Glow plugs

[Des_Beechey]

I am having trouble with glow plugs. My boat has a Nissan ED33 diesel engine - 2 litre, 100 hp - and the glow plugs only last a few months.
This engine has a 24V system, which is supplied by two 12V batteries in series. Power to the glow plugs is from the batteries via a relay controlled by a simple switch. So the glow plugs get whatever two 12V batteries produce - between 24 and 26V I suppose, depending on battery charge.
The plugs I have been using are marked Bosch 250 0250202 111 23V. Does this mean they are intended for 23V supply?
My questions:
1. Are glow plugs very sensitive to over voltage? Will 26V kill these plugs quickly?
2. If so, how do I reduce the voltage supply to 23V?
3. There is a notice beside the glow plug switch left by the previous owner that says "use glow plugs 5 seconds only!". I thought that glow plugs were usually on for 10-15 seconds. What gives here?
4. Where can I contact Bosch to ask them?


Des Beechey

 

[oldharry]

If the plugs were designed to run on a 24v system then slight overvoltage should not harm them. I am not familiar with Bosch codings, but there should be an agent in your Yellow pages under Diesel engineers or whatever, and yes they should last a great deal longer.

The length of time required to bring them up to temperature varies with different engines: I have a Peugeot diesel which needs at least 7 seconds from cold, whereas the old Perkins Prima in my van never needs more than 2 or 3 seconds. 15 seconds does seem a bit over the top.

Glow plugs take a lot of power, so it is not advisable to run them for longer than is necessary before starting, for risk of reducing battery voltage - 12v plugs in a 4 cylinder engine will take a hefty 40amps - killing the battery for the start if its at all 'iffy'!

Frequent failure may be due to an engine or injector fault creating 'hot spots' while running, and an engineer will be able to tell by looking at the plug tips whether there is an external problem.

 

[J_Priedkalns]

There are two types of glow plugs available; the "standard" types take about 10-15 seconds and the fast type about 3-5 seconds and are used in automotive applications. If you have a manual switch glow plug system, you may be preheating when it is unnecessary especially if the engine is still warm and thus shortening the life of the glow plugs. I installed the automatic circuit used in VW automotive diesel engines for preheat starting and have found that the circuit only switches on when doing a true cold start and the plugs have lasted many years.