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Old 11-09-2008, 09:16   #1
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Glow Plug Issues

My boat has been going through glow plugs. Yesterday, while changing them again, I discovered that there is 1 millivolt present, even when the engine is running and the glow plugs should be off. Is this small amount of voltage enough, over time to "burn out the plugs"? Is this a problem with the glow plug relay needing replacement?
I have had 2 different mechanics trouble shoot the glow plug issue, with no success in finding the cause of the problem. Maybe they missed the small amount of voltage still present when there should be none? By the way this is now the 5th set of glow plugs and the engine has just over 700 hours on it.
Thanks for your help,


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Old 11-09-2008, 11:15   #2
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Aloha Tom,
It might help to know what engine you have and what brand of glow plugs. I know my old Mercedes has two different options and it depends on which way they are wired as to which set of glow plugs you are supposed to use. Also, any missfiring of the cylinders will cause glow plug burnout and pitting of the head.
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Old 11-09-2008, 12:42   #3
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1 mV will not light one up. Be sure the glow plugs are not on continiously when the engine is running. My current set are about 2 thousand hours old on my Mitsubishi.
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Old 11-09-2008, 13:22   #4
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A glow plug is a high current/voltage user. mV's won't be of any concern.
Two area's to check. Firstly that you are not over timing the plug. Depending on your engine, you may have a plug timer, or you may manually heat with the key turning the opposite direction for instance. If the time the plug is on longer than needed, they will not last. The other issue can be injector issues. If the injector tip is dirty, the spray pattern can be directed to an area of the cylinder it is not meant to be. This can rsult in hotspots and often those hotspots can be around the glow plug resulting in them burning out.

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Old 11-09-2008, 13:25   #5
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If the glow plugs stay lit up (“on”), for too long, or worse yet, “on” with the engine running, they can overheat & burn out. The preheat time interval is temperature dependant, often determined by a temperature sensor.

Less common, leaking or miss-timed (too much advance) injectors can also burn out glow-plugs.

Otherwise, a glow-plug should last a couple thousand hours or even until an engine overhaul (10,000 Hrs?).

As you suspect, the pre-heat relay is the most common component failure, and yours should be checked. Yours may intermittantly stick "closed" (if stuck "open", you'd be complaining of hard starting).

Notwithstanding, I suspect that the 1mv you found is a metering artefact, rather than an actual applied voltage. As Will (or Muffin) indicated, 1mV won’t appreciably heat a 1 Ohm (approx.) glow-plug.

A 1 Ohm 13 Volt pre-heater will draw about 13 amps (each). Depending upon whether your engine uses “hot” or “Cold” glow-plugs (per JohnL’s Mercedes). An operating heater should draw between 10 & 20 Amps at 13 Volts.

See also Wheels comments, posted whilst I wrote.
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Old 11-09-2008, 18:43   #6
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What is the original complaint that lead you to believe the glow plugs were bad? do you have pics of the bad glow plugs? are the tips burned off?
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Old 11-09-2008, 19:06   #7
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Answers to Questions

First off, thanks for your replies. It is appreciated. The engine is a Volvo MD2030, 2004 model (3 cylinder, 29 HP). The first condition that alerted me to the problem was that the engine was difficult (read that to mean would not) start. I had a Volvo certifed mechanic check it out and all 3 plugs were "burnt out". They look in good condition, on the exterior, but when checked on a multi-meter, they read 1950 ohms (on the 2000 ohm setting) instead of zero resistance like the new ones.

When contacted, Volvo said it must be user error. The owner's manual states that the plugs should be activated (using a toggle switch) for 10 seconds prior to cranking. I have never exceeded this time. I count it off like a basketball referee. By the way, I have shortened this time down to 5 to 6 seconds after the first glow plug change. The plugs are also activated when the engine is cranked. When the plugs are working correctly, the engine starts within 5 seconds of cranking (or less). I had not heard (before) that injector issues could be the cause of this. The engine has run great (up to today but that is another story) after starting. As I stated earlier, the engine has about 750 hours on it, and I have been diligent in my maintenance (oil and filter changes, fuel filter changes, valve adjustment, impeller and coolant changes, etc.). I hope that gives you guys the additional info needed, and again, thanks for your help, I really do appreciate it alot.

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Old 11-09-2008, 19:15   #8
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probably an electrical problem if the old and new GP's looked the same physically. Wire them to their own key on switched circuit. They draw 30+ amps so gauge your wire accordingly.

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