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Old 08-27-2017, 11:47 AM   #1
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Cummins 4BT 3.9M - good engine?

I know that Cummins generally has a pretty good reputation, but I know nothing about this particular engine. I would appreciate any comments/info on this engine from those who have direct experience with one. Things to watch out for, maintenance issues, you know, the good, the bad, and the ugly!
The one I am looking at is a 1989 with a little over 2,000 hours on it.
Thanks for any input.
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Old 08-27-2017, 02:08 PM   #2
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Rock solid engine. Very reliable. Downside is it tends to shake (itself and the boat) at low idle. Well designed mounts can isolate those vibes somewhat, but still shaky. 900rpm up, reasonably smooth. Hard to kill, parts available anywhere.
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Old 08-27-2017, 02:33 PM   #3
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Quote:
Originally Posted by Ski in NC View Post
Rock solid engine. Very reliable. Downside is it tends to shake (itself and the boat) at low idle. Well designed mounts can isolate those vibes somewhat, but still shaky. 900rpm up, reasonably smooth. Hard to kill, parts available anywhere.
+1 to what he said

Also depends a little on the HP rating. The high limit is 250 HP but those tended to have some problems, at lower HP's very good.

Nicknamed the "Bread truck engine" because they've been installed in countless delivery vans.
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Old 08-27-2017, 06:48 PM   #4
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Is there a balance shaft version? Or are they all the same?
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Old 08-27-2017, 09:11 PM   #5
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No balance shaft version. It's luck of the draw how bad they vibrate at low RPM. Considered one when doing my refit. Glad I went with the Deere.

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Old 08-28-2017, 01:36 AM   #6
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You have one of mid production models(came out in 1983)Commonly known by enthusiasts simply as the 4BT, this family of engines was built to offer long-lasting and trouble-free diesel power, and they still deliver.

The 4BT marked an important milestone for Cummins , expanding Cummins power well beyond the heavy-duty engine market. The engine gained such a reputation for reliability that it retains a loyal following well after production ended.

The Cummins 4bt marine engines are usually 150 and 250 hp but are not designed to operate at those power levels but a few minutes per hour. The 150 hp may only be operating at 100 hp most of the time. Once the load is moving you throttle back and just cruise.

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Old 08-28-2017, 05:52 AM   #7
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The 250hp version is aftercooled, called a 4BTA. Completely different beast than the 4BT which most were rated at 150hp. The 250 was not in production in the 80's.

The 4BT at 150 is basically the same as the 6BT 210 missing two holes.

I don't think any 4BT's came with balance shafts. Note that the balance shafts don't do anything much for the low rpm shake, but are for the second order vibe inherent in 4cyl's at high revs. Below about 2000rpm the second order vibe is not likely a problem.

The low rpm shake is just widely spaced compression/firing pulses which want to make the motor rock side to side. With well designed mounts it can be made tolerable in worst case, and downright smooth if really well done. Put stiff mounts on one and you are going to rattle the boat.

The popularity of the 6BT is in part due to the inherent smoothness of a six at both high and low rpm. Many builders would rather be a little overpowered than deal with a shaky four.

But with a little work, the four can be made very social. And it is a rock solid engine just like the 6BT.
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Old 08-28-2017, 06:59 AM   #8
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Ski nailed it. The only thing I can add is that 6BT was used in Dodge 3/4 ton P/U trucks.
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Old 08-28-2017, 07:35 AM   #9
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Unless space is a real restriction I would still want a low hp 6B model instead of the 4B. Dock neighbor had a pair of them and good number of brackets fatigued off, then again I think he would leave it at idle too long as well.
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Old 08-28-2017, 07:59 AM   #10
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Thank you, everyone!
That is exactly the kind of information I was looking for. Parts should be readily available, and not ridiculously priced. Apparently the boat reaches hull speed at about 1900 rpm, and 150 hp is way overkill anyway, so it should be easy to cruise at the sweet spot. Interesting that a number of owners have had problems with engine mounts! Now I think I see why. Since I never low idle a diesel anyway, I should be able to live with this engine.
I do like the reputation for rock solid reliability!

This thread is another example of the TF community coming through!
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