I think you're right in your assesment that its probably coming from the turbo related circuit. The '85 models had a different turbo set up compared to the earlier turbodiesels, so I can't offer any help on the subject. Could it be the EGR circuit? It operates at half throttle, so that might be another thing to check. Good luck.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold