Check Engine Light

truck has been running rough when in park, but when it's in gear with my foot on the brake then it doesn't have any of the rough idle characteristics. RPM both in park and drive is 500-600 RPM.

I don't really recall this problem since I replaced the plugs & #7 coil. I cleaned the MAS, so I guess the next step is pull a couple of plugs to check proper gap.

I called the local auto parts store and they said that all their OBD II code readers really only look for CEL codes and they wouldn't detect a non-CEL problem.

He said that a dealership is probably the only real way to go, but are there consumer OBD II readers that can detect a issue that doesn't always result in an CEL being triggered?

My experience has been, once the first coil goes you can expect the other 7 to go (Usually one at a time) over the next 3 or 4 years.

UPDATE. Changed the plugs. Thought that solved the problem but threw another CEL about a week later. Had it checked and said was still the #7 coil. Went ahead and replaced it and haven't had another CEL in almost 2 weeks. Does seem to have an occasional intermittent period of running kinda rough when I put it in park. Can't duplicate the problem and is kinda hit or miss. Can't notice it when I'm driving so I guess the next thing will be to clean the MAF sensor.

All the plugs looked good for 120K wiles and couldn't visibly detect any difference in any cylinder.

"CEL = Check Engine Light. Its a fault indicator that something in the emissions system is faulting."

Or engine, or transmission, or other electronic components.
Generally mechanical or hard part issues or failures don't trigger a CEL unless there is a sensor or another component that reads, uses or records data or conditions that the hard part is affecting.

IE: A bad belt driven power steering pump will for the most part not trigger a CEL. But if it's also a hydroboost pump for the brakes and the brake system detects a fault, it could throw a code, but that code may only be a clue as to the actual problem.
Biggest problem (although it's never changed, there always has and always will be competent and incompetent people in every line of work) is those that assume or believe that codes and CELs have replaced cognitive diagnosis needs. They haven't. They just warn or help diagnose issues that can't be ascertained by mechanical means.

Usually the computer will have to register the issue on 3 consecutive "trips" to set a code. Likewise it will put the light out with 3 consecutive trips without the fault appearing. Be aware that the computer's definition of a "trip" is different than ours. I would start with a set of plugs but I have seen vehicles with over 200,000 KM on the factory ones and running fine.

To answer your original question, the CEL comes on when conditions detected by the computer call for it to be turned on. When those conditions are no longer present for a set amount of starts or miles, the light goes off.

So if the code came up that the #7 was misfiring, and the next time the light went off, that means the Autozone person reset the code, or the computer no longer thinks the #7 cylinder is misfiring.

A scan tool is as cheap as $30, and handy. Everyone should own one. I never would have made it to 218K on my last truck without one, and people ask me to scan their vehicles with it all the time.

OBDII code readers are inexpensive and offer peace of mind on some occasions. From knowing before dropping your vehicle off what the primary issue is, to finding an emmission leak which is just a loose gas cap.
Dont buy the cheapest ones ($20) but when you step up one level they are pretty comprehensive tools.
And if nothing else, when on the road and having a CEL come on, being able to verify that its minor or being able to clear the code to keep the computer from making additional changes to engine settings which can exacerbate the problem for a short time to allow you to get home becomes invaluable.

theoldwizard1 wrote:

enblethen wrote:
Plugs are not monitored by the ECM. the coil pack is the last point.

That is not an accurate statement.

What is actually monitored is does each cylinder give an equal amount of power for each combustion event. There are literally dozens of reasons why a cylinder is not "contributing" equally. Bad plug or bad coil are two. Admittedly they are easy items to replace.

If you have not replaced the plugs, do that first. Do all 8 as you are overdue. Stick with the EXACT SAME PLUGS ! (Motorcraft SP548 Why not ? The first set gave you good life !) Before you reinstall the coils, carefully inspect the inside of each boot. If you see any carbon tracking YOU MUST REPLACE THE BOOT ! Many auto parts stores do not stock just the boots (they would rather sell you the much more expensive coil) but they are available online.

Also put a dab of dielectric grease on the inside of each boot. They will side on easier and will be less likely to arc. If you do choose to replace the coil or can not find just the boot, again, stick with the exact same Motorcraft coil (DG542).

Amazon has good prices on both.

Best answer. Replace the plugs for sure. Ditto to use Motorcraft coil.

Interesting topic. I had a '91 GMC w/a 454 and it developed a skip. Out of warranty, took it to a local shop I heard advertised nearby. They concluded a plug wire was bad and replaced it. Problem gone. HOWEVER, the same shop wanted to still probe into why it was skipping. Well if a plug wire was bad, skip went away, why continue? Oh I know, $$$$$$$$. Paid them, walked away and never set foot in there again.

Take care of all deferred maintenance first, before throwing parts at it!

Worn plugs will cause a misfire. Worn plugs can also damage the coils, but start with the plugs first. Changing just the coil later is easy.

Don't put in Autozone coils (or any bargain priced auto house brand). They don't fire as strong as the OEMs, and have a much shorter life. They're good for getting you on the road if when goes out while you're on vacation, but that's about it.