Question about Cummins 4BT-3.9?

whistlerxc · Jul 18, 2024

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?

Comodave · Jul 18, 2024

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.

DavidM · Jul 18, 2024

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

jleonard · Jul 18, 2024

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.

whistlerxc · Jul 18, 2024

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

Comodave · Jul 18, 2024

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.

whistlerxc · Jul 18, 2024

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.

38ark · Sep 17, 2024

whistlerxc said:
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.

I have a 38’ diesel Duck with a 4b3.9. No turbo. 75 hp. It’s been a great motor. It shakes only at lowest idle. Noise hasn’t been an issue. Moves my steel trawler right along.

Question about Cummins 4BT-3.9?

whistlerxc

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