There are many people who are looking at buying a new truck (New to them!) or considering keeping their old truck for a few more years. Everyone sees these late model F150's with the 5.4L Triton engine as a risky investment but I'm here to tell you this engine can be solid. It does have some common reoccurring problems you should be aware of. These problems range from easy to change coils, to the dreaded spark plug, all the way to the scary and expensive timing chain issues.
This article is mainly written with the 5.4L truck engine in mind, but it also applies to the 4.6L used in trucks, vans, and cars. This article is written so that the easiest repairs are first, and the harder repairs are last. So keep reading or skip to the bottom to get to the juicy parts. The most common thing we need to mention, but it is the easiest to fix, is the ignition system. The 5.4L Triton features a coil-on-plug (COP) design; a separate coil for each cylinder that sits atop a specialized spark plug (more on that adventure in a minute!). COP systems are now very commonplace in the automotive industry. The general consensus is that this type of ignition system is the most efficient design offering independent control, but the coils do suffer from failures likely related to heat. Most trucks will need multiple replacements in it's lifetime.
The most notable tend to be failures of the coil boot, the piece of rubber that covers the spring that goes from the coil output prong to the spark plug. These deteriorate with age and weak spots in the rubber can cause the spark to jump to the plug well instead of the spark plug. These wells can also fill up with oil or coolant and that can also affect the performance of the coil. Sometimes the ignition coil can fail internally as well. The most common symptoms of this issue is cylinder misfires under load accelerating or most notably cruising around 55mph in overdrive. It will cause a light jerk or bucking that some say feels as if you may have transmission issues. They are pretty easy to replace though. Generally at the Ford dealership, we will diagnose the cylinders with the misfire and replace the plug and coil for those cylinders.
Spark plugs are a whole different animal on this engine. They decided to place each of these sensitive coils upon a specialized spark plug that tends to break when removed. Due to the design of the combustion cylinder, essentially these engines are semi-hemispherical, the plugs needed to protrude down through the head to the combustion chamber. The situation is slightly ironic since earlier 2 valve 5.4L (97-03) tended to blow spark plugs out of the head but later 3 valve 5.4L (04-07) tended to break off in the head.
These plugs generally last around 100k miles but they are a bear to remove when you do go to replace them. They have extended sleeves that reside down inside the head that tend to break off because of possible carbon/rust build up. This issue was fixed by a redesign of the spark plug in the 2008+ years. More details are available in another article I wrote which include step by step instructions on Replacing the spark plugs on the 5.4L Triton at home.
Be wary of driving with this engine if you are experiencing a misfire of any kind. Some people ignore the felt misfire or think it is a transmission issue. At the dealer, we generally see these trucks go a long time with this misfire condition. Eventually, the truck will start to backfire through the intake and even fail to accelerate to over 30mph in severe cases. If this happens, likely you have a catalytic converter that has been damaged due to misfires. Always be on the lookout if you have a converter failure because engine drivability issues kill catalytic converters.
As this model ages, a very common issue in these trucks equipped with a 5.4L Triton 3 valve engine is a failure of the fuel pump driver module. This specific vehicle platform uses a module to control fuel line pressure by pulse width modulation of the fuel pump. The fuel pump driver module is located under the vehicle exposed to the elements. Specifically, on a F150, it is located above the spare tire attached to the frame. Originally Ford attached the aluminum driver module directly in contact with the steel frame. This led to extreme corrosion build up on the module. With extended time the corrosion would eat a hole through the aluminum housing and expose the electronics to water and road grime. The module will short out temporarily causing drive-ability issues and eventually could cause permanent damage to the smart junction box. This usually causes the dreaded code P1233 Fuel pump Driver Module (FDM) Offline. Ford later released an updated replacement fuel pump driver module that comes with mounting studs to space the module away from the frame slightly. The most common symptoms, aside from a no start, is lack of power, the code P1233, and hard starting.
Lastly, the scariest part for most late model 5.4L Triton owners, is the problems related to the timing chain, tensioners, and the variable valve timing. When you hear a 3 valve 5.4L Triton running, you generally can tell by the sound of the engine. Especially on light acceleration most noticeable to the driver with the window down while pulling forward in a drive or down an alley. Most of these issues are caused by low oil pressure do to pump failure or lack of following service intervals, but phasers and tensioners can fail from age as well. As this problem progressively gets worse, when engine is hot and you let off the throttle the engine may loose the ability to maintain engine timing causing it to run rough and rattle like crazy. Sometimes the rattle will go away within a few seconds, sometimes it will not go away until you rev the truck up. The base problem usually being low oil pressure when hot.
Eventually the timing guides will fail from the chain slapping around. It's all down hill from there. That leads to an out of time engine with valve damage and metal particles throughout the engine. This system is fairly complicated so check out the full description an operation of the Ford 5.4L Variable Cam Timing. I also wrote a guide to replacement of all these components in another article Changing Ford 5.4L Phasers and Timing Chain.
One thing that has picked up in popularity is deleting the VCT all together. A link I provided in the paragraph above explains how the system works, and it is very easy to eliminate the VCT. You should check out Tuning the 5.4L 3V.