Quick impression about used diesel engines

[George Samaha]

Hello 🙂

I am new on this forum and new to diesel engines except a little experience with a small diesel generator. My previous boat had a gas engine. Now looking to purchase a 1998 Maxum 4600 SCB with 2x about 1600hours Cummins 6CTA8.3-M3, S/N :45713973 date 24JUN98 (could not get the info about the other engine yet).

Broker said engines are good but during our test drive, they could not go over 2000RPM at WOT and about 18kn, one boat commercial said top speed 24kn. It was not overloaded and I know one of the tanks was near empty.

Port engine’s exhaust gave light water vapor with no smoke. Starboard engine exhaust gave dark smoke (black/dark grey), lightly though, similar but about half the smoke in the picture here: https://www.sbmar.com/articles/understanding-low-power-troubleshooting/

It started with the dark smoke on startup and kept doing it all the time.

The mechanic I hired, after only visual checking everything (not using any tools/instruments), said both engines need to be overhauled. I need to understand how he came with this quick impression. Engine room was clean, no oil leaks, and temperature and oil pressure were not bad according to him.

The captain (from the broker side) said the reason of low speed and low rpm is the dirty hull. Later, looking on the side I saw marine and plant growth. I felt the captain is giving honest information, he even admitted that “a few days ago, after a 3-4 hours out to sea, the starboard engine shutdown by itself when parking. But they now adjusted it.”. He also confirmed that oil and all filters have been changed a few weeks ago. We agreed that they will clean the hull and we will redo the test in a few days.

We tried both engines at “neutral”: port did 2710RPM… starboard: 2400RPM

I have been reading a lot about the 450 diamond for many days now, and am very glad that the boat has got them, but the mechanic made me very afraid.

I understand it may be over-propped according to Tony’s recommendations, and will fix it if I buy it…. A big IF… after the mechanic’s comment….

1) should I look away?
2) how can I find the CPL number on the engine. Plate says “(illisible) SO 81733” is that it?
3) when these engines get older, do their HP ratings go down?
4) should I get them compression checked?
5) Do these engines have gph/load/ boost/EGT sensors from the manufacturer? the digital readout was in a different language, the technician and I could not understand to read any of these.
6) What should I check on the next trial? Or how should I proceeed?

Thank you for reading 🙂

Topic

[George Samaha]

Hi Rick, Glenn, thank you both for your suggestions. I will post more info here when available.

[glenn111]

George, First I would check those “high idle, in neutral” rpm’s with a digital IR tach/gun. What you observed sounds real low and the analog gauges from Maxum in the late 90’s were very often not accurate (I have a ’99 SCA). You want to make sure you are observing accurate rpm data.

Here is an old response from Paul Foultson at Boat Diesel regarding some rpm specs on the 6CTA Diamonds.

“Once you have strobbed the tacho and confirmed that it is 100% accurate check high idle. I thought I forwarded you a spec sheet, however no matter spec is 2,920 to 3020.

“Run engine until nice and warm with thermostat open having checked oil and water. Open the throttle steadily until you are against the stop. Do not get into a panic, just let the governor stabilize, you are doing engine zero harm. If you are getting rpm in the above range you are in good shape. If you are shy on spec high idle drop the control cable off the fuel pump throttle lever and try again. You should now get high idle within spec, now adjust Morse cable in order to get high idle.

I would go out and expect to see AT LEAST 2,700 rpm WOT on your calibrated tacho,five up. You need to allow for the times when you are heavily loaded and still be able reach WOT.”

[Rick Smith]

Dirty Bottom will without question slow it down

If you can see growth on the bottom I assure you that can knock down the RPM’s by 20 – 30% That would be the first place to start. Then a second opinion on the engines

[Author]

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