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Engine struggles after a few hours cruising

Hi there. I have a Narrowboat with a Nanni diesel 4110 with a PRM150 gearbox. I usually go along at about 1500 - 1800 revs quite happily. Recently, after a while cruising the engine gets more and more laboured until at 2500 revs it is struggling to keep going with smoky exhaust. It's as if the prop is tangled (which it isn't) or like it's dragging something heavy behind.
The engine runs cool, at around 45-50 deg C instead of the recommended 71 deg C and I have identified that the thermostat has been removed by the previous owner (I've ordered a new one). Oil pressure seems within bounds and in neutral the engine sounds fine, it has recently had the injectors reconditions.
Gearbox oil seems fine and having had new engine mounts recently the prop has been aligned. I have had an oil and all filters changed too.
Any thoughts? Could the low engine temperature be causing this issue?

Asked by: Keith R Harris  | 11.26am, Tuesday 21 August

WW says:

This seems a tricky problem and can suggest a couple of courses of action.
The low engine temperature certainly will not help, but this should not directly cause the symptoms you are reporting.
Two main areas spring to mind- air supply and fuel. With the air, check that the air filter is clean (try running the engine without for a short while) and that any oil return pipework (breather line) is not depositing large amounts of oil in the engine air intake. Sometimes air filters collapse and partly block the air intake (or corrosion occurs inside the intake causing an obstruction).
The second one might be a combination of fuel starvation and/or water in the fuel. If you have a primary filter, try draining this of any water, and then do the same on the final engine filter. it might be worth checking that there is no water in the base of the tank (or a jelly-like substance, which would suggest the dreaded "diesel bug"). Check that, when you prime the engine, that fuel is easily pulled through. A partly blocked filter may be able to supply enough fuel at low engine speeds and load, but struggle once the engine tries to draw more fuel. Also ensure that the control valves for the fuel tank (and return supply, if fitted) are fully open and not "stuck" partly closed- which does happen.
You might be able to try and "jury-rig" a temporary supply of diesel from a jerry can directly to the engine fuel pump (and take a line back from the spill-rail return as well) to see if this cures the problem; which may suggest problems with either the fuel filter(s), fuel supply/quality, engine lift pump or injector pump.
Also, when the engine is running and has been under load, check the temperature of the gearbox- you should be able to touch it, without it being too hot- the maximum temperature should be around 70C- and it shouldn't be cold (the gearbox will have an oil cooler fitted somewhere, usually on the return from the keel cooling tank- assuming you are not freshwater cooled/wet exhaust). Usually though, any failure in a RPM150 is noticeable by lack of drive, rather than increased load on the engine.
Final thought- you could check that the stern gland is not too tight- it should not run hot when the boat is moving along, but should never get warmer than body temperature- if it is, then it may be too tight. with the engine off (and in neutral), open the weedhatch and try to turn to prop by hand; if it is very difficult, then the stern gland may be the issue.
Hope that helps- please let us know if you manage to track down the source, or if we can be of more help

Mark Langley  | 10.06AM, Wednesday 22 August

The gearbox shouldn't reach above 90C for any length of time- normal working temp is 50 to 80 deg C. If it is, then the oil cooler might not be working properly.
However, to get that hot, it does suggest internal issues to the gearbox- usually the oil never particularly darkens, as there is no carbon deposits, like you would have in engine oil.
The rough metallic noises don't sound good to me- i would suggest, if at all possible, that you have the engine seen to sooner rather than later. Misalignment might be the problem, as could worn gears or clutch inside the 'box.
One thing you might try, is to ensure that, when the control lever is set to ahead or astern, that the end attached to the hearbox does not come hard up against the stop- there needs to be 2mm clearance between the lever and the stop, when selected- adjust the control cables to achieve this.
It might be worth draining the gearbox and unscrewing the cooling line connections (the two high-pressure oil pipes that lead to the gearbox oil cooler)) and check that liquid can flow through, by pouring oil into one, and ensuring it comes out the other.
If all is OK, refit the connectors, and refil the gearbox with engine oil of 10W/30 or 15W/40 SAE C engine oil (standard mineral engine oil- not ATF fluid), run the engine for a few minutes, then check the gearbox oil level again, to account for what has been pulled into the cooler.
If that doesn't wort it, I am afraid your gearbox may need professional intention- although it may not be terminal (and reconditioned gearboxes can be very good value if you do need to replace it).
Let us know how you get on.

Mark Langley  | 5.38PM, Wednesday 22 August

Readers say:

Thanks for the response.
The excessive engine labouring has got much worse and is present all the time now, even just moored at tick over in gear.
I have tried most of the things suggested, e.g. removing the air filter, draining fuel filters etc. I am reasonably confident in the fuel supply as I had some work done on the tank recently following an incident with some Armco getting caught in the prop and causing a leak, hence it has been completely drained, cleaned and new fuel put in in June. I also had all the fuel pipes to the engine replaced at the same time. I realise there could be issues with these repairs but fuel does seem to be coming through.
I can turn the prop shaft freely by hand from inside as well as out so that seems free enough.
One thing that you have said that made me think is the gearbox temperature, it seemed very hot after the last run, too hot to touch. So I ran it earlier today and there appears to be rough metallic scraping/grinding noises coming from that area. I have just emptied the oil from it and it is almost black. Don't know if that means anything?

Keith R Harris  | 12.47PM, Wednesday 22 August

Thanks for the help again, and sorry for the delay in replying, I have been trying various things and think we have a result. It seems there were a number of separate problems that were clouding the issue.
I decided to bite the bullet and go for a new gearbox and most of the problem then went away. Turning the output flange of the old one was like it was lubed with treacle so there was clearly something wrong. The engine was still a little rough but at least the boat moved well enough now.
So I got a local mobile mechanic in and he diagnosed significant engine misalignment, which surprised me since I had just had the engine mounts fitted supposedly by professionals at the local boatyard. I suspect the gearbox was already damaged from years of having badly worn engine mounts and this may have finished it off.
He also found that the tappets were wildly out i.e. instead of 7 thou they were nearer 1/8 inch. I would have thought the recent service I had would have highlighted this and was also surprised to see the oil was already black indicating that it had not been flushed at the service.
A new thermostat has put the engine temperature right and I also did an oil change and engine oil flush out.
With all this put right it seems a lot happier though I'm not convinced the 30hp engine is really strong enough for a 69ft boat, so may take the opportunity to upgrade the engine soon.
Living some way from the boat I had always trusted the maintenance to boatyards thinking they might alert me to any outstanding issues but I think I'll be taking over most of the maintenance myself now that I have the boat more local!
Thanks again for your help

Keith R Harris  | 5.32PM, Tuesday 11 September

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