Loss of Top End Revs

[QBhoy]

All things being well like fuel, oil and air filters being clean, along with oil change...it might just need a good run at full throttle. Likely be noisy, sound harsh and even blow a bit of black smoke, but give it a good hammering to de carbon the thing. Was just replying to another thread there. These wee N/A diesels love a good blast at full throttle every now and then. I could even recommend a wee bit of 2 stroke oil in the diesel tank to lunricate and seal things up a little. Maybe even restore a little lost compression. The fuel quality available these days is very poor with little proper lubricating properties involved.

 

[Tranona]

Thank you tranona and others. I will follow up checking, and possible replacement, of my exhaust hose. (My prop is 17x11 on a 38ft 6t cruiser-racer, so I don't think I am grossly overpropped.)
Blan321, was there a difference in revs/performance when you replaced your collapsed one? Cheers.

Depends on how you define "grossly overpropped". As I suggested to pass the commissioning test the rpm at WOT needs to be above 3050 rpm. It may well be that like the OP your exhaust elbow is blocked or the hose restricted as your prop size does indeed sound right. I have the same engine in a boat that is a bit lighter and the prop is 16*11 and achieved 3100 rpm and 7.6 knots when it was commissioned.

 

[scottie]

How old is the rubber exhaust hose? It is not unknown for this to collapse internally when old.

Running without water will delaminate the hose internally with little outside damage

 

[richardbrennan]

For those interested, I had a look at the old elbow today. I am surprised that the engine was running at all as the diameter had been reduced very unevenly to about that of a 20p piece, and the water jacket had also corroded through. These elbows do seem to be the "Achilles Heel" of Volvo engines, but at £250 a pop, I don't suppose Volvo mind flogging the spares!

 

[Tranona]

For those interested, I had a look at the old elbow today. I am surprised that the engine was running at all as the diameter had been reduced very unevenly to about that of a 20p piece, and the water jacket had also corroded through. These elbows do seem to be the "Achilles Heel" of Volvo engines, but at £250 a pop, I don't suppose Volvo mind flogging the spares!

Not just Volvos. You find the same thing with some models of Yanmar and Bukh for example. Sorry for going on about it but these little high speed diesels prefer to run hard. Light loading and short runs without getting fully hot leads to incomplete combustion and deposits building up in that area of rapid temperature change. Once there is a coating of a mixture of hydrocarbons and seawater leads to more deposits and corrosion.

Generally you don't find the same short life on engines that are run for longer periods at good loading - approx 70% maximum revs which is around 2300 on a D1 30.

 

[Dipper]

For those interested, I had a look at the old elbow today. I am surprised that the engine was running at all as the diameter had been reduced very unevenly to about that of a 20p piece, and the water jacket had also corroded through. These elbows do seem to be the "Achilles Heel" of Volvo engines, but at £250 a pop, I don't suppose Volvo mind flogging the spares!

Mine was even narrower. As you say, it's surprising they run at all like that.

 

[Venus1]

Not just Volvos. You find the same thing with some models of Yanmar and Bukh for example. Sorry for going on about it but these little high speed diesels prefer to run hard. Light loading and short runs without getting fully hot leads to incomplete combustion and deposits building up in that area of rapid temperature change. Once there is a coating of a mixture of hydrocarbons and seawater leads to more deposits and corrosion.

Generally you don't find the same short life on engines that are run for longer periods at good loading - approx 70% maximum revs which is around 2300 on a D1 30.

Being aware of this (but not generally knowledgable), I try to give 10mins of near full throttle at the end of my trip. This means that the engine is definitely nice and hot as I approach my mooring.
So my next question is: what's a reasonable time to allow for a gradual cooling down period? I believe shutting down too soon would not be good. Is 5 minutes of idle on the mooring too much? Or once the temperature drops from the normal 90 degrees? (Am I overthinking this?)

 

[Tranona]

(Am I overthinking this?)

Probably! I usually idle for a few minutes when in the berth mainly because it takes that amount of time to make sure all the lines are secure then switch off. The danger points seem to me to be starting and running for a few minutes before getting the sails up then switching off, using the engine for battery charging, motorsailing at low revs when the sails are providing most of the speed - that is the engine is not being loaded by the prop.

 

[lw395]

....
Generally you don't find the same short life on engines that are run for longer periods at good loading - approx 70% maximum revs which is around 2300 on a D1 30.

Loading the engine is not the same as revving it.
But I think the clogging of mixing elbows depends a lot on the qualities of the local seawater. Lots of plankton or whatever = more deposits?

The other thing that often gets neglected on small diesels is the tappet gaps, it makes a difference.

 

[jac]

So whilst the "Italian tune up" gets good anecdotal feedback ( and feels good) has anyone ever done a test to see what effect it has? DOes it reverse carbon build up by consuming carbon that has already been laid down ( for example in the elbow,) does it just slow down the deposit of more carbon or is it just a waste of time?

 

[Dipper]

My blocked elbow was virtually free of carbon despite mainly using the engine for short periods at a time. The entire blockage was very hard rust and this was at the bend. A cursory look through either end wouldn't have revealed the problem.