[camaraderie]

You may have a loss of compression causing it not to come up to full temp.
Cool temps can be the equivilent of idling the diesel for extended periods which can be damaging as follows:

This is because the engine is not allowed to reach operating temperature, where harmful combustion by-products are eliminated. This acidic by-product accumulates in the oil, where it chemically attacks the engine bearings.

Here's what happens when a diesel engine is idled:

1. Timing gear backlash [gear rattle] is maximized, leading to increased wear
2. The cylinders get glazed, leading to low compression
3. The engine is run at below operating temp, leading to carbon accumulation in the combustion chamber, injector tips, piston rings, piston crown, valves heads, seats, and stems, exhaust manifold, turbocharger, etc. Carbon on these parts will lead to accelerated wear, and possible piston ring sticking and low engine compression.
4. Also, the injected fuel gets past the piston rings, because the engine is not at the correct temperature, and dilutes the lube oil. This compromised oil is sent to ALL the engine bearings, where it increases bearing wear.
   

There's more, but isn't this enough to convince one that engine idling should be avoided?

Yanmar [for one] says not to idle their engines for more than 5 minutes. Bottom line on engine idling - it's great for the engine repair business.

If you've checked out the obvious issues...get it to a mechanic before you really end up with a big bill.