Spark plugs keep going bad

I purchased a 2000 Ford Ranger last year. Within the first two months of ownership, the vehicle was having a cylinder 4 misfire. To fix it, I replaced the fuel filter, plugs, wires and cap - replacing the wires, plugs and cap fixed the issue. The issue went away for four months when cylinder 4 misfired again. I brought it to a repair shop and they again replaced the wires and plugs. The vehicle now has a cylinder 6 misfire.

There has to be something causing these plugs to continually go bad. Rather than just replacing the plugs again, I’d like to attempt to be proactive and fix the root of the issue. I’ve googled the issue and some of the forums seem to indicate a bad EGR valve. Can someone please help me?!?!

It Could Help Me Help You If You’d Indicate Whether Or Not The “Check Engine” Light Illuminates/Has Illuminated And Whether Any DTCs (Diagnostic Trouble Codes) Have Been Obtained And What They Were, Specifically.

Lots of things can cause this. We need some clues to narrow it down some more.

What is the evidence that the plugs are going bad ?
Has the engine compression been checked ?
How much oil volume/miles does this engine consume ?
Codes ?


Please Describe The Conditions Under Which It Misfires ? Does It Do It Cold, At Operating Temperature, At Idle, Low Speed, Cruise Speed, Etcetera, ?

Under what conditions is it better/worse ?


CSA thank you for your help.

The check engine light does come on. The code from 4 months ago was P0304 (cylinder 4 misfire), the code right now is P0306 (cylinder 6 misfire). I’m not certain how the plugs looked, I was just told that they went bad. I was also told that engine compression was fine. The engine doesn’t really consume a lot of oil; I never have to add oil between changes.

The misfires basically occur at all accelerating speeds; it’s especially bad if I shift gears a little too early. It is not noticable at idle.

The past three times this has occurred, the misfiring gets progressively worse but never to the point where it won’t drive. When the problem first starts the misfire is not that noticable, but after about a week it is much worse.

“I was also told that engine compression was fine.”

Fine to one person might be crappy to another.
We need actual numbers to say anything meaningful.

How the bad plugs looked is also essential information.

You can take an “up close and personal” picture of one of the “bad” plugs and post it, too.

Compare it to this chart:

How’s it look?

Thank you chaissos! It looks like insulator glazing. I tried to attach a picture but it wouldn’t take.

I just replaced the plug in question and the truck is running normal. Based on my history with the vehicle, one of these plugs will go bad in 4 mo.

I’d like to know what the compression numbers are. Many mechanics and even many service manuals are flat out wrong with the numbers they quote.

If compression is low then this problem will never stop short of fixing that compression problem, as in valve job or new engine.

Fouled plugs are usually caused by excessive oil or carbon build-up. Carbon build-up would commonly be caused by poor combustion. Oil fouling is caused by excessive leaking somewhere or perhaps the oil ring on the piston gone bad. If the plug has gone bad because of a cracked insulator, then you need to make sure that the temp. range of the plug is right for the engine as a starting point. If that is correct, I would suggest, that it could be theorhtically possible that if there a was leakly in take valve, then perhaps cool air would be getting into the clyinder on the exhaut stroke cooling the plug too rapidly causing the insulator to crack. It is a suggestion but I do not know if this could actually happen or not. If that situation could occur, the temp. differential could put enough thermal stress on the insulator to physically crack it.

Honestly, it sounds to me like you’re replacing one when it goes bad, chances are they’re all old. If one gets worn out, chances are the rest will soon.
When you replace one, replace tyem all. If you still have the issue then it’s time to look into other options, at which point could be damaged seals if the plugs are fouled and dirty, could be impropper spark timing, could be impropper fuel to air ratio.
Good rule of thumb though, when one goes, replace them all. Check the wires too, if they’re cracking or worn may be time to replace those as well.
(6 years later… lol oops)

6 years later…

That heading says a lot. I’d bet that by now this problem has been resolved, forgotten, doesn’t matter…
CSA :palm_tree::sunglasses::palm_tree:

Yeah, mobile shows the month of the postings but not the year, didnt know how ancient this post was until after i posted lol.

That’s wild. Well, it could still be fouling plugs and perhaps the owner will check in soon. Thanks for contributing.
CSA :palm_tree: :sunglasses: :palm_tree:

Pretty sure a failing TPS can bring both your plugs and coil down with it, and not throw any codes cause its a variable sensor. Might want to get that replaced, its all of a $40.

Keeping in mind that we are responding to a 7 year-old thread and this vehicle (and possibly the OP) may no longer be with us - new spark plugs will hide a lot of sins.

I would have suspected weak spark, from corroded primary circuit connections or a failing coil.