Spark Plug Issues and Testing - Mick's Tips of The Trade

By: Mick McCrudden

Date: 20.08.2017

Look after those spark plus, says Mick McCrudden

Spark Plugs

Your car is running a bit odd and, this is probably a funny thing to say, the first thing to do is establish which cylinder the problem is with. That can be a minefield. Some of us older folks with older cars are happy to pull off individual ignition leads while the engine is running, to determine if it makes a difference. But of course you can get zapped.

Buy a spark plug tester, which you can plug in to the lead, and get someone to crank the engine over – that way you don’t hurt yourself.

In modern cars the ignition systems have been upgraded to much higher voltages to try to get a better burn and meet emission targets, and they are susceptible to mistakes such as unintended earthing. In older cars, you’re talking about a 48-50,000 volt set-up and they’re a lot easier to work with.

Once you pull the plugs out, you can use them to get an idea of how each cylinder is working. Oily and damp means you could have an issue with rings, valve guides or valve stem seal. Very black and sooty might indicate rich running, which could be a faulty choke or poor tuning. There are a host of visual guides out there to help you with this. One thing we’re seeing a lot more of (and my pet hate) in older cars is a tell-tale red stain on the centre insulator. That’s an indicator of someone using fuel additives that foul the plugs and I’m not a fan of them. You won’t get a mechanic in a bottle. If your car is having trouble running on modern fuels, there are ways to set it up properly rather than wasting money on something that will create more problems.

Now if you’re a very keen home mechanic, there’s a little trick you can do which is probably worth a couple of horsepower – that’s indexing your spark plugs. The aim is to have the open end of the grounding plate of the spark plug pointing at the exhaust valve. The reasoning is this has it pointing away from the ‘wet’ turbulence of the intake.   This is a very simple task, though very time consuming. You mark where the open end is higher up on the plug where you can see it and, with a box full of plugs (no two are the same), work your way across each cylinder, getting them pointed the right way. You can also get indexing washers from your local speed shop, to help this process.

Now you may have heard of people using different grade plugs for different times of year. This works on some engines, particularly anywhere where there are big temperature differences across the season. Just make sure you understand the numbering system for different brands. For example NGK works in the opposite direction to many, with a lower number meaning a warmer plug. We have an old Cleveland that likes 4s in winter and 5s in summer.

How often should you change plugs over? Every 10,000km used to be the rule of thumb, but these days you can get away with 15-20,000km. Some high-end brands claim 100,000 but I wouldn’t be comfortable leaving them in that long. So, how are your plugs?

Mick owns Glenlyon Motors, an expert workshop and car storage facility in Brunswick, Melbourne. Call him on (03) 9380 5082.

Unique Cars magazine Value Guides

Sell your car for free right here