Volvo penta MD6A will not rev past 1600rpm under load.

[nigelmercier]

Sounds like your prop is too big ...

Do props suddenly grow in size then?

 

[mitiempo]

If it used to rev to or near max rpm and the prop is clean or hasn't been changed the problem is elsewhere. A diesel needs compression, clean fuel, and to breathe - both in and out. The air intake is not likely a problem. That leaves the exhaust. On any engine forum I have been on the answer to this question is invariably exhaust elbow. At the point where water is injected into a hot exhaust build-up occurs and most owners never think about it, but it is one of the major causes of engine problems.

 

[Icarus39]

Do props suddenly grow in size then?

The chap has only had the boat a year and only just got the rev counter going .. my money is on the prop

 

[GH29]

All the above is good advice - could be any of these factors so well worth doing your diagnostics to eliminate each one in turn. I'd also add restrictions in raw water flow - under load when it's needed most - and look at any restriction in your raw (sea) water inlet such as barnacles. Also your heat exchanger to make sure it's clean with no deposits in the tubes. I change my exhaust elbow (high riser version)
every 5 years.

 

[yellowspinnaker]

I have a Volvo MD6a, and I can reassure you that it's not a heat exchanger problem as this is a raw water cooled motor that doesn't have a heat exchange.

Just to add to the debate about the exhaust elbow - yes it's worth checking, so is the exhaust manifold itself. Mine has just been cleared out after removing the core plugs to have a look. It was chock full of rusty crud with only a small passage available for water flow.

I reckon the others are right with the checklist. Barnacles on the prop, blocked exhaust, blocked air intake.

 

[colvic987]

If the exhaust elbow is at fault its the same as the md7a, you might be able to get one for around £100 incl vat, ask your volvo penta dealer if any summer deals are on for md7a. here is a thread i posted when i did my exhaust elbow and exhaust, check out how small the water injection hole has sooted up.also the wall between the elbow exhaust and water has corroded and the extenal wall had a split in it, it then collapsed when i removed the gland it screws onto and bolts to the exhaust manifold.

http://www.ybw.com/forums/showthread.php?t=244686&highlight=md7a+exhaust+elbow


I would also remove the air filter and see if it helps the revs reach the prescribed max revs, and if it doesnt make any difference, check the engine filters for any clogging up with a black sludge.

 

[rob2]

The whole thing about fuel, compression and breathing is correct. Apart from checking the fuel and air filters are clear, it is worth considering whether your fuel lift pump is working. A lot of installations are effectively gravity fed and a certain amount of fuel can be fed by siphon effect. after all the injection pump does create suction on the line.

The fact that the engine starts well and revs to near specification without load bodes well, but one of the factors of fuel and breathing is insuficient for it to develop full power under load. Happy hunting!

Rob.

 

[nigelmercier]

I would also remove the air filter...

On my engine there is no filter, but the intake was full of gunk!

If you decide to go down the RydLyme (or RydLime Marine) route, have a look at the MSDS before you buy it. Despite their claims that it is not acid, the MSDS says:

MATERIAL OR COMPONENT ... %
HYDROGEN CHLORIDE ... LESS THAN 10

In other words, it's just 10% Hydrochloric Acid!

PS The prop size suggestion is a red-herring, the OP said here that the engine used to sound faster.

 

[cptpugwash]

Thanks guys for all your recent reply s on this topic,

On removing the exhaust elbow last week it was found to be very badly corroded inside and out, the build up of carbon and corrosion in the ports was unbelievable, it also had a crack on the outside were it had been welded before in the past. I`m currently looking for a replacement, I have e-mailed a few company's and are waiting for reply s, Trickett marine products can do one for £130.

 

[Katouf]

Low revs in gear

On removing the exhaust elbow last week it was found to be very badly corroded inside and out, the build up of carbon and corrosion in the ports was unbelievable, it also had a crack on the outside were it had been welded before in the past. I`m currently looking for a replacement, I have e-mailed a few company's and are waiting for reply s, Trickett marine products can do one for £130.

I'm not sure that this is the cause of your problem. I had a similar scenario with my MD2B and it turned out to be a slipping lock nut on the 'throttle' cable. See the current post by Fascadale - How to adjust Morse Controls.
But if a new exhaust elbow solves your problem.......

Good luck

Alan

 

[cptpugwash]

Thanks for that Alan,
But the engine does rev upto maximium RPM out of gear,
Throttle linkage seems to be fine.

Regards
Paul.

 

[colvic987]

james troop of runcorn is an authorised volvo penta dealer..

 

[Topcat47]

Running up to max unloaded isn't much of a diagnostic aid. It requires more fuel/air mixture to run up under load. Restrictions to the inlet tract, fuel system or exhaust port wouldn't have much effect on the "no load" case, unless it was severe.

 

[cptpugwash]

james troop of runcorn is an authorised volvo penta dealer..

Thanks for that mate, I`ll give them a try.

 

[cptpugwash]

Running up to max unloaded isn't much of a diagnostic aid. It requires more fuel/air mixture to run up under load. Restrictions to the inlet tract, fuel system or exhaust port wouldn't have much effect on the "no load" case, unless it was severe.

Thanks Topcat47,
I agree with you on that, Exhaust elbow has basically fallen apart, hopefully going to source a replacement this week. If that doesn't work then it must be the prop.