Used to warm indirect injected engines for cold starting and on very modern direct injected engines to give a quieter running during the warm up period.

There are three main types of plugs in marine use:

LOW VOLTAGE TYPES - These are used in older (say pre 1970 designs). Each heating element is a small loop of wire at the end of the threaded portion and as the plugs operate on far less than 12volts they are connected in series, both between themselves and a resistor and fuse.

These plugs should only be tested by use of an ohmmeter or continuity tester - otherwise they are likely to melt. In extreme circumstances if you wipe a dirty hand around the element and then quickly connect it to 12volts you will find you can test them if you disconnect the plug as soon as it starts to smoke - if you are not quick enough the plug will burn out.

If one of the plugs has failed they all refuse to work, so poor cold starting should always initiate a test of the heater plugs.

 

12 VOLT TYPES - Each plug is rated at 12 volts and they are connected in parallel. That is the wire is at 12 volts and the electricity returns via the engine block (so much for fully insulated circuits).

Each plug can be tested at 12 volts and the failure of one plug allows the others to continue working. If one does fail it is may be detected by the engine suddenly taking to only cold starting on 3 cylinders instead of 4 etc.

The extended "noses" on the plugs tend to become stuck in the head over time, so they snap off in the head when they are removed.

To counter this remove the plugs, clean, & refit every year or two.

If one does snap off it can be drilled out with a HAND drill.

 

MODERN "TIMED" SYSTEMS

The heater plugs on modern (especialy automotive derived) systems are likley to use a complicated relay/control system which keeps the heaters on, at reduced power, for a while after the engine starts. This helps prevent diesel knock and polution which is worst when the engine is cold. A typical circuit is shown below.

 

Even more modern systems use simple computers or transistors to control the heater operation.