2005 Ford F-150 rough idle

  • 2005 FORD F-150
  • V8
  • 2WD
  • 57,000 MILES
A ford mech is working on my truck and the diag. Was that cylinder #4 was misfiring--after new coil and sparkplug--the problem is still there--He is sying that thry have to tear down engine to find out the what is causing the problem--If a bent rod is the problem then ford will cover repair under warrenty even though my warrenty is up, but if it is a broken valve spring they will not cover repair and the cost is over 5000 to tear engine down and replace w/ anew head and all new valves? Is there any other possible fix? Any help would be greatly appreciated
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Wednesday, May 20th, 2009 AT 8:54 PM

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Well, if he is a Ford Mechanic he should know what tests to perform without having to tear down the engine. So here is more info for you.

In cases where there is a cylinder misfire code but no other codes, the ignition or fuel delivery system may be borderline and not yet bad enough to set a code of its own. A shorted or open fuel injector solenoid, or a shorted or open coil, will usually set a code, but a dirty or weak fuel injector or a weak coil probably will not set a code.

Bad spark plug wires are a common cause of misfire codes. After 50,000 or so miles, the OEM wires may be leaking current to ground or other wires, shorting the spark before it can reach the plug. Also, internal resistance may be increased, raising the required firing voltage to the point where the engine may misfire under load. Check the resistance and if it exceeds specifications, replace the wire set.

An oscilloscope is a good tool for identifying and analyzing misfires, but using a scope takes some know-how and experience. Suffice it to say that anybody can see a misfire on a scope, but figuring out whether the cause is ignition or fuel related requires an understanding of ignition waveforms.

Once you have identified the problem cylinder, you can look at the primary and secondary patterns for that cylinder to get more diagnostic information. The dwell section of the firing line just ahead of the spark will tell you what is going on in the primary side of the system. The spark line in the firing section after the spark occurs will tell you what is happening in the cylinder and coil.

Misfires can be spotted by observing a secondary parade pattern that shows the firing lines for all the cylinders on one trace. The peak firing voltage for a misfiring cylinder with a shorted plug or plug wire will be lower than the others. But do not forget that low compression and an overly rich fuel condition can also cause the same kind of readings.

If a normal, or lower-than-normal, firing voltage drops even lower when the throttle is snapped, it may indicate a plug wire that is shorting to ground.

If one of the peak firing voltages is spiking significantly higher than the others, the misfiring cylinder may have a bad plug wire with excessive resistance, a badly worn spark plug or a plug with an excessively wide gap. Lean fuel conditions can also cause firing voltages to jump.

Note: If the spark burn lines for all the cylinders are higher than normal (indicating a lean fuel mixture), the underlying cause would point to something that affects all cylinders such as an intake manifold leak, leaky vacuum hose, leaky EGR valve, leaky throttle gasket or low fuel pressure (weak fuel pump or bad pressure regulator).

A rich fuel mixture in an individual cylinder is less common, but can occur if the fuel injector leaks. A more common condition would be a rich condition in all cylinders caused by a dead oxygen sensor or coolant sensor that prevents the computer from going into closed loop, or a faulty fuel pressure regulator that feeds too much pressure to the injectors.

Something else to look at in the spark burn line is the amount of hash it contains. A good cylinder will show a relatively clean line with little hash. A lot of hash, on the other hand, occurs when ignition misfire or lean misfire conditions are present.

The duration or length of the spark burn line can provide more clues about what is going on inside the cylinder. The duration of the spark line for a good cylinder should usually be 1.3 milliseconds or longer at idle with a 0.045- to 0.050-inch plug gap. A shorter spark line would indicate a weak spark, possibly due to a lean fuel condition (dirty injectors or vacuum leaks) or low compression (burned exhaust valve or bad head gasket). If the duration of the spark line is longer than about 2 milliseconds, the air/fuel mixture is running abnormally rich. If the burn line is shorter than about 0.75 milliseconds, the cylinder is running too lean.

The last thing you want to look at in the primary ignition pattern is coil oscillations. If the coil is good, there should be at least two, and preferably three or more, oscillations after the burn line. Fewer oscillations would indicate a faulty coil.

Hands-on checks you can do include removing and inspecting the spark plugs, plug wires and ignition coil (and cap and rotor if the engine has a distributor). Look for obvious problems such as worn or fouled spark plugs, cracked insulators, loose or damaged plug wires, and cracks or carbon tracks on the coil, rotor and distributor cap. If a spark plug is wet when you remove it, it probably is not getting any spark.

If firing voltage is low (weak spark), measure the coil primary and secondary resistance with an ohmmeter. If it is out of specifications, replace the coil. Also, make sure the coil is receiving normal voltage from the PCM or ignition module. If it is low, check the charging system for anything that may be reducing alternator output (battery and alternator cable connections, ground connections and voltage output).

One thing to keep in mind here is that low coil output can also be caused by a faulty ignition module or coil driver circuit in the PCM. A lot of coils are replaced unnecessarily because problems like these are overlooked or misdiagnosed.

Simple checks that can be completed to identify a misfire caused by a fuel delivery problem include checking the injectors and fuel pressure. Are the injectors getting normal voltage? Is the PCM driver circuit energizing the injectors? You should hear a buzzing noise if the injectors are pulsing. A test light or logic probe should also flash when connected to each injector circuit.

You can also observe injector performance on a scope and check its response to changes in the air/fuel mixture. First of all, a flat line would tell you the injector is dead or not receiving voltage (depending on where the line falls on the screen). If the injector is working, the line should drop when the injector turns on, then peak when the current is switched off.

The injector scope pattern will tell you how long the injector is on. If you make the air/fuel mixture artificially lean by momentarily pulling off a vacuum hose, and/or artificially rich by feeding some propane into the manifold, you should see a corresponding change in the injector on-time as the computer responds to input from the oxygen sensor. No change would tell you either the O2 sensor is dead or that there is a problem in the computer.

Injectors can also be checked with an ohmmeter with the engine and ignition off. If resistance is not within specifications, you have found a bad injector that needs to be replaced. Over time, the insulation around the injector coil can corrode, causing the coil to short out. Depending on how the PCM driver circuit is wired, a shorted injector may kill all the other injectors too, so it is important to figure out if the problem is the injector(s) or the PCM.

Dirty injectors and low fuel pressure can also starve the engine for fuel and create a lean fuel mixture that misfires. If fuel pressure is within specifications, the fuel pressure drop across each injector should be tested to see if one or more are clogged. There are some really cool testers available for such testing that will automatically test each injector without having to remove the injectors from the engine. This can be a real time-saver when injectors are hard to reach.

If a lean misfire condition is due to dirty injectors, cleaning the injectors on or off the engine may solve the problem. If the injectors are too badly clogged to be cleaned, they will have to be replaced.

If ignition and fuel delivery appear to be okay, that leaves compression (or the lack thereof) as the underlying cause of the misfire. The air/fuel mixture will not fire normally if compression is leaking from the cylinder due to a burned exhaust valve, bent valve or leaking head gasket. A weak or broken valve spring can also cause a loss of compression.

A leakdown test or cranking compression test can be used to see if the cylinder is holding or pumping up normal pressure. Low or no compression in a cylinder means the head will have to come off so the leaky head gasket or bad valves can be repaired.

Another reason for removing the head is if you find spark plugs that are heavily fouled with oily deposits. Oil leaking past worn valve guides is usually the culprit here. Installing new plugs will cure the symptom but not the cause. The permanent fix is to replace or reline the valve guides and to install new valve guide seals.

A spark plug that shows heavy whitish to brown deposits may indicate a coolant leak either past the head gasket or through a crack in the combustion chamber. This type of problem will only get worse and may soon lead to even greater problems if the leak is not fixed.

A cylinder also will not fire normally if the valves are not opening and closing properly due to a weak or broken valve spring, or a rounded off cam lobe.

If a misfire is not ignition-related and you suspect the fuel system, here are some checks you can perform to diagnose the fuel system.

First, make sure fuel pressure is within specifications, and that the fuel pump is delivering the required amount of fuel (typically a pint in about 15 seconds). Sometimes a pump will deliver normal pressure at idle but not enough fuel at wide open throttle to keep up with the engines appetite for gas.

Check injector voltage. Backprobe each injector connector with a voltmeter or digital storage oscilloscope (DSO). The injector supply voltage with the key on, engine off should be the same as battery voltage. If you see a difference of more than 0.4 volts, there is a problem in the supply circuit. Check the wiring and the injector power relay.

When the PCM energizes (grounds) the injector, you should see the voltage drop to near zero. This verifies the PCM ground driver circuit. If you are using a scope, you can also note the milliseconds of injector on-time and make sure it is changing when you goose the throttle.

Something else to watch for on the scope pattern is a little bump in the waveform that occurs after the ending voltage spike when the injector pintle closes. The bump should occur in the same place if the injector is working properly. If you see multiple bumps or the location of the bump is varying, it means the injector pintle is sticking or hanging up, or the injector is dirty. Note: Some scopes maynot show this bump in the injector waveform.

Another injector test is to check the current flow in the injector circuit on your scope when the injector is energized. This can be done by attaching a low-amp probe to one of the two injector wires. When the injector is energized, the current waveform should start to ramp up as the magnetic field builds in the injector solenoid. When the injector pintle pops opens, it will create a slight dip in the trace, then continue to climb. No such bump in the trace would tell you the injector is not opening.

When the PCM turns off the injector, the current waveform drops back to zero. What to look for here: A sharp vertical rise in the current trace when it starts to ramp up would tell you the injector has an internal short. Also note the peak current draw. If it is over specifications, replace the injector.

Another simple check is to measure the internal resistance of each injector with an ohmmeter. Disconnect each injector from its harness and measure the resistance across its terminals. If resistance is not within specifications (typically 3 to 5 ohms for a low-resistance injector, or 12 to 17 ohms for a high-resistance injector), the injector needs to be replaced. A difference of only 1 ohm less than specifications may be enough to prevent the injector from opening reliably under some operating conditions.

Note: Injector resistance typically increases a couple of ohms as it warms up. Therefore, you should check the values twice: Once when cold and once at normal operating temperature. If resistance goes out of specifications when hot or cold, replace the injector.
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Wednesday, May 20th, 2009 AT 9:11 PM

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