CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Spark Plug Replacement

1999 Lexus 300 with V6 engine.
105k miles. Bought used. Runs smooth as silk, no perceptible miss.
Would you change the plugs?

Only if you fly me to your house and provide the beer.
I can’t do this weekend, though.

Seriously, first law of engineering states that if it works, you don’t mess with it.

I’m going to partially disagree with Remco–not about changing your plugs for you (I’m not interested in the job), but about whether or not they need to be changed.

But, first, we need more information from the OP.
Since this vehicle was bought as a used one–Did you obtain its prior service records?
Do you know when the spark plugs were last changed–if ever?
Do you know what interval Toyota specifies for spark plug replacement with this engine?
Are the present spark plugs platinum, or copper, or are they perhaps iridium?

What I am driving at is, if you are not sure about whether these plugs have been replaced already, then they need to be replaced.
If you know that they were replaced, but they were replaced more than…let’s say…60k miles or 5 years ago, they need to be replaced.

Even if the engine seems to be running well, and even if the spark plugs are in decent condition, when plugs are left in place for a very long time, they can become essentially “frozen” in place. As a result of that situation, it is not unusual for the threads of the spark plug mounting to become stripped when the spark plug is forcibly removed.

If you don’t have maintenance records for this vehicle, there are other items that you should be concerned about, in addition to possibly overaged spark plugs. Based on the vehicle’s age and its low mileage, it appears that it was driven only about 8k miles per year, and that might be a bad thing if the owner did not change the oil frequently enough. Engine sludge can result from someone changing oil on the basis of odometer mileage, rather than on the basis of elapsed time. Also–if you don’t know for sure that the transmission fluid was changed every 3 years, you really need to have the trans serviced a.s.a.p.

Also of concern is the timing belt. Whether your “Lexus 300” is an ES 300, a GS 300, or an RX 300, the timing belt was due for changing at ~7 years of use, and would be coming up very soon for its second timing belt change, on the basis of elapsed time. The good news is that this engine is not of the interference type, so a broken timing belt will not destroy the engine, but it will result in the sudden loss of engine power, power steering, and power assist for your brakes, and that might not be a good thing if you are in the left lane, in the midst of a bunch of 18 wheelers when it breaks without warning!

Do you know for sure if the timing belt was ever replaced?

thanks, virtual driver. Yeah, I have taken care of all that other stuff. And I have no clue about the plugs previous history, so I guess I will go ahead and have that done.

You plugs if original are overdue for a change. Sparkplug electrodes do erode over time, and left too long the plugs can get stuck in the holes. The spark pulse arcs from the center electrode to the plug’s casing, and grounds to the head. That arc is very hot and evaporates minute amounts of electrode material with each arc, and the current traveling from the plug body to the head with every spark pulse combined with the high temperatures does tend to cause the materials to bond over enough time.

VDC has given excellent advice, as usualy. Read his reply in full.

I purchased a used '01 Toyota Sequoia in 2008. It was running fine, but I pulled a plug to take a look. It looked really worn down. I put in a set of new plugs at 90K miles. I really couldn’t tell you if the plugs I pulled had gone 30K, 60K, or were still the originals at 90K.

Modern cars have computers that compensate for plug wear up to a point. If you haven’t reached that point the car runs fine, but the plugs could be pretty much worn out. I suggest you pull at least one plug and take a look at it. Look at the tip of the electrode for wear and signs of deterioration of the ceramic. On the outside of the plug look at the base (where the ceramic meets the metal base material) and see if there is corrosion, or evidence of some “blow by” meaning the gasket between the metal and ceramic is getting tired. If in doubt, get some new plugs and use the same plugs originally supplied by the factory for the motor.

Check your owners manual, it may call for a service interval of 120k miles. If you have those plugs, you do not need to worry about them freezing in the holes, the threads were plated to prevent this from happening, and it works.

Yes I would. I changed my 100K plugs at 60K to be done with it. If you buy a used car and you don’t have the maintenance and repair history, first thing is starting to replace everything to bring it up to standard. Be very careful of the wires though or change them out too at the same time.

If they’re the originals I would without hesitation and hopefully they will come out without a fight. It’s quite possible to have sublte misfires with no codes and no noticeable symptoms due to aged plugs and just speaking from my personal experience, I’ve never seen a 100k miles spark plug that did not need changing.