depending on where this thing broke off (I think the threaded portion came out, leaving just the long shaft on the GP?) would depend on whether the threads would need to be reconstructed.

I'm going to venture a guess and say that a leakdown would be pointless...if there is coolant pouring out, the jacket has been breached. At this point the injector needs to be pulled, and a scope inserted through both the GP and injector holes to asses the damage. From there, post up the pictures and we will all mull over the best solution.

If the only alternative to an unconventional repair like I detailed is replacing the head, which the OP says is not an option, then we should get creative! If nothing else, we will gain knowledge of what the options for repairs are as this type of thing becomes more common as more GPs fail in the 606 engine out there.

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Garage:

2017 Chevy Colorado Diesel (nanny state emissions)
2005 Volvo S40 T5 AWD, 77k
1987 Mercedes-Benz 300D turbodiesel, 4 sp auto, 156k - 28.7 mpg
1996 Tracker 4x4, 2 door, 16v, 3 sp auto. 113k - 28.6 mpg

WARNING: this post may contain dangerous free thinking.