Burning out glow plugs

[smokin150]

Got an odd one here... Two weeks ago I replaced the glow plugs on the four corners of the engine they are now burnt out again but just on the four corners. What's going on here? is there a way to see why the corners are burning out before the center ones do they were replaced with the original Motorcraft ZD9 plugs it just doesn't make sense any help would be appreciated

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[ToughOldFord]

Don't know why it would specifically hit those four corners, but if your controller is working correctly and not running the GPs too long have you checked your timing? Timing being off is known to burn the ends off the GPs.

 

[smokin150]

Haven't checked timing, but the plugs are still in one piece its not like the ends are being burned off they're just burning out

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[Shadetreemechanic]

Glow plug resistance changes over time with use. Glow plug controller senses resistance across the glowplug circuit to know when plugs are hot and turn them off. By replacing just some of the glowplugs, you have the potential to have plugs in the circuit heating at different rates and since the controller cant tell individually what is happening it cooks some while others remain too cool.
You need to replace all the glowplugs as a set. I have rarely been able to get away with just replacing a few because of above.

 

[smokin150]

Shadetree.... I replace all 8 six months ago, then one morning the wts light came on then went right off... Figured I'd do a resistance test, found the four corners burnt out (infinate resistance) the others ohm out just fine... As I'm laid off money's kinda tight so I replaced the 4 burnt ones. The system works well now but this is the 3rd time that the corners have fouled. I'm with wa at replacing as a set of 8... But I'm really looking for input as to what if anything could cause the above issue

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[Shadetreemechanic]

Check the harness. Look for corrosion on any of the lines headed to those plugs that are not burning out either at the connector to the glowplug or somewhere else in the harness that might change resistance values. My guess is there is some reason more juice is getting to those 4 plugs.
I can't think of any engine related reason that would happen.

 

[smokin150]

Soon as the weather warms ill check the harness

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