Spark Plug Interval

[AZ Mike]

The owner's manual says to change the spark plugs at 90,000 miles/72 months. Just so happens I have a 10 year old LS400 with only 63,000 miles on it, so have never changed the plugs. Out of curiosity, I pulled one of the plugs (original; Denso Iridium Long Life) today to have a look. Gap is perfect, just a little light brown over the upper portion of the ceramic....in other words, looks perfect....especially for 10 years and 63,000 miles! I can't see how time alone can deteriorate a plug........but then again, I certainly don't know everything. And, it seems kind of weird since I have never had plugs in for 10 years in any motors I have ever owned. Any reason to change good looking plugs just cuz they've been in for 10 years? I venture to guess the answer is "no", but I am allowing my anal retentive side to ask......

[Jeffrey Davis]

The owner's manual says to change the spark plugs at 90,000 miles/72 months. Just so happens I have a 10 year old LS400 with only 63,000 miles on it, so have never changed the plugs. Out of curiosity, I pulled one of the plugs (original; Denso Iridium Long Life) today to have a look. Gap is perfect, just a little light brown over the upper portion of the ceramic....in other words, looks perfect....especially for 10 years and 63,000 miles! I can't see how time alone can deteriorate a plug........but then again, I certainly don't know everything. And, it seems kind of weird since I have never had plugs in for 10 years in any motors I have ever owned. Any reason to change good looking plugs just cuz they've been in for 10 years? I venture to guess the answer is "no", but I am allowing my anal retentive side to ask......

I've got 250,000 miles on my '98 LS400. I changed the plugs at 90K but not since. I'm getting 27+ mpg on the highway so I'm leaving well enough alone.

Jeff

[1990LS400]

According to the maintenance booklet for my 2000 LS400, the spark plug change interval 120,000 miles. I don't think the spark plugs used in the 2000 model year are different from those used for the 1999 model year. There was a minor change to the emission control system on LS400 for the 2000 model year but I don't think it affected spark plug life. I think it is doubtful that spark plugs deteriorate with age like some components do.

I had the spark plugs replaced on my first LS400 at 90,000 and 180,000 miles. I remember looking at the old spark plugs one of those times, seeing that they still looked like new and wondering if they really needed to replaced that soon.

[Sando]

Hi, I am new to this forum.  My intuition is telling me to question my mechanic.  I own a 1999 LS400 with 68k original miles.  The car is like brand new inside and out but I haven't driven it in about a year.   It's sort of been sitting.  My husband replaced the battery recently but when we tried driving it, we had to pull over as it was missing and backfiring on the fwy!  I had it towed after that.  Mechanic said it needs new spark plugs and new ignition coil.  Can this happen with only 68k miles?   Thank you kindly in advance for any help!   

[paulo57509]

If you feel the need to question the mechanic, by all means do so.  My father ran his own shop for +40 years in the Bay Area and would always try to explain how and why whenever a customer had questions regarding his diagnosis and fix.

Did the Check Engine light come on?  If it did, did the mechanic read the trouble codes that were set (if any)?  If it indeed needs plugs and a coil, did he tell you why these things needed replacing?  From your description, it sounds like he found a bad coil (the engine has eight coils, one for each plug).  If the coil was bad, it's plug probably fouled.  In these cases, it's pretty much SOP to change all the plugs and not just the one.

When was the last time the plugs were changed before the rough running on the freeway (replacement interval is 90k miles)?  Plugs can go bad but these days they usually go bad because something caused them to go bad.

It may also be a fuel issue.  I doubt fuel goes bad in a year, but it could be a "stale" fuel problem.  Was the tank refilled recently?  If not, remove the fill cap and give the filler neck a whiff.  When gasoline goes bad it begins to smell like turpentine.

[unknwn]

Looking at the center electrode with a magnifier ought to help you discern whether the edges of the cylindrical area are 'sharp' 90 degree -or- rounded/worn. The fixed electrode (welded to the body of the plug (by the threads) are generally NOT iridium (except in the very highest cost versions), and will be an area that would likely exhibit erosion.

If your well-enough off to feed it $100 of plugs (just because), then I wouldn't hesitate, but put your take-outs in the new boxes and store them somewhere out of the way. You might wish you had one day.

 

[Sando]

On 12/7/2023 at 12:19 PM, Sando said:

Hi, I am new to this forum.  My intuition is telling me to question my mechanic.  I own a 1999 LS400 with 68k original miles.  The car is like brand new inside and out but I haven't driven it in about a year.   It's sort of been sitting.  My husband replaced the battery recently but when we tried driving it, we had to pull over as it was missing and backfiring on the fwy!  I had it towed after that.  Mechanic said it needs new spark plugs and new ignition coil.  Can this happen with only 68k miles?   Thank you kindly in advance for any help!   

On 12/12/2023 at 6:58 PM, unknwn said:

Looking at the center electrode with a magnifier ought to help you discern whether the edges of the cylindrical area are 'sharp' 90 degree -or- rounded/worn. The fixed electrode (welded to the body of the plug (by the threads) are generally NOT iridium (except in the very highest cost versions), and will be an area that would likely exhibit erosion.

If your well-enough off to feed it $100 of plugs (just because), then I wouldn't hesitate, but put your take-outs in the new boxes and store them somewhere out of the way. You might wish you had one day.

 

Thank you so much.   I forgot I joined the forum...stupid me!  : )   At this point, car is still in the shop and he found yet another issue.   The catylitic converter was only working 50%.  He put new spark plugs in as they had oil on them...and also installed the ign. coil.   I'm not sure if he replaced all 8 though.  I'm going to see mechanic tomorrow and mention what the forum has told me.   What bothers me is the fact this car was running fine and then this...just very strange.   Thank you again!

[Sando]

On 12/11/2023 at 5:57 PM, paulo57509 said:

If you feel the need to question the mechanic, by all means do so.  My father ran his own shop for +40 years in the Bay Area and would always try to explain how and why whenever a customer had questions regarding his diagnosis and fix.

Did the Check Engine light come on?  If it did, did the mechanic read the trouble codes that were set (if any)?  If it indeed needs plugs and a coil, did he tell you why these things needed replacing?  From your description, it sounds like he found a bad coil (the engine has eight coils, one for each plug).  If the coil was bad, it's plug probably fouled.  In these cases, it's pretty much SOP to change all the plugs and not just the one.

When was the last time the plugs were changed before the rough running on the freeway (replacement interval is 90k miles)?  Plugs can go bad but these days they usually go bad because something caused them to go bad.

It may also be a fuel issue.  I doubt fuel goes bad in a year, but it could be a "stale" fuel problem.  Was the tank refilled recently?  If not, remove the fill cap and give the filler neck a whiff.  When gasoline goes bad it begins to smell like turpentine.

Yes, the check engine light came on and the VSC.   Yes, it showed codes for what I mentioned.  The only thing he mentioned was the spark plugs had oil on them.   I'm not sure if he replaced 8 coils or 1, however, I will ask tomorrow.   Car is still in the shop and it's making me so nervous.  I just don't understand why all of a sudden it started to run bad.  It was fine 6 months ago, which was the last time we drove it.  Today he said the catylitic convertor is only working 50%.  So now that also has to be replaced and he's telling me that he still won't know for sure if it will run properly.   Before my husband replaced the battery, he put some kind of fuel cleaner in the tank and then drove it on the fwy.   We're not sure if this made things worse!    Thank you for sharing about the smell of old gas, my husband said he did not know that and said thank you as well!  Thank you for all of your time and help, so appreciated.

[unknwn]

There are -other- aspects- of engine geometry that will vastly affect how it performs, and when you mention that things changed -somewhat(?)- abruptly, then the solenoids (&-or-) sensors intimately involved in cam timing ought to be investigated,  electrical resistance (and visible function of the solenoids) along with resistance & signal aspects of the sensor s can dramatically affect performance, discoloration of the plugs is another symptom I saw prior to a sensor issue.

in-person and hands-on nearly always trumps interwebs forum opinions, -but- I've gone through some somewhat similar diagnostics since landing here, and my RX is better for my efforts.

[Sando]

Thank you Charles I appreciate all of your input.    I rec a call today to pick up my car.   Mechanic told me the car is running good and he put an aftermarket convertor on because the dealer said they cannot get them any longer.  I'm keeping my fingers crossed.   I am now thinking of selling it because of the original miles and due to the cond it's in.   I am going to see what I can find on the forum re pricing and research what I can.   I still have my other Lexus a 98 which has 260k miles on it.  Since I never drive the other one I feel it needs to be driven more than twice a year!   Any how, thanks again and Happy Holiday's.

[mikaelse]

On 12/7/2023 at 9:19 PM, Sando said:

Hi, I am new to this forum.  My intuition is telling me to question my mechanic.  I own a 1999 LS400 with 68k original miles.  The car is like brand new inside and out but I haven't driven it in about a year.   It's sort of been sitting.  My husband replaced the battery recently but when we tried driving it, we had to pull over as it was missing and backfiring on the fwy!  I had it towed after that.  Mechanic said it needs new spark plugs and new ignition coil.  Can this happen with only 68k miles?   Thank you kindly in advance for any help!   

Your 1999 is a very fine low mileage vehicle. Here in Sweden they are highly sought after and now sell for up to 15.000US$. Doubled at least in value last 10 years. Hard to find a more reliable and long life vehicle.
I would have guessed more likely a fuel related issue than anything else. Fuel does not like being kept over long time in the car tank. Depends on fuel quality, type of fuel, Climate, type of car  etc but more than six months is not a good idea at all.  There are additives you can add to make it last longer that absorbs moisture etc. Most sediment and varnish when you started driving was reasonably caught by the fuel filter but there is risk for some dirt after the filter etc clogging fuel injection nozzles.  With some luck they can be professionally cleaned if that happens. 

It seems very common that when you take these car to a "normal mechanic" you get proposals to change stuff that later is shown to have nothing to do with the actual problem.
It is not easy for the inexperienced to know that these cars parts often last twice as long as "normal" car parts. CAT converter dying after 68 k miles is not normal. 
Not seldom you remove a OEM japanese part made by AISIN, NIPPODENSO or SUMITOMO designed extremely long life expectancy and put in a new third party thing with less life left in it. 

I have also not driven my LS400 cars for more than a year but I let it varm up for 30 minutes and move around the block about every month. Stil works fine on the old gas but I live in a cold climate. When you run the engine the fuel is circulated and passes through the fuel filter and most of it goes back to tank again and it repeats. Hence it filters out most dirt if you do it regularly is my theory.