Replace plugs by mileage or wait for failure?

[OhioMazda]

I have two cars that I bought used, and I don't know if either has ever had new spark plugs installed. My '08 Infiniti M35 has 125,000 miles, and my '09 Mazda 6 2.5 has 191,000. I don't have a scanner, but both cars run well, with no CEL.

Should I replace the plugs or wait for a CEL to appear? How difficult is it to do the replacement myself? What tools would I need? Should the plug boots be replaced at the same time?

 

[fdcg27]

With any used car, plug replacement is probably a good idea.
Plug failure can result in the loss of an electrode, which will likely pass out of the exhaust valve but a bit of metal banging around in the combustion chamber is never a good thing.
The real problem you might face is in getting the plugs out.
If the Mazda is on its original plugs, the gaps will be worn very wide which could in turn cause a coil failure.
You should absolutely replace the plugs, but getting them out might not be all that easy.
Work them just a bit loose and then tighten them.
Rinse and repeat until they turn freely.
The real risk is that you could strip the threads and then need to repair them with one of the commercially available inserts, which might well be beyond your DIY skills and tool compliment.
Depending upon your level of skill and experience, you might want to farm this job out to a good indie.

 

[HangFire]

I would pull one from each car and take a look at them, check the condition, compare the gap to factory, post the results here and let the experts guestimate the age of them.

 

[demarpaint]

I go by miles stated in the OM, or just under that amount. I'd rather not wait for a failure.

 

[JeffKeryk]

Yeah, check 'em. I once replaced the plugs in an '83 Monte Carlo with the Buick V6 engine.
The back plugs were buried.
The car was really clean and had been serviced professionally.
The front 4 plugs were worn but OK; the rears were original.

 

[Snagglefoot]

If you wait for failure it might happen during a well deserved familly vacation trip. Who wants that?

 

[Snagglefoot]

If you have 100,000 miles or more, might consider doing the boots. If not, at least put some dielectric grease on the inside of them so they come off easy when you do decide to replace them.

 

[The Critic]

Not sure about the Mazda.

Plugs are due at 105K on the M35 and yes, they do wear out. We replace them with Denso #3417, which is an upgrade over the originals.

Leaking spark plug tube seals are common the 3.5s. Especially at your mileage. Unfortunately the spark plug tube seals are integrated into the valve cover and are not sold (or replaceable) separately. List price for both valve covers, total, is about $600. You will also need valve cover gaskets and gaskets for the plenum (which needs to come off in order to replace the valve covers).

 

[Oro_O]

Originally Posted by HangFire
I would pull one from each car and take a look at them, check the condition, compare the gap to factory, post the results here and let the experts guestimate the age of them.


This is the smart move.

Both of those cars almost certainly came with high-end iridium plugs when new. They are extremely long lived - generally far longer than the stated service interval. However, "waiting for a CEL" is never a smart move with regular wear parts that can cause damage like plugs. Check 'em. Plugs are one part you can judge wear on more easily than most.

 

[boom10ful]

Your coils/ coil will go bad after replacing very worn out plugs. I've seen it many times. Sometimes, the plugs will be seized into the heads. Follow the maintenance interval and you'll be fine.

 

[qdeezie]

One thing I recently learned when pulling plugs. If they are extremely tight, work the plugs back and forth and eventually you will be able to remove them without any sort of damage or breakage. They work themselves loose using this technique.

I realize my Ford 5.4 is a different animal from most on spark plugs, but I used this technique without breaking a single plug and will be using it moving forward on anything I encounter that has tight or nearly seized plugs.

Exercise some patience and untighten/tighten each plug until they come out. Sometimes, they will only move a little bit when you pull on them to loosen it. If this happens, tighten it back down that same amount and work it back and forth, while attempting to loosen it more and more each time. It will take awhile and will be extremely tedious, but it will work loose.

Hopefully your plugs won't be a headache.

 

[pitzel]

In the extremely cold winters (ie: once you get beneath -15C), there can be a significant difference between brand new and worn plugs cold-start wise. Even if you pull 'em out and re-gap them. Simply due to how sharp the electrodes are.

Ohio might not be as cold as Montana, North Dakota, or Western Canada, but still could help you on those extremely cold days...

 

[Number_35]

Originally Posted by fdcg27
With any used car, plug replacement is probably a good idea.
Plug failure can result in the loss of an electrode, which will likely pass out of the exhaust valve but a bit of metal banging around in the combustion chamber is never a good thing.
The real problem you might face is in getting the plugs out.
If the Mazda is on its original plugs, the gaps will be worn very wide which could in turn cause a coil failure.
You should absolutely replace the plugs, but getting them out might not be all that easy.
Work them just a bit loose and then tighten them.
Rinse and repeat until they turn freely.
The real risk is that you could strip the threads and then need to repair them with one of the commercially available inserts, which might well be beyond your DIY skills and tool compliment.
Depending upon your level of skill and experience, you might want to farm this job out to a good indie.

Excellent advice.

 

[Snagglefoot]

After blowing the plug well with a jet of air, I give it a shot of Liquid Wrench. I then give it a bit of torque to unseat it ( sometimes with a loud squeak). I then let it sit for 10 minutes to let the penetrant contact the threads. Then I unscrew it the rest of the way.

 

[Chris142]

Change them before they fail. If they start misfiring the can damage your catalytic converter.

 

[littlehulkster]

Don't wait for failure on a cheap part that can damage others when it fails. Just do them on the recommended schedule, even with the high dollar plugs most manufacturers are using these days it's not THAT expensive of a job, especially if you do it yourself.

 

[Linctex]

Some engines will have a rougher idle.... but still NOT throw a code

I did a full OBD scan on my F150 - - no "stored" misfires anywhere... but it sure idled rough before plugs were swapped!!!

 

[slacktide_bitog]

If you can't be certain they've been replaced recently or at all, replace them. The job is easy on the Mazda, not sure about the Infiniti

 

[Snagglefoot]

Originally Posted by Or
Originally Posted by HangFire
I would pull one from each car and take a look at them, check the condition, compare the gap to factory, post the results here and let the experts guestimate the age of them.


This is the smart move.

Both of those cars almost certainly came with high-end iridium plugs when new. They are extremely long lived - generally far longer than the stated service interval. However, "waiting for a CEL" is never a smart move with regular wear parts that can cause damage like plugs. Check 'em. Plugs are one part you can judge wear on more easily than most.



Iridiums with 180,000 miles on the left. New AC Delco Iridiums on the right.

 

[Gebo]

I'd remove the plugs. I would gently try to remove them first. If I sensed any difficulty in removing them I would get the engine warm. Remove the wires and definitely give the plug well a shot of air to blow out most of the trash. You can buy a can of compressed air if you don't have a mechanics air tank and such. You might blow out some oil so holding a rag over the hole would catch most of the oil and not let it blow all over you and the engine area. If oil blows oil out, you will have to replace the valve cover or what ever prevents the oil from getting into the spark plug tube. Then I would spray some type of PB, Penetrol, or Liquid Wrench down the hole and let it sit an hour. This should seep down around the threads and make spark plug removal go the way you want it to go.

The reason I know it is important to blow out the plug well is because I didn't one time and I let oil and dirt run down into my engine. How do I know this? Because one of the removed plugs was virtually covered in motor oil where it had been sitting in oil.

Please blow out the hole.....