Do you read spark plugs?

Just changed the spark plugs on my 2000 Town Car. Huge difference in the performance. I know that the way the old plugs look can tell you about potential problems with the engine and wanted to see if anyone had experience deciphering spark plug wear?

The four from the driver’s side of the engine had some kind of crusty, ashen deposit on the electrode. One from the passenger’s side was black. I’m posting pics along with this.

Read plugs?

They’re totally worn out!


They're totally worn out!

Yeah man, I replaced them! By reading them I meant that spark plugs are supposed to be able to tell you about the health of the engine, like this page explains here:

But I don’t see any thing like the crusty ashen deposits that I had on three of the plugs from the driver’s side.

All show significant erosion of the electrodes from normal wear, as evidenced by the rounded center electrodes, thinned ground electrodes, and large gaps. One of the ones on the left shows a wee bit of oil deposition, but nothing that would suggest a problem. The ones on the right have (in addition to the aforementioned erosion) some “oil ash” deposition, but again nothing that would suggest a problem. This crust deposition isn’t abnormal.

Other than the fact that they look shot, they look pretty good, like the engine has been operating properly. I attached a link to charts for reading spark pugs that you might find educational.

The whitish deposits are ash from various additives in the fuel and the black deposits are likely from missfire. How many miles were on the plugs? If less than 10,000 the engine needs some attention. If 100,000+ pull the new plugs out in 10,000 miles and we can “read” them. There is a reason that replacing plugs is considered regular preventative maintenance. They get dirty and deteriorated.

Are those the originals? If so, were they hard to remove?


You wanna read plugs?

You just installed new ones.

After a thousand miles pull the plugs and compare them to each other.

If they all look normal and the same, you can stop reading plugs.


I always look at mine and hope to see a nice tan or brown deposit.

Me too. But I accept imperfection as long as it’s not in a manner that suggests an abnormal internal problem.

I just changed my own plugs recently and while they told me that the engine was operating great, they also told me I need a new valvecover gasket. Toyota molds the spark plug tube O-rings and the valvecover gasket as one piece. Two of the shells had wet oil on the outside. Bummer. Fortunately, Toyota also uses flat plug bases with soft metal crush washers, so there’s no way to the oil to get drawn into the cylinder. By the way, the engine is 10 years old with 227,000 miles.

I have the gasket in my inventory in the garage and am waiting for my neighbor’s grandson to show him the process. He’s interested in cars, and just bought a new Dart, but his dad knows absolutely nothing about cars. I think I can teach him a lot in this process, primarily the importance of getting the proper documents, familiarizing yourself with the process, the proper use of torque wrenches and good beck & forth torqueing practices, how the gasket (w/O rings) works, what camshafts look like and how they work, and good workmanship practices. I’m looking forward to changing the gasket. I enjoy teaching a lot more than actually wrenching.

As a minimum, I’d pull the one that replaced the black one at 1,000; 5,000; and 10,000 more miles to see what might be going on with that cylinder. If it goes dark quickly, I’d throw a new coil on it.

The spark plugs look normal to me, with a little bit of oil deposits on some of them, but nothing out of the ordinary for a 15 year old car with what I assume is well over 100,000 miles on them.

As for the article you linked, you won’t find most of those problems on a modern day car anymore. That, in addition to the fact that on many cars removing the spark plugs can be an hours-long process, means that we (shops) rely on other means to determine the operating condition of an engine. A few minutes with a scan tool will give us far more information than a spark plug will.

Most of the problems in the charts shouldn’t have been found on engines 40 years ago either… {:slight_smile:

Lead glazing? Detonation? Carbon fouling? These things could happen 40 years ago but are nigh unto impossible today.

Good points.
But hey, if many of the conditions shown in the charts no longer exist, than he won’t see them on his spark plugs, will he? That doesn’t make the rest of the chart useless.

I agree with the above posters, those plugs don’t indicate any problem. The sort of grey look indicates things are sparking along normally. I usually don’t get quite as much as ash build up, but that might be b/c of the gasoline I use or that I change my plugs more frequently.

By “no problem”, I mean for well-used plugs. It’s not surprising you’re noticing a performance improvement. If you measure the electrode gap I expect it will be considerably more on the old plugs than the new ones you just installed. When the gap gets too wide, the spark can’t make the jump sometimes. An overly wide gap will also stress the coils, causing them to overheat or even start sparking where the spark shouldn’t occur, like on the inside of the coil.

OP, if you have a chance, suggest to measure the gap of the used plugs, and compare it to a new plug or the recommended plug gap for your vehicle and let us know what you measure. I’m guessing the used plug gap will be too big and considerably out of spec.

But hey, if many of the conditions shown in the charts no longer exist, than he won’t see them on his spark plugs, will he? That doesn’t make the rest of the chart useless.

yeah, well, there you go making sense again.

It’s a habit. Not 100%, mind you, but with a fair batting average. I shoot for over .500.

Other than worn out and needing to be changed eons ago, I don’t see a problem with them. The darkish colored plug could be one that was misfiring badly.

It’s a habit. Not 100%, mind you, but with a fair batting average. I shoot for over .500.

Well that’s better than my beloved Mariners. 2/3 through the season and they’re 11 games back and dead last in the AL West.

Here’s how I read spark plugs. If I go to the trouble to remove old plugs…they better look brand new before I would ever re-install them. That’s never happened before and new plugs always make my engine run better. Some things you just can’t see…remember that.