Ford F-150/F-250: Why is My Truck Knocking?

Engine knock is one of those really annoying things that causes much worry with car owners. Is it something simple to fix, like using better gas, or something more costly and complex? Learn what to listen for to diagnose engine knock before spending money on unnecessary engine repairs.

By Jeffrey Smith - November 18, 2014

Engine knock occurs when the air-fuel mixture inside a cylinder is not optimal, and as such, makes the fuel burn unevenly. This uneven burning causes shock waves at the wrong time that can damage the cylinder and piston. This shock is the characteristic rattling or pinging noise that is heard. Learn how to determine the likely causes of your engine knock here and what you can do about it.

Tools Needed

  • Stethoscope

Step 1 - Determine if bad gas is your culprit

One of the most common causes of knocking is using an inferior gasoline product. This could just be "bad gas" or the fuel might have too low of an octane rating. Too low octane fuel is less stable and will explode before it's supposed to creating a knocking, pinging noise and great stress on your engine components. The technical term for this is called detonation. Gasoline may have a higher or lower rating, depending on how its anti-knock performance compares to the performance of pure hydrocarbon octane. The higher the octane rating, the more heat, air and pressure the fuel can take before spontaneously combusting. An octane boosting additive in your tank of gas could very well be all that is needed to stop the knocking temporarily. Any brand of octane boost product will work; these products can be found at any hardware, automotive or big box retailer.

All of Ford's gasoline engines recommend 87 octane—even the turbocharged EcoBoost. That said, if you experience engine pinging under heavy loads, try using a higher grade of gasoline temporarily and take the truck into the shop later. The F-250 and other Super Duties that run diesel don't have different grades of fuel and tend to make more noise than normal anyway. Pinging and knocking will happen more frequently under severe duty, but you probably won't hear it from inside the cabin. In the case of both the F-250 and and the F-150, the engine computer will retard the ignition timing if it senses pinging.

Figure 1. Use the manufacturer's recommended octane level in your vehicle at every fill up.

If you are using the correct octane level of gasoline or using an octane booster and knock persists, go on to Step 2.

Pro Tip

If you think you're going to get extra power from using a higher grade fuel, think again. Higher octane gasoline only has a higher resistance against detonation, not a higher energy potential. If you want to get an extra benefit from running higher grade gasoline, consider installing a high-performance engine tune. Higher octane allows for advanced engine timing and higher air/fuel ratios without putting the engine in danger. This is especially important when tuning turbocharged vehicles (like the EcoBoost).

Step 2 - Use a gasoline detergent additive

Another common cause of knocking is carbon buildup on the cylinder walls. U.S. gasoline is required to have a certain level of detergent additives, but in some cases of really bad buildup, this may not be enough. Purchase an over the counter additive from an auto parts store to help clean your cylinder walls and stop the knocking.

Figure 2. Gasoline detergent levels.

If you have cleaned your gas tank using a detergent additive and that has not helped your knocking, go on to Step 3.

Step 3 - Check your spark plugs

A slightly more mechanical reason for engine knock is the use of incorrect spark plug type. Manufacturers recommend a specific spark plug to use and may also endorse an alternative or two. If you do not follow the manufacturer's guidelines, the plug may not spark at the optimal time causing premature detonation of the gasoline creating knock.

Figure 3. Use the manufacturer's recommended spark plug for your engine.

Continue to Step 4 if your problem is not fixed with using the correct spark plugs in your engine

Step 4 - Check your rockers

If you experience a really bad knock and a corresponding loss of power, you could have badly worn rockers. To diagnose this problem, use a medical stethoscope and listen to different parts of the top end with the engine idling. You should distinctly hear which cylinder has the problem. Remove the valve cover to find the worn rockers. Replace those and you should have an engine that runs like new.

  • Figure 5. A rocker arm is highlighted above. These make a knocking sound that changes with engine speed if they're worn out.
  • Figure 6. The rocker arm panels have been removed from this F-250's engine. Poor oiling will cause these to wear out.

If the rockers aren't the culprit, you could have a bad timing chain tensioner. Proceed to Step 5.

Step 5 - Replace timing chain tensioner

A less common, but more serious reason for knock is a bad timing chain tensioner. A tell-tale sign for this is if your knocking is accompanied by a fluctuating oil pressure. This usually means that an o-ring is shot on the timing chain tensioner, which works off oil pressure. If you experience this, stop driving your truck immediately and get a tow to a garage or, if you have skills, you can do your own repairs. If you keep driving your truck with this problem, you will most certainly damage your engine beyond repair.

Figure 7. Ford F-150 timing chain tensioner.

If the timing chain tensioner doesn't cure your knocking, see the next step.

Step 6 - Determine when the knock occurs

In order to figure out exactly what is making your truck knock, you need to determine exactly when your truck starts knocking. Is your knocking occurring at idle or above? Does it knock only if the engine is hot or cold(or both)? Compare your engine noise with another F-150 or Super Duty engine with a build date of 4/18/2005 or later. If the knocking is very light, or more like a ticking noise it could be the fuel injectors or the lash adjustors in the valve train. These noises are quite normal for your Ford Super Duty.

However, if the knock is not prevalent at cold temperature, it may be your VCT phasers. A little noise is normal, but loud noise inside the passenger compartment or in the wheel well is not normal. Place the truck in park, bring the oil temperature to 160 degrees as indicated by a scan tool. Allow the engine to idle and determine if the noise is noticeable. Increase engine speed to over 1200 RPM. If the VCT is the culprit, the noise should greatly dissipate at higher RPMs. Return the engine speed to idle and verify if the knock returns.

If the noise intensity is more than a lightly audible knock at hot idle under 1200 RPM at engine operating temperature, replace the cam phaser. If your super duty has a start up rattle in the front of the engine that lasts for 1 to 3 seconds, replace the VCT Phaser Kit. If the engine continues to make the noise after the initial start-up, do not exchange VCT.

Figure 8. Ford cam phaser.

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