Just had the t-stat replaced in my E420 (simliar warm-running characteristics as your modified M119) which fixed the same issue - rarely getting up to 80C even after extended idling. Get it looked at immediately as an engine running too-cool can cause severe damage quickly.

See my recent post "M119 Running Too Cool?" For other repsonses on the same subject.

Regards,
- Ryan

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'95 E420 - 'Shadowfax' 138kmi.
'92 Volvo 740Turbo Bard 193kmi
'74 240D - 'Ol' Green' 4spd Manual 104kmi. (sold )
'77 300D - 'Red' 223kmi (sold)
'75 240D - 'Bianca the Blue Bomber' (sold)