Cummins Mercruiser 120 diesel inboard

longjohnsilver

longjohnsilver

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Does anybody owns a boat powered by the Cummins MerCruiser Diesel 1.7 120 HP?

Is it a good engine?
Is it reliable?

What is the overall experience?

We as a diving group are looking at buying a 27' semi displacement Aquastar which was re-engined with one about 6 years ago. It now has 550 hrs on the clock and looks in good nick but a mechanic friend has told us it's one of the worst engines ever made! Which understandably puts us off buying the boat, which is a shame as everything else is good. I have done some research on the net, and it looks as though most of these engines were put in planing outdrive boats which I guess are generally run at close to full revs (4400) but the owner tells us he generally runs at about 3200rpm with no problems.

That leaves us in a bit of a quandry as to what to do. Do we put in a much lower offer on the basis the engine may be a problem in the future or forget about it completely? Or are we being paranoid about an engine which is running perfectly at present?

Thanks in advance for any observations.
 
Don't own one, but plenty of experience around them.

The engine has a long list of people who are out of favour with it.
Almost exclusively they will be people who have the engine in small speed boats or other craft where the engine is run hard for sustained periods.

Run at WOT it really suffers and you are very likely to have major and probably catastrophic issues. Run at cruising revs and only briefly harder to pop it up on the plane and it will last as long as any other good diesel lump.
 
Thanks. That's re-assuring but suspect the fellow members of my dive group have already been put off. I don't see any huge problems if we stick to about 3200rpm.
 
Just a thought...

Hi John,

Perhaps have a chat with Sheppy as he has the 2.4 165hp Mercruiser in his 25'. It's a full planing hull, but I know when it's got any amount of 'slime' build up that he's struggles to get her to plane. The previous owner used most of the time at displacement speed so had few issues! My gut feeling would be that a 1.7 would be under powered and as a result you'll notice any build up very quickly. If however you don't intend to cruise above displacement speed then it would prove frugal and reliable.

Best regards,

Robin
 
longjohnsilver said:
Does anybody owns a boat powered by the Cummins MerCruiser Diesel 1.7 120 HP?

Is it a good engine?
Is it reliable?

What is the overall experience?

We as a diving group are looking at buying a 27' semi displacement Aquastar which was re-engined with one about 6 years ago. It now has 550 hrs on the clock and looks in good nick but a mechanic friend has told us it's one of the worst engines ever made! Which understandably puts us off buying the boat, which is a shame as everything else is good. I have done some research on the net, and it looks as though most of these engines were put in planing outdrive boats which I guess are generally run at close to full revs (4400) but the owner tells us he generally runs at about 3200rpm with no problems.

That leaves us in a bit of a quandry as to what to do. Do we put in a much lower offer on the basis the engine may be a problem in the future or forget about it completely? Or are we being paranoid about an engine which is running perfectly at present?

Thanks in advance for any observations.
Click to expand...

As far as I'm aware it's n Isuzu base unit from the vauxhall corsa.

In my opinion a strange engine to fit to an aqua star boat as its a high revving unit, the boat would be suited to a more lower revving unit in my view. High revving motors are best suited to sterndrive boats where the torque is at the higher end.

Speak to aqua star maybe they didn't fit it from new.
 
volvopaul said:
As far as I'm aware it's n Isuzu base unit from the vauxhall corsa.

In my opinion a strange engine to fit to an aqua star boat as its a high revving unit, the boat would be suited to a more lower revving unit in my view. High revving motors are best suited to sterndrive boats where the torque is at the higher end.

Speak to aqua star maybe they didn't fit it from new.
Click to expand...

No definitely not fitted from when it was built. Engine is about 6 years old and the boat about 30. Strangely enough the chap selling the boat worked for Aquastar when it was built in the early 80s.
 
rbcoomer said:
Hi John,

Perhaps have a chat with Sheppy as he has the 2.4 165hp Mercruiser in his 25'. It's a full planing hull, but I know when it's got any amount of 'slime' build up that he's struggles to get her to plane. The previous owner used most of the time at displacement speed so had few issues! My gut feeling would be that a 1.7 would be under powered and as a result you'll notice any build up very quickly. If however you don't intend to cruise above displacement speed then it would prove frugal and reliable.

Best regards,

Robin
Click to expand...

Thanks Robin. Yes almost all anticipated use would be at or below displacement speed, and as divers we can and will keep the hull pretty clean!
 
I have a pair of these in Rafiki, and after 4 seasons am very happy. Easy to maintain, reliable and economical. They are Isuzu units as used in the GM Astra amongst other apps. There have been issues with fishee boats mainly in the USA where the engine is exposed to sea water and spray, where the wastegate rod sticks at WOT and can cause piston 3 to overheat. Mercuryman is the forum expert.
 
rafiki_ said:
I have a pair of these in Rafiki, and after 4 seasons am very happy. Easy to maintain, reliable and economical. They are Isuzu units as used in the GM Astra amongst other apps. There have been issues with fishee boats mainly in the USA where the engine is exposed to sea water and spray, where the wastegate rod sticks at WOT and can cause piston 3 to overheat. Mercuryman is the forum expert.
Click to expand...

That's good news. Doesn't look as if Mercuryman has visited ybw for a few months, do you have a contact number for him? (by PM)
 
They are good motors if maintained correctly, but that goes for all engines. I have installed about 20 of them and the main problems are they run a bit hot if you don't clean the heat excanger every couple of years and the main thing to be aware of is keeping the wastegate actuator rod lubricated where the spindle goes into the waste gate housing.
It doesn't stick open at WOT, what Rafiki means is that it just rusts and sticks so the waste gate doesn't open at all and dump the exhaust through the bypass when it reaches the preset boost, so all the exhaust goes through the turbine causing the boost to go too high, and this is what burns holes in the pistons, I've only ever seen a couple melt down in whole time the engine has been in production.
A lot depends on the installation and design of the engine room as to whether it gets moisture at the back of the engine.
Also seen a couple where the engine harness was incorrectly routed and it melted the insulation causing all sorts of weird instrument readings and needed a new harness.
 
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