Engines not making top revs, suggestions please (bit FAQ I know...but)

[stefan_r]

I have recently had my MAN engines rebuilt as my revs were down and they were both smoking more than I would have liked.

Rebuild is completed with several problems found including valves and seats caked with crap and fuel injection pump timing out.

Everything now assembled and tested, stuff tightened and run for a few hours to run them in.

The largest underlying problem, however, is still there in that I am not achieving my book rpm - and previously achieved (some years ago) 2,300rpm.

It is not a problem with throttle movement - without load they will rev to 2,600...but with load 1,850 port and 2,000 starboard. RPM gauges checked and accurate.

Good news is the smoke has cleared...except from a bit of grey from the port engine...and that is clearing with each run so suspect a slight overfill of oil perhaps.

The engines are NOT smoking black (too much fuel) at any revs.

So, the big question - what's my rev problem?

I say the props are not sized correctly but at a couple of grand a throw it is a big gamble to take to find out it is something else.

Most cruising gear removed, spare fuel tanks (1,500 litres) emptied and running on 1/4 water and 1/2 fuel with 2 people.

Vital statistics:
Mochi 46 flybridge
Displacement unknown...going to calculate on haul out
LOA 46'9" / 14.25m
Beam 14'8" / 4.47m
LWL can't remember...have to wait for haul out
Engines: MAN D2848LXE rated 680hp each
Gearbox: Twin Disc MG5111 ratio 1.50:1
Running through Vee drives (no gearing)
60mm shafts
4 blade 26" props...unknown pitch...trying to find a man locally who can tell me.

Has anyone had experience re-propping?

Any decent shops out there?

One further problem - boat is now in Greece...so this is a bit of a one off and sending them back and forth to Steel Developments or whoever is a no-no. I don't mind buying them in the UK but on the basis they will be "right first time".

Cheers
Stef

 

[Divemaster1]

From the top of my head ....

Fuel pickup restricted.... anything from tank to too fine filters, both water seperators, low and high pressure...

Fuel stop valve restricts flow somehow...

Vacum in tank (how is it vented )

Fuel hose kinks, or degraded hoses...

 

[lookout]

surely if it once managed the right revs with those props.... the props are fine, and summat else is up?

how was the injection pump timing sposed to have been out? something that would have happened in use- cuz again, she used to pull correct revs, right?

 

[volvopaul]

Your revs are well down but as a boat is designed to give a certain speed with a certain hp as the combined hp from 2 engines, as one is down on rpm further than the other its that engine which has the underlying fault though thats not to say the other one is perfect at 300 down, the problem could be lack of turbo boost as your not really revving enough to boost in pressure, I had this on a v8 cat 3208 it simply turned out to be the inlet gasket was missing on the side of the engine you could not see.

Have they altered the timing much as it was ok before, have they got the correct figures, the other thing is the advance unit on the pump, if its that system is working had this also on a cat v8 the units weights had broken up, though I think your on a bosch system it may be a fixed typr of pump.

Are they man agents or just general engineers? if man they should know all the correct settings, if your not getting balck smoke then maybe its underfuelling as the boost pressure pipe from the inlet manifold may be leaking pressure and not telling the pump to give extra fuel when boost air is introduced at the correct rpm, I suggest a rethink at get some sea trial boost figures if you dont have gauges at the helm.
Obviously you have all new fuel filters fitted after this big job? so its not a delivery problem to the injection pump.
Summary is that if the 1850 rpm engine is holding back then the other one cannot be expected to exceed what rpm its making so I would look at that one first.

 

[BlueJasper]

Now that's what you call an answer!

 

[stefan_r]

Cheers loads...
[ QUOTE ]

Fuel pickup restricted.... anything from tank to too fine filters, both water seperators, low and high pressure...

[/ QUOTE ]
...hmmm good one. Will try with RACOR filters out...new fuel from tanker so risk is minimal.
[ QUOTE ]

Fuel stop valve restricts flow somehow...

[/ QUOTE ]
Less likely...especially on both engines
[ QUOTE ]

Vacum in tank (how is it vented)

[/ QUOTE ]
Not an issue...2x38mm hoses from tank port and starboard, but good idea.
[ QUOTE ]

Fuel hose kinks, or degraded hoses...

[/ QUOTE ]
Checked...though "degraded?". The hoses are 15 years old...is this a problem?

 

[stefan_r]

[ QUOTE ]
surely if it once managed the right revs with those props.... the props are fine, and summat else is up?

[/ QUOTE ]

Problem was old owner gave me two sets of props and I have tried both sets as one set got dinged. Not sure what else has changed but performance has slowly been down over the years. I used to put it down to trim and the fact that my spare fuel tanks (tonne+++) were upsetting prop pitch - so I was happy to run slower.

Now not sure if:

Original props: - dinged, not sure if they were repaired right or not.

Spare props: - were they “right” in the first place


Everything else has been fixed, engines are done, injectors brand new, fuel timing 100% according to Bosch, valves sealing...so what else?????

aaarrrrrrrrrrrrggggggggggghhhhhhhhh

 

[stefan_r]

SUPERSTAR!!!! Cheers.

[ QUOTE ]
Are they man agents or just general engineers? if man they should know all the correct settings

[/ QUOTE ]

Fuel system was done by Bosch main agents, rebuild by MAN.

[ QUOTE ]
if your not getting balck smoke then maybe its underfuelling as the boost pressure pipe from the inlet manifold may be leaking pressure and not telling the pump to give extra fuel when boost air is introduced at the correct rpm, I suggest a rethink at get some sea trial boost figures if you dont have gauges at the helm.


[/ QUOTE ]

This is interesting....and worth more attention.

[ QUOTE ]
Obviously you have all new fuel filters fitted after this big job? so its not a delivery problem to the injection pump.

[/ QUOTE ]
yup...unless - as suggested elsewhere - the wrong filters, I am using RACOR 2020 filters as that is what it had on when I bought it 7 years ago.


[ QUOTE ]
Summary is that if the 1850 rpm engine is holding back then the other one cannot be expected to exceed what rpm its making so I would look at that one first.

[/ QUOTE ]

Good thoughts...thanks very much!!!!!

Stef

 

[Divemaster1]

Checked...though "degraded?". The hoses are 15 years old...is this a problem?

Hoses can delaminate from the inside, which makes them prone to collapse as it looses structural integrity... Just a guess though..

Fuel pickup.... if you have a gauze filter inside the tank, this can get clogged up ... reverse flow by introducing air to systen and flow it back into tank with force, could be a temporary fix if that is the case... but problem will come back at some stage...

Would also echo the issue around lack of turbo boost ... check this as well ... small leak will cause big loss of power..