Question about Cummins 4BT-3.9?

I am interested in a 30 foot trawler style boat that is otherwise in good condition and style with what I am looking for. It is a 1990 version and has an original 4BT-3.9 Cummins engine, with 4131 hours on it. By the looks of the overall good condition of the boat, it probably has been well maintained. Although I am familiar with the popular QSB-5.9 engine and its dependability, etc., I do wonder about this particular engine. I assume it is noisier but also wonder if I should steer away from it for any other reasons. Anyone have any experience with this engine and/or care to comment?

I don’t believe it has a balancer in the engine so at idle it runs rougher. But above idle it is fine. This is per Tony Athens.

I only know what I have read on boatdiesel.

First it is a mechanically injected engine unlike the QSB you mention.. Nothing wrong with that though.

The 4BT is rougher at low rpm’s will be noisier than the QSB.

But all in all a good, strong engine.

David

There are a LOT of those engines in delivery trucks. They have a good reputation for longevity. Plus they are mechanical meaning not electronically controlled.

Thank you for your feedback. It gives me more information to think about. I will also check in at Boat Diesel.

I used to patrol on an ex Navy tender with a 4BT in it. It was a bit rough at idle but thinking back about it, it wasn’t terrible. And it did smooth out above idle.

Seems to be a common theme- rough and a bit loud at idle, fine above idle. It is just that it is hard to give up my Perkins Sabre 135 in my current boat for a louder, 4 cylinder and older unknown-to-me engine. Thanks again to all.