3 dead glow plugs in 1 day???
 

Car started fine yesterday. All glow plugs were good a month ago (replaced one and checked the others). Granted all the glow plugs I've put in are used ones with unknown age but what's the chance of 3 burning out in 24 hours? Which leads to the question what are causes of premature glow plug failures?

1. Packed carbon in the glow plug hole?

2. Bad glow plug relay keeping plugs on longer than needed?

Anything else you can think of?

Good news is that I bought a brand new set of Bosch Duraterm chromiums and will install them today and be ready for winter coming. May as well check/adjust the valves while the injector hardlines are off.

I am also thinking of bypassing the automatic glow plug relay control and splice in a push button so it's totally manual so the glow plugs are not on longer than neccessary to prolong their life.

all of the above.
it's a resistive heating element. who knows? if you do have carbon in the chamber, now is a good time to get a reamer and remove it all. you don't wanna overheat the new plugs, and you want ALL the heat from the new plugs to heat the chamber.

When do you think the relay opens in the current configuration? Unless you have an aftermarket "afterglow" timer, an manual switch isn't going to make a material difference in the glow plug cycle duration.

unless his relay is broken in some way that is...

I wouldn't question the system until I'd used new glow plugs and saw the same issue.

I'm not condemning used parts, goodness knows I use enough of them myself and couldn't get by if I didn't, but GP's are one of those things that can fail pretty easy and unless you're sure that all your GP's are identical you could be introducing some unknowns into the system.

I would want to see it misbehave on new GP's before I rule them out. I would however time the GP's first to see exactly how long they're running, and that doesn't mean trusting the dash light, I'd test it at the GP itself with a test light.

steve a

Hogging
 

Another interesting glow plug story, like mine where I had a hard to start issue and all glow plugs checked fine and then the next day 2 checked bad.
I think this may be an issue of current hogging... .
I can't explain why 2 of my glow plugs failed at the same time actually, but since it is a parallel circuit, I vote for hogging caused by a cascade .
I just replaced them all and hope they last a few years.
The hogging is common in semiconductors with a low dynamic resistance and can cause this sort of thing to occur without proper safeguards.

I agree with vstech.

The manual also has that a bad Injector Spray pattern can cause early Glow Plug Failure.

What brand were the Glow Plugs that Failed.

You can make your own Reamer from one of the no good Glow Plugs. In the DIY section somewhere.

What does the dash light indicate with regard to glow cycle duration?

The relay is quite audible when it opens/closes. No light required.

I believe that Carbon buildup causes the heat to be conducted into the Cylinder Head instead of the Prechamber Air. This keeps the Glow Plug cooler and when they are cooler they draw more amps for a longer amount of time than the would if they heated up normal.

I tested 1 good Glow Plug across the Battery Terminals with and Automotive Dash type Ammeter. It pegged out the 60amp Gauge for about 1.5-2 seconds and as it got hot the needle dropped to 16 amps and held there.
So if the Glow Plug cannot heat up like it is supposed to due to heat conduction through the Carbon it will continue to pull the higher amperage.

I can see that where carbon (if the junk is mostly carbon) has a fairly high thermal conductivity and might allow the heat from the glow plug to transfer to the pre-chamber wall and into the cylinder head more easily than if the pre-chamber is clean. Air is a very poor conductor of heat compared to carbons.
If the glow plug is giving up its heat more quickly (compared to air) to the surrounding metal through carbon then it would tend to operate at a lower temp. and operate a higher currents - but how much and how long is the thing. If the carbon gets the majority of the heat from the glow plug, how long before the conductivity gain is overcome and the glow plug simply returns to the same temp. as it would operate at in air?
I think it is a matter of different evils operating without a safeguard that will allow excessive current in a single or multiple gp's.

the dash light just signals that they've heated long enough to start the car, not that the full heat cycle has ended.


Perhaps when you test and don't start the car, but I would still put a volt meter or a light on one of the GP's to verify current and amount.

I don't disagree with you on how audible the controller is, but I would put a volt meter on the GP anyway.

steve a

yeah, the relay is loud clicking on and off, but once you've started the car I challenge you to hear the click

second hand GP's could have a very checkered past, knocking around in the bottom of your tool box or on the floor. I have used a couple in a situation to get a motor to start but they are only a temporary fix. There is an increased risk of failure. There is plenty on here about stuck GP's , best not increase the risk. If you need to have spares, get a new set & keep them some place safe, not in your tool box or trunk, maybe in the glove box in their original box.
When you replace them, toss the old ones out. You dont keep your old oil in case you want to re use it !!

It would not only be trying to heat the part of the Cylinder head it would also be trying to heat up the Coolant.
Under the carboned up conditions I do not know how long it would take for the Glow Plug to Heat up to the normal temp.


See the first pic in post #52
http://www.peachparts.com/shopforum/showthread.php?t=262543&page=4
You see in the pic that only about 1/4 of the Glow Plug sticks into the Prechamber. You can also see where the Carbon can fill the space between the Element and the Cylinder Head.
While my comment on the conduction and heat transfering into the Cylinder Head and causing the Glow Plugs to continue to pull high amperage for a longer period of time is speculation; it seems to be pretty well know that Carbon build up shortens the life of the Glow Plugs.

Yup. The that vintage glow relay cuts when you hit the starter. Why would you want to stop the glow cycle before hitting the starter?

Sixto
87 300D