911,
I think when you have a carbon build up & the motor is up to temp. Any of that carbon that starts to glow or burn as it will on a good hard run, will weaken the surface of the GP that it is near & so lead to premature GP failure.
The increased high current time you mention increases the internal temp of the GP and can cause them to burst open & so cause a stuck GP.

Actually, the cycle ends when you release the key from the start position.

You need to glow longer if you know that :D

Sixto
87 300D

hogging???
 

"Current hogging", "hogging caused by a cascade " in a glow plug ??? Never heard of it before. Care to explain?

They are all used Bosch glow plugs. Mechanically they look fine, no stripped threads, no tip erosions etc.

How would a bad injector spray pattern casue early glow plug failure?

4 out of 5 glow plugs were difficult to remove! They didn't want to come out even totall unthreaded due to carbon holding it back. What cause carbon to form in the prechamber? It is a dead-headed cavity with no combustion flow through it. You'd think the red hot glow plug would burn whatever carbon gets in there and keep the area clear. Is it due to poor injector spray pattern? injection timing off? Anyway I had to pry the glow plugs with long nose pliers as I turn the glow plug with a gear wrench to get them out. I happen to have a glow plug reamer I made (7 mm dril bit + old glow plug) and it came to good use. It took a long time to ream out all 5 holes as I had to apply forward pressure with one hand while I turn the reamer with a gear wrench with other hand in an awkward position. Wheel bearing grease on the reamer helped it stick to the carbon.

I'll be checking out the glow plug controller next to see if it is misbehaving.

On my 84 the Glow Plugs get Voltage while the Engine is Cranking.

On the Termial Block infront of the Battery in the most forward part of the Terminal Block is a one White Wire (that comes up from the Starter Solenoid) and a Purple Wire (that goes to the Glow Plug Relay) are screwed together on the Terminal Block.

When you Crank the current comes up from the Starter Solenoid by way of the white Wire and into the Purple Wire and on to the Glow Plug Relay.

I am guessing it is done that way so that if a person is lazy to use the Glow Plug Light and just jumps in the Car and starts it if he/she cranks long enough the Voltage to the Glow Plugs will also heat them and aid in starting. The Glow Plugs working during cranking must be way better than just cranking alone during starting.

I suspect that carbon build up is also related to how the car is used. Our diesels when ever used always get a run for at least 15 minutes at 60mph. We never see carbon build up. If you are mainly using yours for around town trips mainly stuck in traffic, st a couple of miles do to the shop& back, expect problems with things claging up.

With the doors and windows closed you will still hear it faintly from inside the cabin, if you listen hard enough.

That said, I wouldn't put used glow plugs in my car.

Why not? It's not a critical mechanical part and there is little risk. The worst that can happen is they burn out. I have been using used glow plugs for 20 years and run them till they burn up and put another used one in. It's rare that I bought 5 new glow plugs a few weeks ago (as an experiment to see how long new ones will last). If they burn up in less than a year I am going back to used plugs.

Edit: Forgot to mention when I pulled the old glow plugs, I checked each one again with an ohm meter and this time only 2 plugs are bad. I am certain I did not make a mistake the first time when I found 3 were open. I think it was just a case of an old intermittent glow plug.