Please start by replacing the thermostat! Diesels DON'T generate much heat when not loaded and in below zero (F) temperaturers, blocking off part of the radiator can be desirable, but in ordinary driving the engine is designed to come up to the 80-90 degree (C) operating point in about 3-4 minutes of driving. If it doesn't, the most common problem is a thermostat that's stuck open.

Most people think of the thermostat as a device involved in cooling the engine, but it's PRIMARY function is to PREVENT cooling until the engine reaches the lower limit of proper operating temperature range.

If the temperature runs low, engine wear increases and combustion efficiency declines. I've seen fuel economy drop 20% in a Mercedes diesel that's running 15-20 degrees (C) too cold.

Marshall