MTU 12v396: smokes when cold

They all do.
If twin installation, any meaningful difference between port and stbd?
You might consider a block heater, anyway.

What color is the smoke?

Un-burned fuel.
Low compression, cold block, leaking injectors.

In south FL, block heaters usually is not a norm. If you have them, turn them on several hours before starting. See what happens.

I'm leaning towards leaking injectors.
When was the last injector service? Not replacement but in the shop, tested and popped.

Nozzle, Injector ;
If they leak, it's not rite.
Quickly found a corny kid making a video with a crack pump.
Not to technical but gives you a good idea.

At 6:00 gives you one example of leaking.


Another issue is just leaking when not running. A few extra drops in the lung and exhaust manifold takes a while to clear out.

Before our 12V71 rebuild, our smoke could block the sun, worst than old misquote control trucks.

Sounds like a Smoking Deal already.
Walk on.
When I was shopping for a whole 396 engine for a Hatteras project boat a few months ago (blown, not rebuildable), GoldCoast Power found a set out of country and one on the left coast. Running take outs. Near cheaper for the ones over seas, freight and a spare.
Shame, they gave that fancy Hatt 60 away to get rid of her.

Yep, sold way cheaper than that.
I did get a bid in, was going to scrap her.
More info in the Hatt rag.

$20k
Boat & Breakfeast.