Sept 10, 2020

a spark plug fouled with carbon deposits

My Dad is not a mechanical guy. He is educated and well-read, and handy around the house – but not mechanical. When I was 12, I joined a trip to the local mechanic to get our family car fixed. Dad struggled to describe the issues – he didn’t know the terminology or the parts or how various engine systems work. The best he could do is what many of us find ourselves doing: making noises and blindly guessing.

I wasn’t in the room when the problem was described. But I imagine that Dad talked about a strange noise. Kind of like a ‘rrrrrruuuuummmm ruuuummmmm’, followed by a ‘chuuuugawuga chuuuugawuga’, and then a slight hiccup and a cough. But not every time. Sometimes it didn’t make that noise at all. Come to think of it, it only happens when the engine is cold. Or when it rains. Sometimes on a warm afternoon, but only on Tuesdays.

If you find yourself describing your engine’s symptoms in this way: don’t be surprised or embarrassed. You’re not expected to be a mechanic or to understand the sounds or symptoms associated with potential or ongoing problems.

Often, our engine is telling us things without us being aware – one of my favourite diagnostic tools is looking at the exhaust. It gives us a wealth of information about what’s going on inside the engine as fuel is burned. The exhaust tells of is the engine is getting enough fuel, or too much, or too early or late. It tells us is the explosions are happening at the right time, and if all the elements of smooth running are aligned. It rarely tells us how to fix the problem but it will point us in the right direction.

When we talk about the exhaust, we’re really looking at the by-products and left-overs of the combustion process. On a diesel engine, this can most readily been seen in the exhaust gases. On a gasoline engine, we can get a bit closer by looking at the spark plugs in combination with the exhaust.

Here are some examples of what diesel exhaust will tell us:

white smoke from a Diesel engine

No apparent exhaust – means all is running well. A diesel’s exhaust should be colourless, with a slight trace of a diesel smell (without being powerful)

White smoke – indicates that raw diesel fuel has come through the exhaust completely intact and unburnt. There are a few reasons that this might occur:

• faulty or damaged injectors – injecting too much or too little fuel at wrong moment in the combustion cycle
• Incorrect injection timing – a worn or damaged timing gear, or perhaps mis-aligned
• Low cylinder compression – leaking or broken valves, worn cylinder walls/pistons, or piston rings
• Water entering the combustion space – caused by broken head gaskets or damaged cylinder heads
• Fouling deposits or cylinder glazing can cause white smoke at start-up, which then disappears as the engine warms up

Blue Smoke – indicates that oil is being burnt. Oil can enter the combustion chamber in a few ways:

• Worn valve guides or seals
• Cylinder, piston ring or ring-groove wear
• Stuck or sticking piston rings
• Faulty seals in the turbocharger
• Incorrect grade of oil (oil is too thin)

Blowby – this is when compressed air and fuel in the cylinder bore is greater than the pressure in the oil pan. Fuel leaks past piston rings and into the crankcase. When the pressure becomes too great, it has to escape, and finds any route to ‘blow by’ seals and gaskets. In most cases this is due to worn (or broken/missing) piston rings. It’s identified by:

• Rough idling or misfiring
• White smoke billowing from the valve cover or the oil fill tube
• Diluted oil (by diesel fuel)

a wet spark plug - either water or unburned fuel

In a gasoline engine, the spark plugs tell us similar diagnostic stories. The spark plugs are threaded into the top of the engine’s cylinders. Air running into the engine, and exhaust gases running out pass by the spark plugs. The way that the controlled explosions inside the engine occurs, can be evidenced on the tip of the spark plug.

Spark plugs are specific to their application: They have to be the right length (to sit at the correct depth in the combustion chamber), and the gap between the electrode and the tip has to be the correct distance to maximize the amount of electrical current that jumps across that gap. Engine problems can affect spark plug performance, and spark plug maintenance can affect engine performance. Here are a few spark plug conditions and their performance indicators:

• Dry or wet fouling: the spark plug end is covered (fouled) with debris – wet or dry. This could be unburnt fuel, water, or exhaust deposits.
• Overheating: the insulator tip has a melted or glossy look from the build up of hot engine deposits
• Deposit build-up: The accumulation of deposits can indicate that oil is leaking, or poor fuel quality
• Melting – When an abnormally high current arcs across the spark plug tip, it can melt and effectively weld the electrode to the tip
• Gummed – old fuel can build up and gum the gap between the electrode and tip, preventing it from functioning.

Engines can tell us a lot. The look, feel, smells and sounds that are evidenced during normal operation (as well as when problems happen) can tell us a lot about the engine’s health. Don’t be shy to let your mechanic know what you’re hearing, seeing or smelling. These diagnostic signs can help in many ways. Learning some of these signs can also alert you to problems, and let you tackle maintenance items to prevent problems in the long-run.

Andrew McDonald is the owner of Lakeside Marine Services – a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College ‘Mechanical Techniques - Marine Engine Mechanic’ program.

Questions or comments for Andrew? Email him directly via: This email address is being protected from spambots. You need JavaScript enabled to view it.