7-10 sec. is perfectly normal. The newer plugs heat up much faster. I still have the old slow glow plugs in my volvo and they take about 15-20 seconds to reach full temp. Just depends on the type of plug and application.

Also, not to discount WHunters warning but on a cold day I have found a good quick way to nail down a bad glow plug is to feel the hex portion of the body after a couple glow cycles. A bad plug will be stone cold and the others will be warm to the touch. This is in the head of course where there is enough heat transfer to prevent the plug from heating up enough to burn you. Testing out of the head in the open air is another story.

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1982 240D auto
1978 300CD auto
1985 300D auto
1983 300TD auto
1984 Porsche 944 5 speed
1973 Opel GT 1900 4 speed
1967 Chevy C30 350 Sm420
1973 Mustang Grande Convertible 302 C4
1981 VW Pickup 1.6D Turbo 5 speed
1983 Rabbit 1.6D Factory Turbo 5 speed
Plus parts vehicles...