Not to bring doom and gloom but some here have said they think there is a correlation between the Trap Ox being left on and head failure due to excessive heat. If you can check the upper radiator hose for hardness and the burp tank for any traces of oil that will be helpful. A compression check will also tell you if the head gasket is leaking. Did you check the casting number on the head? Is it a #14? If it is there is a chance that it could crack or already have a small crack that will only get bigger, eventually causing head failure.

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Doug

1987 300TD x 3
2005 E320CDI