or electrical issue. Fuel pressure can be checked with a pressure gage and if there is pressure, pulse can be checked at the fuel injectors in several ways including the most common, with the use of noid lights. If the injectors don't have pulse the engine won't start.

Broken timing belts also caused about 40% of the tow ins in my experience. A broken timing belt can fail causing the car to seem as if it ran out of fuel while driving. When trying to start the car though, the engine typically spins over faster than normal due to the engine having low compression from the timing being off and the valves being open at the wrong time. If the timing belt breaks, there sometimes could be a code set in the vehicles ECU (Electronic Control Unit) for no signal from the cam sensor. If this code is present, you can sometimes check to see if the cam is moving by removing the oil cap while someone engages the starter. Also the upper timing cover can be removed to allow access to the timing belt for inspection. Most of the time when a timing belt breaks the teeth (or cogs) are stripped down at the crank pulley the bottom sprocket preventing the belt from turning the camshaft.

Lack of pulse from the fuel injectors as stated before can prevent a car from running. This brings me to the third most common cause of a car that cranks over but doesn't start, approximately 20% of the tow ins that I worked on had the following problem. Many times the cause of lack of pulse is because of a bad cam or crank sensor. If the computer doesn't see either the crank or the cam turning, the fuel injectors won't be commanded to pulse. One quick indication that a cam or crank sensor may be bad is to check for RPM movement on the tachometer (if equipped) when trying to start the engine. If you have a scan tool not just a code reader, the cam and crank sensor data can be checked on many vehicles by accessing the appropriate screen. If no revolutions are shown when attempting to start the car, the respective sensor could be bad.

Obviously there are many vehicle makes, with different ignition and fuel delivery systems that may require a service manual to properly diagnose or to delve deeper. But with the above tips and tricks that I've used over the years you'll be checking the most likely causes, that hopefully will send you in the right direction.