Ford F-150 Coil on Plug Failures and How to Troubleshoot Them

My friend has been having trouble with his 2000 Ford F-150 running rough and misfiring occasionally. One of the symptoms is that the engine stumbles when he is going up any hill while in overdrive. No matter speed, the engine misfires and the truck begins to buck violently until he backs off the gas and drops her out of overdrive. There are times he has to go to second gear to get the sputtering to clear up.

One of the problems with the F-150 is that the Coil On Plug design is prone to failure in a number of ways. First the coil can be going bad, which shows up when the engine is under heavy load, and the coil cannot produce enough spark to keep the engine revolutions up. The other thing that I have seen is the rubber boot that covers the spark plug end can get burnt and brittle, or it gets coated with oil or water and this makes the spark short out to the cylinder head before it makes its way into the spark plug. This is an easy thing to remedy as you can purchase new rubber boots at almost any parts store in person or on-line.

New coils are not a bad idea if your Ford is more than five years old. These parts live in a harsh environment and can become weakened from the abuse. There is also the simple fact that the F-150 and many other Ford models used a specific coil which was prone to failure and as a result an improved coil was used when warranty repairs were made. On this 2000 F-150 the factory coil equivalent is a Motorcraft part number DG491 the better and correct replacement is a Motorcraft part number DG508. There are cross references for each alternate brand and the person or shop selling the item should have access to these data charts, use them to make sure you select the correct COP for your engine.

So what is special about these COPs? First of all they reside directly above the spark plug hence the "Coil On Plug" designation. Second they are designed to prevent moisture from making its way down into the bore where the spark plug lives. The problem comes when the boot is old dried out and cracked, the smallest amount of moisture can cause it to misfire as the spark is actually shorting to the head instead. To add insult to injury there have been some ford models which have minor coolant leaks into the spark plug hole and the misfire can come and go making it difficult to track the problem down.

Now on the 5.4 triton V-8 the plugs are buried deep in the inside upper part of the heads, in the valley of the intake manifold area, and in there are all kinds of obstructions. Couple that with a very small hold down bolt that tends to rust and can break off if you are not exceptionally careful and this job can become a nightmare fast. The boot is very long and if you are not careful there is a chance you will think the plug is mated and the rubber is instead pushed to one side causing a constant misfire. Since it is impossible to see into the plug bore when you are putting the COP back on this job can be quite frustrating. If you have never done one of these before I would advise taking it to a professional and watch them very carefully while he/she is doing the job. Your goal is to learn how to work on this yourself next time.

There are some basic electrical resistance tests you can do that prove the coil is not an open circuit, but the condition is usually evident only under load, and can be caused by moisture and heat so bench testing the COP does not always tell you what is wrong. If you are interested in checking the coil with a voltmeter you need to check across the two terminals in the connector body looking for 0.55 ohms, and then test either terminal with the spring that goes to the spark plug, this should be 5,500 ohms. These values are for the 2000 Triton 5.4 Liter V-8.

Whenever you replace or remove one of these COP's you need to make sure to put dielectric grease on the inside of the boot where it goes over the spark plug. This helps resist water getting between the COP and the plug, plus it helps keep the high voltage spark in the spark plug and not shorting out across the nearest metal it can find.

There are now a number of tools you can purchase that may make this testing easier, but if you are not doing this job all the time then a simple voltmeter check should be sufficient. Some owners keep a spare COP in the glove box. I have found that a spray can of water displacing penetrating fluid is always a good thing to carry if you own one of these Ford engines.

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