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One of the best indicators of your engine’s health is the spark plugs. Keeping an eye on them could help prevent more serious damage so read this guide on how to check spark plugs.
Now this might seem like a ludicrously simple job, but here we’re looking at more than just unscrewing the old spark plugs and screwing in new ones, for the condition of your spark plugs can tell you a great deal about how well your engine is running, you just need to know what you’re looking for.
First, lift the engine lid, look in the engine bay and take a moment to orientate yourself with the position of the plugs in your engine bay. Removing them is tricky at the best of times, particularly numbers 1 and 3 near the back of the engine bay. If your car has twin carbs it can be a real headache, but persevere, try different combinations of extensions and UJs (universal joints) on your ratchet and you’ll get there.
The most important thing of all is to make sure you have the spark plug socket firmly on the plug when you’re undoing them and always to start screwing the plugs back in by hand. If you feel any resistance, back them out and start again, then only use the ratchet for the final tightening and then don’t overtighten them.
Only do this job when the engine is cold else you’ll almost certainly burn yourself and there’s more chance of damaging the threads screwing spark plugs into hot cylinder heads. The last thing you want it is to cross thread a spark plug in its hole as, while it is possible to have the threads restored, the engine has to be taken out and the heads removed, which generally means money paid to a specialist.
How to check spark plugs: Job info
- Cost: Approx. $15 dlls for a set of plugs
- Time taken: One hour
- Tools used: 21mm spark plug socket, ratchet and various extensions, brass brush, feeler gauges
We can’t emphasise this enough. Always start a plug back into a head by hand.
A cheap brass brush is all you need to clean up your plugs. Put one in your in-car tool kit today
How to check spark plugs: Step 1 of 8
First, pull off the HT (high tension) leads that go to the spark plugs and tuck them away roughly in the position they came off so you don’t get them mixed up (or mark them with masking tape). Check the condition of the rubber seals at the ends and the bakelite connectors. Any damage like this and you need new ones right away.
How to check spark plugs: Step 2 of 8
Now, using a proper 21mm spark plug socket with a rubber liner, feed it down into one of the holes in the cylinder head shrouds and locate the end of the first plug. Only then should you fit whatever combination of extension, universal joint and ratchet you find easiest and start undoing it. If it seems tight, make sure the socket is properly seated before applying too much pressure.
How to check spark plugs: Step 3 of 8
Once out, have a good look at the plug. What does it look like? Brown, grey, sooty, oily? Ideally, the end of the plug wants to be light brown or grey in colour with an even pattern and no broken or molten bits. This one isn’t perfect, but it’s pretty good, suggesting the engine’s running alright and things are okay in this cylinder.
How to check spark plugs: Step 4 of 8
If it’s sooty but dry, like this one, your engine is running rich, meaning that it’s not burning all the fuel that’s going through it effectively. In which case, refer to our guide on how to tune your carburettor to sort this out.
How to check spark plugs: Step 5 of 8
If the plug is wet and oily your engine is in trouble and most likely has worn piston rings and/or valve guides, which indicates a rebuild is on the cards.
How to check spark plugs: Step 6 of 8
If the insulator in the centre is broken, damaged or has major carbon desposits on it like this then your timing is out and needs rectifying immediately. Refer our guide on engine timing in order to fix this. If the insulator is white and flaky then your engine is running lean, so check the timing and then adjust your carburettor as well.
How to check spark plugs: Step 7 of 8(Strictly Aircooled San Diego)
Whatever your plugs look like, they’ll benefit greatly from a vigorous clean up with a brass brush. You should have one of these in your on the road tool kit as cleaning the plugs will often help start a reluctant engine. Remember though, if they’re in a bad way, it’s only indicative of another problem, so you should try and solve that problem.
How to check spark plugs: Step 8 of 8
If you think they’re good to go again, check the gap with a feeler gauge. For most Beetles it should be 0.024in (0.6mm) though it’s worth checking a manual specific to your car as some, 1500s for example, specify a slightly wider 0.028 (0.7mm) gap. The gap is correct when the feeler slides in with minimal resistance. If you’re replacing the plugs, don’t just assume they’ll be right out of the box. Check each one and set it first. You can close up the gap with a light tap on the floor and open it by prising it carefully with a screwdriver.
Remember to visit: Strictly Aircooled Volkswagen Bugs mechanic shop in San Diego.