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Solving a 2007 Lincoln Navigator misfire

Thursday, March 1, 2018 - 09:00
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When a vehicle shows up with a misfire, the first though that comes to mind is something ignition related like a spark plug or an ignition coil because they are such common failures. If that is not the cause, then it’s on to something in the fuel system like a clogged fuel injector or a fuel supply problem. Even though this is not the best approach, it is the one most technicians follow due to previous repairs with the usual suspects. While a good percentage of the time they are able to repair a vehicle with this course of action, when a problem occurs outside of the realm of these common items, the next diagnostic steps become unclear. Even when performing the correct steps, interpretation of the results can be misleading. This vehicle was one of those cases.

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This is a 2007 Lincoln Navigator with a 5.4L V8 with 3 valves per cylinder

The patient history
The vehicle is a 2007 Lincoln Navigator with a 5.4L 3-valve Triton engine and 82,810 miles. This SUV was in approximately a week ago for a misfire concern under acceleration during the time the transmission shifts between third and fourth gear. The technician determined that the misfire was due to failing ignition coils and eight new Motorcraft coils were installed. The technician test drove the vehicle under the same conditions that revealed the problem and found that no further misfires were present. The customer was relieved to find that the vehicle did not have a transmission problem, since it was easy to see why they thought that because the misfire occurred just as the transmission was performing an upshift. The vehicle was returned to the customer and all seemed fine until eight days later. 

This is a common problem that occurs when trying to remove spark plugs on this type of engine.  While the threaded portion separates from the cylinder head, the shell of the spark plug remains seized in the combustion chamber and special tooling is required to remove it.

The vehicle returned with an almost constant misfire that was present at idle and while driving it into the shop. After a visual inspection for a coil connector that may have fallen off, the technician grabbed a scan tool and retrieved a code P0305 (Misfire Cylinder 5). He again inspected the connector by pulling it off, checking the pins on both the connector and coil and reinstalling it but to no avail. He then swapped coils between cylinder #5 and #6 and found that the misfire remained on cylinder #5. Staying with the original plan of “Misfire = Ignition Problem,” he attempted to remove the spark plug from cylinder #5. Anyone who has replaced spark plugs on one of these engines knows what I mean when I said “attempted.” The threaded portion of the spark plug and the body came out, but the rest of the metal shell remained frozen inside the combustion chamber. The spark plug was extracted without any incident and a new Motorcraft spark plug was installed. I inspected the spark plug and could not find any signs of carbon tracking or damage to the nose of the plug other than what was caused by the extraction tool during removal. 

Misfire equals something else
The misfire remained without any change in intensity and I did suggest to the technician to verify the spark plug was actually firing before reinstalling it into the cylinder head. So, if it’s not ignition, it has to be fuel right? (If Misfire ≠ Ignition, then Misfire = Fuel Problem). The fuel injectors on these engines are fairly easy to swap with only a couple of bolts holding down each side of the fuel rail, so the injectors between cylinders #5 and #6 were swapped and the fuel rail tightened back down. Of course, the engine still had the misfire when restarted, but the technician was confident that it had moved to cylinder #6. So he cleared the code and was going to test drive it to let the code prove that the misfire followed the fuel injector. Because the misfire was pretty severe, I offered to let him use my Ford IDS and its power balance test to show a graph of each cylinder and its misfire history. Much to his amazement, cylinder #5 was still the only cylinder misfiring. So he has ruled out spark and fuel, only thing left is compression, right?

An easy first test to perform with the Ford IDS scan tool is a relative compression test. While cylinder 5 is slightly lower than the other cylinders, it did not provide conclusive evidence that an internal engine problem existed.

Unfortunately, Ford does not list an actual specification for compression but rather a Min/Max range between cylinders. After the dilemma with removing the first spark plug, he was not going to remove the other seven to perform a traditional compression test. But he did test cranking compression on cylinder #5. He stated the result was 190 psi so compression was good. At this point, he threw his hands up and asked the service writer to send this one to me.

It’s got to be something!
One of the great benefits of the Ford IDS scan tool is the power balance test; but caution must be used since there has been a time or two when the wrong cylinder has been identified as the one misfiring. The misfire monitoring is a strategy that is learned, unlike a circuit code for a problem with an injector or ignition coil. There can be variances in the tooth spacing of the crankshaft reluctor, so these characteristics must be learned to enable the misfire monitor. This is usually done after the Keep Alive Memory (KAM) has been cleared and is accomplished by performing a few decelerations from 60 mph to 40 mph without braking. The fuel is cut to the engine so no combustion takes place during the learning process eliminating that variable from the calculation. There should also be a code P0315 (Unable to Learn Profile) if the vehicle was not able to learn the profile correction. That code was not present, but that does not guarantee that cylinder #5 was the one misfiring.

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