Too much Timing = broken GPs

[Chevyboy_0]

I have been reading on a few posts about this and im just curious as to why this happens? what is it about to much timing that causes the GPs to fail?

 

[argve]

puts undue stress on the cylinder.

 

[FordGuy100]

It may put more stress on the cylinder, but I also want to know why the glow plugs get pitted and eventually break off. Wonder why it eats em away.

 

[THECACKLER]

I don't know for sure, but I bet we'll find out right after I post... but my assumption would be that advancing the injection would start the ignition event while the cylinder is still on it's compression stroke and would let the burn propagate out of the pre-cup into a more oxygen rich atmosphere and then be forced back into the confines of the pre-cup as the piston reached TDC. Creating a flame and then compressing it would create a super heated mixture that would likely have the thermal effects witnessed by fried glow plugs.

 

[Agnem]

I like that theory! I too am mystified by what causes it, but cause it it does! Generally the tips don't break off, they just errode away.

 

[Chevyboy_0]

Well I know on a gasser that if you put the timing to far advanced then your peak cylinder pressure and your peak combustion pressure are to close together, and by doing this you have your combustion fighting compression. I have a feeling that this is the same concept but I'm still curious as to why the GPs become pitted etc?

 

[THECACKLER]

When I worked in a Heat Treat shop we had High Nickle Stainless Steel carriages that were the skid or base that we racked the parts on to run in and out of the furnaces. These furnaces ran at temps up to 1800*F all day and night and repeatedly cooled by quenching. Over time the carriages would fatigue from the heating/cooling cycles and become brittle and eventually break as if they were made of sandstone. The high strength material would break down along the grain boundaries and look similar to the way the fried glow plugs look. Not burned off like a cutting torch would do but crystallized and subsequently fractured along the grain boundaries.
If you picture the tip of the glow plug setting out in the middle of a superheated chamber with the cyclical shock of the relatively cool fuel spraying on it 15 times per second it would seem to be much the same conditions.

 

[sle2115]

I think Thecackler hit the nail on the head, the heat is contained in the pre-cup longer than it was designed to be, heating the tip of the glow plug so much that when fuel is sprayed, it cools and pits the glow plug.

 

[cetanefreek]

I like the cackler's first theory about the flame propagating back into the pre-cup, just because I don't recall having any glow plug problems from high timing in powerstrokes or duramaxes.