Solving a 2007 Lincoln Navigator misfire

March 1, 2018
This ’07 Navigator had a misfire. Follow along and see if you would have taken these same diagnostic steps.

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.

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.

I have chased an incorrectly identified misfiring cylinder before, and it was a frustrating learning experience. A simple way to verify that the correct cylinder is being identified is to create another misfire or two on different cylinders from the one identified and verify that those cylinders are correctly identified also. I perform this by watching the power balance test and unplugging a coil on a different number cylinder and verifying it is correctly shown on the test. An injector would have the same effect if it is easier to access than the ignition coil. Needless to say, cylinder #5 was the one with the misfire so that eliminated that possibility.

Another test that should be performed on any vehicle with a misfire before delving too deep is a relative compression test. While I usually perform this test with an oscilloscope, a current clamp, and sync it with the cylinder #1 ignition coil, the Ford IDS has a built in test that can be performed by depressing the throttle fully to the floor (to prevent fueling during the test) and cranking the engine for 10 seconds.

What I did find when performing this test initially was that cylinder #5 had 4 percent lower compression than the other cylinders, indicating the possibility of a mechanical engine problem, but it seemed too small of a difference for this scenario. However, since the cylinder was misfiring, not to mention a fuel injector swap between cylinders, there was also the possibility of a cylinder wall that has partially washed down from excessive fueling.

While the other technician already swapped the ignition coil and fuel injector from cylinder 5 without any change in the misfire for that cylinder, I needed to verify that the actual signals which activated the ignition coil and fuel injector were present, since even a known good component will not work correctly without the proper signals.

Even though the previous technician swapped the ignition coil and fuel injector from cylinder #6, that didn’t mean that there wasn’t an issue with the signals from the Powertrain Control Module (PCM) or wiring to the fuel injector or ignition coil for that cylinder. I attached a scope to both the fuel injector and ignition coil of cylinder #5 and found that both were receiving the correct voltage signal and were operating as designed. At this point I am satisfied that there is some type of mechanical problem affecting only cylinder #5. 

Cranking compression isn’t the whole story
An easy test to perform without removing anything other than the vacuum hose is to perform a cranking vacuum test with a First Look Sensor (FLS). I installed the FLS into the hose going to the vacuum brake booster and synced to the Cylinder 1 ignition coil. The FLS measures changes in pressure, so this means that each time an intake valve opens a downward hump or “pull” is indicated in the waveform.

Here is the capture using the First Look Sensor in the intake while synced to the cylinder #1 ignition coil. Note that the intake pull for cylinder #1 starts approximately 360 degrees from when the ignition coil fires.

An engine in good condition will produce a consistent and relatively even pull for each cylinder in the firing order, since each cylinder should be drawing the same amount of air and each intake valve should be open for the same amount of time, but when an engine has a problem the steady repeatable pattern will become erratic. This is where the sync comes in. By triggering off a known good cylinder and using the rotational rulers of the scope, it is possible to divide the period between ignition firings into eight evenly spaced divisions with each representing an individual cylinder. Keep in mind that the intake stroke for the cylinder that has the ignition sync occurs about 360° before the intake stroke. The firing order for this engine is 1-3-7-2-6-5-4-8. Looking at the capture, there is definitely a problem, but it is not something that most can pick out unless they are very familiar with FLS pattern recognition.

Here is the same capture with the relative piston stroke position of cylinder 5. Notice how the symptom of a leaking intake valve does not show up during the intake pull of cylinder 5 but the effect it has on the other cylinder’s intake pulls depending on where the cylinder 5 piston stroke position is during their intake valve opening.

The pull for cylinder #5 looks fine, however where the intake pulls for cylinders #3 and #7 should be, there are actually pushes or upward pulses. One of the reasons behind this type of a pattern is that if the intake side of cylinder #5 is not sealing, it can leak cylinder pressure back into the intake manifold (where the FLS is connected to) and disrupt the normal balance between cylinders. However, I am not confident enough in my abilities to make the call just yet so a little more testing needs to be performed first.

The cranking compression results found while using the Pico WPS-500X pressure transducer matched what the previous technician found using a mechanical gauge.
While there are no definite specifications listed for compression of this vehicle, 190psi would normally be a very good number on most vehicles.  However after a few more revolutions, the number started dropping.

Since the other technician had already broken the original spark plug while trying to remove it and installed a replacement, I was not hesitant to remove the new plug and install a compression hose attached to a pressure transducer for my next test. If that was not the case on this type of engine, I would have opted for different testing methods rather than risking removing and possibly breaking the spark plug unless absolutely necessary. With the pressure transducer installed and the injectors disabled, a cranking compression test is performed, and I was a bit confused by the results. I know the previous technician also performed a test with a mechanical gauge, and I also came up with the same results – 190 psi of cranking compression and did not notice any discernable indications of a mechanical failure.

However, is 190 psi of compression good? Is it high? Is it low? Remember, Ford does not list a min/max specification, only a range between what the highest and lowest cylinders can vary by. Based on what happened with the first plug that was removed, I am definitely not going to remove more spark plugs to perform comparison compression tests. Something interesting to point out is that as the cranking compression test continued, a decrease in pressure occurred by almost 14 psi as shown in the capture. The next test to perform, especially since the pressure transducer and compression hose are already installed, is a running compression test. 

The running compression test is where the problem is revealed.  Normallthe compression would be between 1/3 and 1/2 of the cranking compression reading, but here it does not even reach 16psi.

A look inside
For the previous technician and several others, this is an overlooked test that reveals a lot of information. This is a dynamic test compared to the cranking compression test and some performance and tuning shops use this test to determine how well a particular cylinder is contributing to the engine. The test is performed with the fuel disabled to the cylinder being tested, which on this vehicle was as simple as unplugging the injector and running the engine at idle. The pressure of a good cylinder should be approximately one-third to one-half of the cranking compression test result. A throttle snap is also recommended during a running compression test, which should be approximately 80 percent of cranking compression results for that cylinder; however, when using an in-cylinder pressure transducer instead of a conventional compression gauge, it is not uncommon to see a higher reading than on the original cranking compression test. So, starting the engine and reading the test results showed a startling discovery — the running compression for cylinder #5 was only around 16 psi, a lot less than the 63-95 psi that would be expected and a snap throttle only increased to 70 psi.

While the vacuum achieved on the expansion stroke appears good at 21inHg, the vacuum reached during the intake stroke is around 15inHg, indicating an intake valve is not sealing.

On a healthy cylinder, this would have been 150 psi or higher, but was under half of that number. A good amount of the time, when snap throttle results are lower than expected but the running compression is normal, the problem lies in the intake side of the system, and if they are higher it points to the exhaust side, sometimes a restriction. This, however, showed very low running and snap throttle results. So now we know we definitely have an internal engine mechanical problem on cylinder #5 which is going to be intake related. This is also gathered from looking at the vacuum achieved on the expansion stroke and intake stroke. Usually both vacuum pockets are in the vicinity of a normal engine vacuum and are even with each other. When the expansion stroke vacuum is relatively good and the intake stroke vacuum is low, it generally points to an intake valve not sealing. The reason the expansion stroke vacuum is relatively good is because when the intake valve is leaking, it is open to the intake manifold where a vacuum already exists. An important point to remember that I was taught in pressure transducer training class is that the camshaft lobe opens the valve, but the valve spring closes the valve and keeps it closed.

With the driver side valve cover removed, a broken spring for one of the two intake valves of cylinder 5 is found.

Going by these observations, a phone call was made to the customer and approval was given to remove the Bank #2 valve cover for further inspection. With the valve cover on the driver side removed the test results were confirmed. One of the two intake valve springs for cylinder #5 was broken which was the cause of the misfire. 

An old-school style technique was used to hold the valve in place while changing the spring. Some small nylon rope was fed into the spark plug hole while the piston was at the bottom of the cylinder and then the crankshaft was rotated to compress the rope against the face of the valve which worked perfectly. The rocker arm pops out and back in without any bolts but a specific type of valve spring compressor was needed due to space constraints.

The cause of the misfire, a broken intake valve spring on cylinder 5.

Once everything was reassembled the Navigator idled and drove smoothly without any further misfire complaints. With ignition being such a common cause of misfires it can sometimes be hard to step back and consider basic mechanical failures, especially with modern engine reliability. However, with the increased amount of labor required to remove engine covers and components for inspection, old style tests can be performed with new tools and techniques to help isolate problems before teardown.

About the Author

Michael Miller | Contributing Editor

Michael Miller lives and works in Las Vegas, NV. He is an ASE certified World Class Technician. He holds a degree in Mechanical Engineering and Automotive Technology

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