Mercruiser (VM) 4.2L 320 won't get past approx 3000rpm

jointventureII

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Cummins Mercruiser (VM) 4.2l, 320hp diesel year 2006, Bravo 2 leg, Bertram 25'.

So went out in some moderate conditions the other day. Decided early that this wasn't a great day so turned around around 10 minutes.

On the way back in, around 3300rpm, the engine struggled to maintain revs. Reducing throttle to 2800-3000 seemed to keep her going but then it slowly decreased, at which point I was back into the harbour entrance so decreased to 1400 / 5 knots, it ran fine for the 5 minute run back to the berth. Assuming this was simply some dirt being kicked up in the chop and into the prefilter / filter, I promptly changed both the next day. Whilst at it, I opened the fuel tank and cleared any dirt off the pick up pipe meshed filter too. To be honest there was a bit around the entrance of the pick up pipe, and the prefilter was also quite gunked up. Bled the air out, restarted on the berth, she sat at all rev ranges comfortably for a few minutes. Job done, or so I thought.

Assuming this was the reason I'd experienced the loss of power, I took her out again this morning (much calmer conditions) to test it under load. But the same problem arose after 5 minutes or so - it feels like the fuel is not getting to the engine.

So my question - could this be the lift pump starting to fail, or gunked up too? When bleeding the air out, it did seem to take quite a while to manually pump the fuel to the bleed screw on top of the filter. I've never taken a lift pump apart, but it looks to be a fairly simple job...?

Or is this likely to be turbo related? As the turbo seems to kick in around 2000 rpm - wastegate? The turbo itself is around 2 years old, the wastegate is original.
 

Freebee

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getting good revs with the engine unloaded is no indication as the engine will require more fuel for the same revs when under way. Are you sure there is not another filter in the line somewhere? In my old Bertram 25 I discovered a fuel filter back under the deck on the transom which possibly had never been changed!! check the tank breather make sure its clear , a blocked breather would make it hard for fuel to be pumped out of the tank....next I would check the lift pump.. last the turbo.
 

BB1

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Cummins Mercruiser (VM) 4.2l, 320hp diesel year 2006, Bravo 2 leg, Bertram 25'.

So went out in some moderate conditions the other day. Decided early that this wasn't a great day so turned around around 10 minutes.

On the way back in, around 3300rpm, the engine struggled to maintain revs. Reducing throttle to 2800-3000 seemed to keep her going but then it slowly decreased, at which point I was back into the harbour entrance so decreased to 1400 / 5 knots, it ran fine for the 5 minute run back to the berth. Assuming this was simply some dirt being kicked up in the chop and into the prefilter / filter, I promptly changed both the next day. Whilst at it, I opened the fuel tank and cleared any dirt off the pick up pipe meshed filter too. To be honest there was a bit around the entrance of the pick up pipe, and the prefilter was also quite gunked up. Bled the air out, restarted on the berth, she sat at all rev ranges comfortably for a few minutes. Job done, or so I thought.

Assuming this was the reason I'd experienced the loss of power, I took her out again this morning (much calmer conditions) to test it under load. But the same problem arose after 5 minutes or so - it feels like the fuel is not getting to the engine.

So my question - could this be the lift pump starting to fail, or gunked up too? When bleeding the air out, it did seem to take quite a while to manually pump the fuel to the bleed screw on top of the filter. I've never taken a lift pump apart, but it looks to be a fairly simple job...?

Or is this likely to be turbo related? As the turbo seems to kick in around 2000 rpm - wastegate? The turbo itself is around 2 years old, the wastegate is original.
Never understood the idea of a mesh filter on the pick up tube in the tank, surely you want any dirt to come up the tube and be trapped in the prefilter, which should be replaced annually or every 200 hrs or so. Common sense really.

I think the engine is 220hp
 

Portofino

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Check for air leaks in the turbo and the air filter is pretty clean . Then as others have said the fuel supply side .

Not familiar with its lift pump but if it’s got a rubber diaphragm cam driven type ? - they age and split noticeable symptoms at higher rpms .Part(s) should be available. In the classic car world you can service them pre emptying this .
Or indeed fit an a more modern and reliable off the shelf electric fuel pump .It’s probably got a metal rotary pump which might be crapped up or just worn with age ?
 

jointventureII

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Thanks all

Regarding an additional filter in the way - no there's definitely not. There is a fuel flow meter with a slightly smaller diameter than the rest of the pipe work but I've checked that and it's clean

The mesh filter, well it's not a particularly fine filter at all, also not a particularly hard job to remove it and clean it out every 6 months or so. I can see why they've put it in there even if it isn't the most logical thing.

Yeah Porto it's the cam driven rubber diaphragm type. I'll have it off tomorrow or Monday, already seen they're a cheapish part. Air filter is new too.
 

Portofino

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Ok ( potential split diaphragm) if it turns out that’s the case fuel can get past the diaphragm into the sump diluting the oil .
Dip stick might show a slight rise assuming you can recall it’s normal level ?

Or worse still because of the diesel dilution into the oil if you where to cane it your big end bearings are at risk at it will throw a rod eventually.

So change the oil once the new diaphragm is fitted ……assuming it’s that ?
 

jointventureII

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Well I think I have found the cause...

IMG-20240610-WA0013.jpeg

Didn't know there was a mini filter in there. Not even sure how the engine ran at all!

The rubber diaphragm, after cleaning, also looks like it's starting to deteriorate. €70 for a new pump, so it's been ordered.
 

jointventureII

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Just an update for future reference - this semi-sorted the problem. Another sea trial and it was still behaving in the same manner, but able to maintain 2200rpm before power loss kicked in.

I'd forgotten totally about the fuel flow sensor. Return line sensor was of course fine, supply line however :
sketch-1718741744271.png
Photo doesn't do it justice, there was a sizeable lump of crud in there.

Yet another sea trial today and it's good again. I'll need to get inside this fuel tank though, at some stage. Eep.
 
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