Talking Tech: Finding a blown spark plug

By Jeff Taylor

A while back I had a customer come into the shop with an engine noise complaint. This guy was a contractor, and he took great pride in his 2008 F150 5.4L Triton V8. He kept it in top shape, clean inside and out, and always in perfect running condition.

He was very concerned about a ticking noise in the engine. It was rhythmic and quite noticeable. He was embarrassed, partly because his coworkers had started making comments about it.

I’d performed many tasks on this vehicle but the customer also performed a lot of his own maintenance. He was stumped by the noise… so he came to see me.

When he dropped off the truck, I went outside to listen to it. There definitely was something wrong. The rhythmic nature of the noise pointed me to the valve train in the engine. I brought the truck in and performed a couple of basic tasks. I hooked up a scanner, and pulled out a stethoscope. I could tell that the noise was coming from the right bank of the V8, but the scanner showed me nothing. There were no misfires, and no codes of any kind.

The loose plug causing all the noise. You can easily see the blowby that was escaping by the tapered seat that creates the seal.

I did a cylinder balance so I knew that all the cylinders where performing equally. I couldn’t alter the noise at all, even by shorting cylinders. And a vacuum gauge showed proper and steady readings.

Frankly at this point, I was a tad skunked.

My next step was to start tearing down the engine and remove the valve cover. The Triton 3-valve engine is known to have a few issues with valve train and timing chains… but they’re probably best known for having the spark plugs break off in the heads. I knew pulling the plugs would be a challenge so I was so reluctant to do a compression test.

The real conundrum here was that the truck wasn’t missing and was running smoothly… but there was a noise.

Taking it all apart

I had our service advisor contact the customer to explain the next step: engine disassembly. He had a lot of questions first, with a bunch of ideas about what might be causing the noise. (The Internet is full of good and bad information!) But, in the end, he said go ahead. He just wanted the engine to run quietly.

I went back to the truck and did my due diligence. Before tearing it apart, I would pull out the spark plugs and do a compression check, and a cylinder leak down. Nothing was really pointing me in that direction, but I wanted to be sure. The customer said that he had put new spark plugs in the truck about a year earlier and he’d had no issues getting them out, so I started to dig in.

Two spark plugs removed from a second Ford F150 that had the same ticking noise as the first truck. I knew right away what the noise was this time. But these plugs had been loose so long, the electrode had melted off. Remember, the plug transfers heat from the combustion process to the cylinder head… and if it’s loose, it can’t transfer enough and the plug overheats.

Starting with the most difficult cylinders – the ones at the back – and working forward, I removed the coils. When I got to cylinder #2 it was very discolored from blow by. Could this be the issue? I took out my plug socket and when I tried to remove #2 plug, I found that it wasn’t loose… but it wasn’t tight either. It had the exact same feeling as a broken plug. But when I unscrewed it, I found to my surprise that it was intact!

Looking at the plug, though, you could see that the threads had been polished, and had been working its way out. It was blackened but still in one piece. The wheels in my head were turning pretty fast now. Could this be the cause of the noise? Could it be that simple?

In my 30-plus years as a tech, I’ve never heard a noise from a loose plug that mimicked a valve train noise so closely. I’ve heard many blown-out spark plugs in my time. You can place that sound even when one passes you while you’re standing on the sidewalk. But I’d never heard a tick like this truck made before.

Based on the blow-by damage, I ordered a new set of plugs and a new coil. When the new parts showed up, I took the time to inspect the spark plug holes, remove any dirt and corrosion that these deep wells tend to accumulate. I also ran a proper spark plug chase into the threads to make sure that everything was perfect. After doing a compression and leak down check that showed no internal issues, I installed the new plugs and started the engine. The noise was gone! A loose spark plug had been the culprit.

Dry installation

The lesson here was that the customer had installed his own plugs, and reading on the Internet how prone these engines are to spark plug issues he’d coated the plug with anti-seize. This is a very common practice in our industry… but it is something that should not be done.

Now, before you get into a snit, I’m just telling you what the spark plug and the engine manufacturers say. Spark plugs should be installed dry.

The issue with this particular engine, and the frequency of plugs breaking in the cylinder has to do with deposit build-up in the exposed part of the combustion chamber. It is not the result of dissimilar metal sizing of the housing into the cylinder head.

I know what you’re thinking. You’ve read the TSB and it says to install anti-seize on the spark plug. But read the TSB again. It says install a film coating of a specific nickel anti-seize to the ground electrode shield… a very slight amount. Using too much anti-seize is a well-documented cause of tip-in misfires, especially if you get it on the strap at the end. As for the spark plug threads, the TSB is clear. They should be left dry. No lubrication. Just tighten them to the correct torque.

Today’s spark plugs are made and designed to tackle the issue of dissimilar metals and most have a proprietary coating that is meant to prevent them from seizing into the cylinder heads. In fact, the use of a lubricant or anti-seize compound can cause all kinds of problems. The most serious is over-torquing by as much as 20-30%. That may not seem like much, but on an aluminum cylinder head with only four threads, that can cause galling and enough damage to make the plug very hard to remove when the next replacement is required.

There’s even evidence that some anti-seize products can act as an electrical insulator, affecting the actual spark by interfering with the current’s path to ground.

Here you can see the tapered seat area that had some contamination and the start of the plug losing its seal. The result: a blowtorch effect of hot gases escaping. That usually destroy the coil as well.

Many engine designs have long hidden wells that hold the coil or wire centered deep in the cylinder head for better performance. These wells are perfect for collecting debris, coolant, corrosion, and oil. That all has to be removed even before we remove the spark plugs. If the well is full of oil from a leaking valve cover, that needs to be repaired before new plugs are installed. Failure to do that will invite more issues in the future.

The trick here is to get the area as close to what it was like when it was new: clean and dry with a smooth sealing surface, and threads that are the same. I use compressed air to clean the holes out (covering any areas I don’t want stuff being blown into – and that includes my eyes!).

Any debris that falls into the well during installation can interfere not only with the seating of the spark plug, but can affect the torque of the plug during tightening. The spark plug will seat against the debris and be tight for a while but after some cycles, that debris may cause the plug to come loose.

Changing spark plugs used to be a straightforward task. Maybe not always easy, but definitely straightforward. Nowadays, with the price of many spark plugs climbing higher and higher due to all the exotic compounds they contain, the last thing we want is to have to do the job over. Or, worse, repair a damaged cylinder head for free.

Always take the time to make sure that all the debris is removed before you even start the disassembly. And use a torque wrench to install them.

We have to follow the proper procedures – the ones listed in the service manuals and installation guides – to the letter now. If it says the spark plugs are to be installed dry, then we’d better follow those instructions… or we may be causing internal engine damage and frustrating drivability issues.

Jeff Taylor is a former ACDelco Technician of the Millennium and Canadian Technician of the Year. He’s the senior tech at Eccles Auto Service in Dundas, Ont.