Spark Plugs are found on petrol engines and glow plugs are fitted to diesel engines. Both are key parts in ensureing that your Ford Transit starts and runs. The spark plugs produce a spark to ignite the fuel. If the spark plugs are old they won't be efficient and in some cases won't work at all. Spark plugs should be changed periodically in line with the manufactures guidelies. The glow plugs on a diesel engine are a heating aid in the starting of the engine. When one stops working it will take longer to start the engine. Glow plugs should also be replaced perioically. orangespanner have spark plugs and glow plugs in stock from some of the Worlds best brands.