Ford F-150: Why is My Truck Losing Power?

Find the cause of your F-150's power loss with our diagnostic guide.

By Mark Kovalsky - November 19, 2014

This article applies to the Ford F-150 (2004-2014).

An engine needs four things in order to make power: air, the proper amount of fuel, compression, and a means of igniting the compressed fuel/air mixture. Any F-150 or F-250 that has lost power is deficient in one of those four things. This makes discovering the underlying cause of low power output a relatively logical process. This diagnostic will walk you through the major common causes of weak engine performance in the gasoline and diesel F-150 and F-250 models.

Step 1 - Is the air filter dirty?

If the air filter is so dirty that it restricts airflow to a large degree, the computer controls won't be able to compensate for this, and the air/fuel ratio will be too rich to operate properly. Alternatively, if there is an air leak beyond where the airflow is measured, the air/fuel ratio will be too lean. Either condition can cause a loss of power, or even a misfire.

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Figure 1. Neglected filter clogged with grease and dirt.

Step 2 - Is the fuel filter clogged?

If the fuel filter is plugged, the air/fuel ratio can be too lean. This will cause a loss of power or a misfire. If the fuel pressure regulator were to fail, the air/fuel ratio can be either too rich or too lean, depending on the failure.

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  • Figure 2. Diesel fuel turns into jelly at around 17° F without anti-gel additives.
  • Figure 3. This fuel filter is completely clogged. Filters will turn black with particulate matter over time.

Step 3 - Are the injectors dirty?

Another potential cause of a loss of power is dirty injectors. Residue from fuel combined with heat from the engine can cause deposits on the injectors, which causes several problems. The deposits can block the fuel flow so that the engine doesn't get the proper amount of fuel. Deposits can also interfere with the atomization of fuel. If the fuel isn't atomized it will either burn too slow, or not burn at all. Obviously, this will cause a loss of power.

Figure 4. The injector nozzle on the right is clogged with deposits.

Step 4 - Are the spark plugs worn?

Worn or defective spark plugs can prevent a good spark so that the air/fuel mixture doesn't burn at all. So can deposits on spark plugs. Spark plugs are a wear item and must be replaced according to the manufacturer's schedule to maintain engine performance.

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Figure 5. Spark plug chart.

Step 5 - Are the ignition coils failing?

A weak or failed ignition coil is another potential source of a loss of engine power. Ford Modular engines are well known to have issues with the coil on plug systems. Coils get weak with age and misfire under load. This is often misdiagnosed as torque converter shudder. It is easiest to detect under load at about 45-50 MPH with the torque converter locked. The weak coils show themselves as random jerks in this condition.

Most of the time, a bad coil will cause the truck's computer to store a diagnostic trouble code. Connecting an OBD-II scanner to the truck and checking for codes is a good preliminary step. A misfiring truck likely has a diagnostic code stored that identifies which cylinder is misfiring.

Typically, the coils tend to fail at relatively the same time. The best fix is to replace all of the coils at once. The less expensive, more complicated route is to isolate the bad one(s) and replace them individually as they go out.

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Figure 6. Motorcraft ignition coils from an F-150.

Step 6 - Is the engine mis-timed?

Engine timing that is out of step could be another possibility, but with modern engines this is rarely the cause. Timing is no longer adjustable like it used to be, so if the timing is wrong it is because something major has happened, such as the timing chain has jumped a tooth. This will be apparent as the engine always is very low on power. It probably will have problems accelerating altogether.

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