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How to Understand and Diagnose Misfires

For everyone that already knows about misfires and just need to diagnose theirs, please read the first section here then jump to the purple "quick help" section.

This thread is a work in progress and is here for anyone who needs help figuring out, diagnosing, or understanding a misfire. The point is to reduce the amount of misfire threads that are started, as well as help the general public with misfire problems. Most of these tips apply to both engines, but there may be a few discrepancies that will be noted. This guide does apply mostly to the Taurus and Sable, but will be helpful for other cars as well for a place to start.

Please anyone PM me if you can think of something to add to this thread, as I am always open to ideas! Thanks to many TCCA members for their help on creating this helpful thread.

Ctrl-F (windows) or Command-F (mac) will allow you to search this for a specific topic or keyword.

Thanks to behlinla, Bull Geek, and JW657 for helping me create this. If we helped you, please hit our rep button!

General Information

What is a misfire? A misfire is when one or more cylinders do not fire at all, at the correct time, or incomplete combustion occurs. If the misfire is severe enough, it may generate a flashing CEL or MIL to warn of a severely damaging misfire, or the light may come on solid if a less severe misfire is detected (which should still be fixed ASAP). Unless the light is flashing, the misfire may not be presently occurring because the problem is stored in memory for some time after the event. Misfires may or may not be felt by the driver depending on the severity.

A cylinder needs the proper amount of air to fuel mixture, spark, and compression to fire. If any one of those is absent, mistimed, or sub-optimal, the cylinder will misfire.


First step to diagnosing a misfire is finding out what cylinders are misfiring, and when it occurs (under load, idle, when you let off the gas, etc). If your CEL is on, read the codes and figure out what cylinder is the problem, or if it is a random miss. You can read Check Engine Light (CEL) also known as the MIL (Malfunction Indicator Light) through the OBD-II port under the driver’s side dashboard using a scan tool. Most parts stores such as AutoZone will read these codes for free (except in CA).

Sometimes the PCM will not pickup on a misfire if it is partial or the monitor is not working correctly and the CEL may not light up even though you feel a misfire. If it misfires and doesn’t throw a code, you can do a power balance test to see which cylinder is misfiring. Unplug the plug wires from the coil one by one. Bad cylinders won’t change the idle when unplugged.

By reading your codes, you can help diagnose which cylinder seems to be the problem.

There are 10 basic misfire codes:
P0300 – Random misfire
P0301 – Misfire cylinder 1
P0302 – Misfire cylinder 2
P0303 – Misfire cylinder 3
P0304 – Misfire cylinder 4
P0305 – Misfire cylinder 5
P0306 – Misfire cylinder 6
P0307 – Misfire cylinder 7 (V8 SHO)
P0308 – Misfire cylinder 8 (V8 SHO)
P0316 – Misfire detected on startup (first 1000 revs)

Cylinder numbering (applicable to all Ford V6 engines 1987-2007):
1 2 3
4 5 6
Front of Car

Before just randomly replacing parts, be sure you know which part is creating the problem. Part changing is an irrational way to attempt to fix the problem. Keep in mind purchasing low quality parts will sometimes hurt your problem rather than help it.

Quick help – what to check:
1) Spark plug wire routing
2) Vacuum leaks
3) Worn spark plugs or wires
4) Ignition coil(s)
5) Camshaft synchronizer shaft (OHV only)
6) Fuel pressure
7) Fuel injectors
8) Cylinder compression
9) Oxygen (O2) sensors

Advanced User Misfire Diagnosis (good for those who don't have a CEL on but feel a misfire): http://www.taurusclub.com/forum/82-maintenance-repair/175528-misfire-diagnosis-using-06-advanced.html

Ignition Problems

Older spark plugs do not always fire correctly. Sometimes as plugs get older, they begin to fail at the same time. This may cause a random misfire (P0300). A poor firing spark plug or one that has a poor spark will casue a misfire. The first step would be to replace your spark plugs if they are old and have over 100k miles on them, they are going to need to be replaced anyways. Replacing plug wires is also a good idea at this time. At least inspect the wires for worn insulation and the plug boots for any cracking. You should also check your spark plugs for signs of arcing or carbon tracking on the porcelain when you take them out. If you see arcing, you need to change both the plug and the boot. Sometimes bad wires cannot be discovered without cutting them open. If you are out of ideas and have not replaced the plug wires, do so. On DOHC engines, oil on the outside of the plug can also contribute to misfires. In that case you need to change the valve cover gaskets and the O-rings surrounding the spark plug wells.

Replace your spark plugs with OEM motorcraft (Part # AGSF-32FM), or Autolite double platinum plugs. Gap should be between 0.042 and 0.046 inches.

Check for correct routing of the plug wires, sometimes they may be put back incorrectly after changing the spark plugs. Note correct cylinder numbering:

However, the most common cause of an ignition misfire is a bad ignition coil. If misfire still occurs after replacing plugs and/or wires, or both plugs and wires are new, suspect a bad coil. Engines 1996-2007 generally use a single coil pack (contains three coils) for all six cylinders. For 00, 04, and 05+ Duratec engines, you have Coil On Plug (COP) ignition, which means individual coils are located on top of the spark plug, one per cylinder. Always inspect coils carefully for fine cracks (including the epoxy on the bottom of coil packs), which are a good sign the coil is bad (see video). Replace if you find cracks, or if the coil does not produce a strong spark with a spark tester attached. Some COP's can melt internally; remove and inspect them for melting.

If you have a Vulcan OHV engine, you can perform a cylinder drop test to find which cylinder(s) are not firing. This test will also tell you if your coil is weak or bad. It's important to check for a strong spark (jumps about 1") because a weaker spark might cause misfires under load.

Vacuum Leaks

A vacuum leak can cause a rough idle, misfire, and poor fuel economy. It is time to check if you have a vacuum leak. The most common leak is on 00-03 duratec and 01-03 vulcan motors where the PCV hose connects to the intake manifold with the PCV elbow. These elbows become oil soaked and collapse, creating a leak. This is located to the left and behind the throttle body, under the intake manifold for the Duratec. For the vulcan, PCV snaps/screws/presses (depending on year) into the valve cover nearest the firewall and the 90 degree elbow runs up and connects to the upper intake manifold. Most of the time when you have a vacuum leak, you will be showing lean codes (P0171 and/or P0174) along with the misfire code.

Three easy methods to check for vacuum leaks include the cigar smoke test (blow cigar smoke into a vacuum tube, repeat, find leak), the propane torch test – turn a propane torch on with engine at idle and move it around the engine until the RPM raises, or the brake/carb cleaner test - take brake cleaner NON CHLORINATED or carb cleaner and spray it at all vacuum lines and at the UIM/LIM intake holes (where the gasket is), if idle changes thats where the leak is. Listening for a 'hissing' noise is usually a good indication you have a leak. Be careful not to spray brake or carb cleaner near or on exhaust parts such as the exhaust manifold or catalytic converters - this could cause a fire. Upper and lower intake manifold gaskets have been a problem with leaks on the Duratec, as well as the tubes behind the intake for the EGR system. The smoke test is good for finding large vacuum leaks. If you have a scan tool that can read fuel trims, watch the short term fuel trims on both banks when you spray cleaner or use the propane torch. You will see a quick, large change in the STFT if the cleaner or propane has found a leak.

Cigar Smoke Test Video:

Fuel System

If you have a P0300 random misfire code, start by checking the fuel pressure at the rail with the Schrader valve and an inexpensive fuel pressure gauge. If you have a 03-07, you will need to use a scan tool to check this pressure. Random misfires are a bit harder to diagnose if they are the only misfire codes on the computer. The valve was removed either in april 2002 for vulcans and may 2002 for duratec's.

Okay, lets go check the fuel pressure. The pressure your car should have at idle and under load depends on the year. For instance, 00-03 should be 40psi at idle and slightly more under load. If you notice the pressure is below 35psi, it could indicate a lean burning engine and possibly a misfire. If fuel pressure is low, first replace the fuel filter, if still low, you may need to clean the sock in the fuel tank, or replace the fuel pump. Another way to check for a bad fuel pump is by removing the vacuum line from the fuel pressure regulator. This only works on a 2003 or older vehicle as the newer ones compensate immediately and the scan tool will not show a change in pressure. If the pump is working correctly, you will see a 10psi or so rise in pressure after removing the line. The increase in pressure you notice “under load” is caused in the reduction in the manifold vacuum, so pulling the hose simulates this.

To test if the fuel pump is functional, you should hear the fuel pump “prime”, or run for a second or two when you turn the key to the “on” position with the engine off.

Gen 4 Tauruses (00-07) especially are susceptible to cracked fuel line connections right where the lines clip into the fuel pump assembly (strainer, pump, etc). That plastic is brittle on the assembly, is known to fracture, and can cause low fuel pressure.

Next step if fuel pressure is OK is to check the fuel injectors. You can check for a stuck open injector by turning the car off and watching the fuel pressure. If one is stuck open the pressure will go down fairly quickly. The pressure should not change within 5 minutes.

If you have a bad, clogged, or stuck closed injector, it may not let fuel enter the combustion chamber. You can check for a stuck closed injector by listening for each injector clicking with a screwdriver against the ear or with a mechanic’s stethoscope head to each injector body. A clogged injector might still click open and closed but still have very little or no fuel flow, so the only way to diagnose it is by seeing lean codes and ruling everything else out (ignition, compression, vacuum leaks) and isolating it to a particular cylinder(s) using a drop or power balance test. Injectors may also have electrical problems, which will throw some other codes. Refer to the OBD-II code master list sticky in the Maintenance and Repair forum for other codes.


Perform a dry and wet compression test. The cylinders compression should be within around 10% of the rest of the cylinders. This is an example, not to be taken for real data or real cylinder compression: If cylinder 1 had a dry compression of 100psi, and cylinder 2 has a dry compression of 92 psi you are still in an okay range. However if cylinder 3 has a compression of 80psi, that is more than a 10% difference and there may be serious engine problems.

Compression test video:

Another way to test compression is by performing a leak down test where compressed air is blown into the cylinder and is timed on how fast it leaks out. This is a very useful tool because you can listen to where the air is escaping to see where the leak is. For example hissing in the exhaust=exhaust valve, intake=intake valve, oil cap=cylinder rings, bubbles in coolant=head gasket.

Leak Down Test Video:

A few issues can cause poor engine compression, such as:


1) Head Gasket failure
2) Cracked head
3) Valve problems
4) Worn or damaged piston rings
5) Worn or damaged cylinder walls


1) Damaged Piston
2) Cracked block
3) Engine Out of Time

Videos about Vulcan cylinder head failures: http://www.taurusclub.com/forum/185-instructional-videos-links/214217-hard-find-engine-misfires-testing.html

Other Possible Places to Check

In many cases you will get lean codes (P0171, P0174, P1131, P1151) along with a misfire. If a fuel injector was clogged, the resulting lean mixture would cause the O2 sensor output to drop and remain "stuck lean". However, you have to be very careful interpreting O2 sensor readings when dealing with misfires because they're completely dependent upon whether combustion is occurring or not. O2 sensors only measure oxygen, not fuel. Remember that. In the next case, we have no ignition on a cylinder. The fuel injector is still injecting the correct amount of fuel, but the O2 sensor will also go lean. Why? Because we haven't burned any of the oxygen in the cylinder. Another tricky situation a stuck-open injector. If the mixture is so rich as to not combust, the O2 sensor will go lean again even though the mixture is extremely rich (again because we haven't burned any oxygen). The last (very) common case are vacuum leaks. The ignition and fuel injection are working fine, but the extra unmetered air will cause the engine to run lean.

So whenever you have a lean mixture with a misfire, it's important to first determine whether it's "real" or not. If the exhaust smells rich and the O2 sensors are lean, then you probably have an ignition misfire or a stuck open injector. Another important factor to consider is when the O2 sensors are going lean. If they're only lean at idle but operate normally while the engine is at cruise speed (~2000 RPM), then it's probably a vacuum leak. If the O2 reading is normal at idle but lean under acceleration, then you probably have a fuel pressure or clogged injector problem.

On the 3.0L OHV (Vulcan) engine, a bad camshaft synchronizer (AKA cam syncro, syncro shaft, cam sensor) can also cause misfires. This type of misfire is distinguished because it only will occur at higher engine speeds (above 3000 RPM) and will result in a "bucking or jerking" feeling under acceleration. Usually you won't have any codes and the car will run just fine the rest of the time. The cause is the failure of bearings inside the camshaft synchronizer shaft (which takes the place of the distributor shaft the engine was originally designed with). In many cases, the bad bearings will cause a chirping noise at idle coming from the driver's side of the engine, which is another big clue. The bad bearings allow the shaft to vibrate at higher engine speeds, which destroys the crankshaft position signal and causes the fuel and spark to get out of sync with the engine. The performance of this sensor is not monitored above 3000 RPM, which is why you often won't get any codes for it.

The only way to fix this is to replace the whole syncro shaft, not just the camshaft sensor puck that sits on top. In some cases the rotary vane will also destroy the sensor, so you might have to replace both parts. Please note that the syncro shaft also drives your oil pump, so delaying replacement may cost you your engine! If you want the fix to last, you must buy the Motorcraft part. The other remanufactured parts are inferior. Proper alignment of this shaft is important, so if you still have misfires after replacement you need to realign it correctly.

Cam Synchronizer Replacement Video:

More information on how the misfire detection monitor works in early to mid 2000's Ford cars: http://www.motorcraftservice.com/vdirs/diagnostics/pdf/obdsm00C.pdf

If you still have a misfire after following these steps, or have questions about these steps, start a thread, list that everything you have done and we will try to help you! Please do not start a misfire thread without starting here first!
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