Ford 4.6L SOHC & DOHC Engines

Service Issues on Ford 4.6L SOHC & DOHC Engines

Ford TSB 08-7-6 covers the recommended removal procedure to reduce the risk of breaking the plugs, and covers their repair procedure using Rotunda tool 303-1203 for extracting the broken tip of the spark plug from the head.

Many Ford dealerships are scared to death to change the spark plugs on these vehicles because they know the risk of breakage is so high — and they are charging their customers a small fortune (up to $1,000 or more!) when they break the plugs because of the time and effort it often takes to make the repairs. That means there’s a significant service opportunity for the independent repair shop that knows how to reduce the risk of plug breakage on these engines.

First, don’t wait until the odometer hits 100,000 miles to change the plugs. When the plugs have been in that long, the chance of corrosion and ­carbon buildup binding the plugs in place is extremely high. Many experts recommend changing the original factory spark plugs at no more than 30,000 to 40,000 miles. The longer you wait, the greater the risk of breaking one or more plugs.

Second, run some combustion chamber cleaner though the engine before you attempt to change the spark plugs. This will help loosen up and remove carbon that has built up around the tips of the spark plugs. Also, spray penetrating oil into the spark plug wells from above and give it some time to work into the threads. And when you first loosen the plugs, only turn them about half a turn before applying more penetrating oil and allowing more time for it to work. If a plug starts to bind, rotate it the other way slowly (retighten it) before trying to back it out again. It may squeak in protest as you slowly work it loose, but patience is essential to prevent breakage. The last thing you want to do is muscle it out with excessive force as this will almost always snap off the tip of the plug.

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Third, when you replace the spark plugs do not install the same type of two-piece spark plugs as the original equipment spark plugs. One-piece aftermarket spark plugs are available for these applications that virtually eliminate the risk of repeat breakage. Apply a light coating of nickel anti-seize lubricant to the outer surface of the metal ground electrode shell to help prevent it from binding the next time the plugs are changed. Do not apply lubricant to the very tip of the spark plug (near the electrode gap) as this could cause a misfire.

You should also use a torque wrench to final tighten each spark plug. Note that the early style spark plugs on 2005 to early 2008 engines have a lighter torque specification (9 lb.-ft. or 12 Nm) than those on later engines (which is 25 lb.-ft. or 34 Nm).

On the 32-valve 4V DOHC 4.6L engines, spark plug access is difficult and requires a deep well spark plug tool that can hold the plug securely as it comes out. Drop the plug and you may waste quite a bit of time trying to fish it out.

These engines as well as the earlier 2V SOHC engines have spark plugs with “short” threads (only four or five threads on the plug) so it is relatively easy to damage the threads in the cylinder head if a plug sticks on its way out, or if it is crossthreaded or over-tightened when it is installed. As with the troublesome two-piece spark plugs on the 2004 to early 2008 3V 4.6L engines, use penetrating oil and a gradual back-and-forth loosening technique to remove the plugs. The engine should also be cool to the touch, never hot.

If the plug threads in the cylinder head are damaged, there are various thread repair kits available for restoring the threads, which is far less expensive than removing and replacing a cylinder head! Ford TSB 07-21-2 covers thread repair procedures on these engines.

Ignition misfires can be a problem with any engine, and may be due to multiple causes such as a dirty or dead fuel injector, an air leak in the intake manifold, a leaky EGR valve, a weak or broken valve spring, a blown head gasket, fouled spark plug, bad ignition coil or spark arcing down the boot around the spark plug.

If a misfire problem is ignition related, remove the coil-on-plug and inspect the boot that extends down around the spark plug. Carbon tracks or corrosion on the boot can provide a path to ground for the spark as can water in the spark plug well. Clean or replace as needed. If there is corrosion on the outside of the boot, it could be from an engine coolant leak at the intake manifold.

Another common problem on these engines is a broken coil-on-plug electrical connector. The plastic locking tab that holds the wiring connector in place may be broken, allowing the connector to work loose or make intermittent contact. Some bozo who last worked on the engine probably broke the connector and never repaired it. Replacement pigtails for the connector are relatively inexpensive and simple to install.

The plastic intake manifold can be another source of trouble on some of 4.6L engines. On 1996 to 2001 Crown Vic, Lincoln Town Car and Mercury Grand Marquis models, the plastic manifold can split across the front (right behind the alternator), creating a coolant leak. If not detected, the loss of coolant will eventually cause the engine to overheat.

The front temperature sensor fitting and rear heater hose connections may also loosen on these plastic manifolds, creating additional coolant leaks. In 2002, the intake manifold was redesigned with an aluminum crossover piece in the front to reduce the risk of cracking and coolant loss, and a stronger upper alternator mount. If you are replacing a bad intake manifold on one of these older applications, therefore, install the newer upgraded intake manifold.

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