Back To Basics: Spark plugs fitment
Rob Hawkins explains how to renew spark plugs, and outlines some of the typical problems that can arise.
Renewing spark plugs was once part of the annual routine servicing of a vehicle, but their lifespans have extended by so much that many are now only replaced every 100,000 miles. Modern iridium spark plugs, for instance, are longer lasting and perform better, with more efficient cooling capabilities. Consequently, few vehicle owners have the opportunity to blow the dust off their spark plug spanner or socket, or indeed find their feeler gauges. Even an annual removal of the spark plugs, cleaning with a soft brush and checking the electrode gap isn’t included in most service schedules.
Spark plug removal in many cases isn’t as straightforward as it used to be. Lift the bonnet of any vehicle manufactured in the 1980s (or before) and you’ll find the HT leads and spark plugs almost staring at you in the face. Modern engines have covers to hide them away, and V6/V8 engines have the spark plugs concealed even more. So, the following pages provide some general advice for accessing such spark plugs, but also outline the problems that can arise, such as HT leads that break off. And there are a number of issues concerning the removal of spark plugs and fitting new ones. We’ve had our fair share of trouble, including a damaged thread inside an aluminium cylinderhead, seized plugs, and being unable to fit spark plugs deep down inside a cylinderhead.
Plug colours
The end of a used spark plug can tell a lot about how an engine is running. A used spark plug should be a dry brown/grey colour, helping to indicate the fuelling is correct and is being burnt efficiently. If the end of the spark plug is black and sootylooking, this suggests there’s too much fuel or it’s not being burnt off effectively. It could be caused by a dirty air filter, short running of the engine, low-speed driving or an over-rich fuel mixture.
If the spark plug is wet, the engine may have been flooded with fuel and not fired-up. Clean and dry the spark plug before refitting it. Black, oily deposits on the end of the spark plug often suggest oil has been leaking onto it. This could be caused by worn valve guides or piston rings. Blisters or white deposits indicate the spark plug has been running too hot. This can be caused by the engine overheating, a lean fuel mixture, incorrect ignition timing, a loose spark plug – or a wrong one being fitted.