GLOW PLUGS

IIf slow cranking isn’t the problem, perhaps there’s something wrong with the glow plug system. Most passenger car and light truck diesels have glow plugs to assist cold starts. The glow plugs are powered by a relay and timer that routes voltage to the plugs for the prescribed number of seconds. When the timer runs out, the relay is supposed to turn off the voltage. But relays sometime stick and continue to feed voltage to the glow plugs causing them to burn out. One or two bad glow plugs on a V8 engine might not cause a noticeable starting problem during warm weather, but it can when temperatures drop.

Glow plugs can be checked by measuring their resistance or continuity. Excessive resistance or a lack of continuity would tell you the plug is bad.

If one or more glow plugs have burned out, are heavily coated with carbon or are not receiving their usual dose of start-up voltage, the engine will become progressively harder to start as temperatures drop, and will idle roughly and produce white smoke in the exhaust for several minutes once it finally starts. If all the glow plugs are burned on the end, you’d better check the injection timing because it is probably overadvanced.

To see if the glow plug module is providing power to the glow plugs, use a voltmeter to check each plug for the specified voltage when the ignition key is turned on. No voltage? Check the glow plug control module connections, ground and wiring harness. The glow plugs themselves can be checked by measuring their resistance. Replace any plugs that read out of specifications.

Hard starting can sometimes be caused by a glow plug module that fails to turn the glow plugs on or doesn’t keep the plugs on long enough when the weather is cold. On GM 6.2/6.5L diesels, there have been reports of heat from a still-warm engine causing the 125-degree inhibit switch inside the controller to shut off making the engine hard to restart. The cure here is to relocate the control module away from the engine. On Ford 7.3L diesels, the control module can cut off early if there are two or more bad glow plugs. We have also heard of control modules that do not keep the glow plugs on long enough for easy cold weather starting. The on-time is sufficient for warm weather, but not cold weather.

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