I have a 2006 Chevy 1500 with 4.8 5 speed and I get a random misfire code in #8 cylinder.

jac1962

I have a 2006 chevy 1500 with 4.8 5 speed and I get a random miss fire code in #8 cylinder doesn't do this all the time just sometimes I have changed wires plugs coil and injectors around and still get miss fire in cyilinder #8 could this be the computer only throws code when driving and engine light does not stay on only blinks when missing. on this server.

Mr_Shiftright

I think you should run a compression test on #8 and start from there.

Other possibilities

Vacuum leak(s)
Low or weak fuel pressure (including plugged fuel filter)
Improperly functioning EGR system
Defective Mass Air Flow Sensor
Defective Crankshaft and/or Camshaft Sensor
Defective Throttle Position Sensor
Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems

If this misfire only occurs at high speed, you also have to look at the less likely possibility of a powertrain misfire, especially if you experience any kind of violent jerking while decelerating.

thecardoc3

This is a good example for how complicated this can be. The report is a random misfire, any failure that could be detected with a standard compression test would be misfiring all of the time. When performing this kind of diagnostics, the ability to logically rule in/out possibilities is important otherwise you could be testing things all week and never find anything.

The first thing that needs to be known is;

Under what operating conditions is the misfiring occurring?
Idle? Which is no-load.
Light engine load such as cruising below 30mph?
Normal load which would be cruising at highway speeds?
Some combination of the above?

Then we need to examine the fuel trim corrections during the conditions that cause the misfire. Which correspond to these generic sources. Is the fuel trim adding or taking away fuel from one or both banks? Is this correction happening all of the time, or only when the misfire is occurring? So what about the above possibilities? Are they really where you should be looking for the cause? To answer that we have to look at what kinds of problems they cause and then compare that to the symptoms that are being observed.

Vacuum leak(s)

That would be prevalent at an idle, no-load and the fuel trims would be making a correction to add fuel which would diminish as the engine load increases. At a heavy engine load there is no manifold vacuum so a vacuum leak has no impact on the engines air/fuel ratio.

Low or weak fuel pressure (including plugged fuel filter)

This would have a greater impact at higher engine loads and again the fuel control system would be adding fuel to try and correct the condition. Even so this would have an impact on the entire engine, not just one cylinder so that is effectively ruled out already.

Improperly functioning EGR system

On some engine designs we would have restrictions that could deliver excessive EGR gasses to one or two cylinders causing them to misfire but that would only occur when the EGR was being commanded on. EGR flow when it shouldn't be occurring disturbs the whole engine and not just one cylinder, so that is ruled out as well.

Defective Mass Air Flow Sensor

The engine load calculation would be incorrect causing the fuel trims to have to adjust for it. This usually results in fuel trim codes well before a misfire is detected and even so would have an impact on the whole engine, not just one cylinder. Both the fuel supply issues (filter and/or pump) as well as a MAF under reporting cause drivability symptoms that are engine wide and would not be related to a single cylinder issue.

Defective Crankshaft and/or Camshaft Sensor

The systems are very good at coding f one of these signals drop out or become erratic. The other thing the system does should that occur is it disables the misfire counter since it needs these inputs to recognize crankshaft speed variation (which is what a misfire is). While these cannot be completely ruled out, logically a crank sensor signal issue that would appear to suggest a misfire would not be cylinder specific, it would and could occur at anytime during 720 degrees of crank rotation (2 revolutions) so that makes it highly unlikely.

Defective Throttle Position Sensor

You know all of the complaints about the loss of power and ESC and Stability system lights coming on? A large majority of those are a throttle position sensor starting to fail and the PCM see's signals from the two sensors that aren't plausible so it shuts the engine down.

Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems

If the problem is a burned valve or most head gasket issues it would misfire all of the time, and these kinds of problems are easy to confirm with the compression test. Issues where a valve can randomly stick and not close fast enough, or a lifter issue where the valve could even fail to open far enough or not close properly could genuinely be in play with the reported symptom. Testing to prove this requires the skilled usage of pressure transducers and a digital storage oscilloscope. Regular compression tests are a static test and if the failure isn't there 100% of the time that you are testing it isn't going to reveal anything.

The whole point right now is one must try and logically rule possibilities on or out as a potential cause. Then what is left dictates the kind of pinpoint testing that must be done in order to prove why the misfire is occurring. For example, you wouldn't want to blindly tear the engine down and replace lifters and a camshaft unless you had real proof that something is occurring that is causing the valves to not open and close correctly all of the time. A lifter that is having roller wear can easily cause a random misfire as sometimes the valve might not open enough, and other times not close at all.