Diesel engine speed mystery

[DSdriver]

I thought I would throw this one at you as there is a world of experience with old diesel engines here and I have a problem with a boat engine which it seems no-one has had before. which is a bit wierd when the engine is nearly forty years old.

The engine works fine in neutral, you can set the throttle to whatever revs you like. However when you put it into gear it just runs at somewhere between 1300 and 1500 revs and moving the throttle lever has no effect. 

Take it out of gear again and back to full control.

Yes the throttle lever does move the lever on the engine when in gear but with no effect so it doesn't seem to be a gearshift/throttle fouling problem.

 

The engine is a 2 cylinder SABB (not Saab) Norwegian engine of mid seventies vintage. It was used a lot in lifeboats because it is very basic, normally reliable and can be started by hand cranking if you have sufficient adrenaline pumping through your veins.

 

Useless fact: Sabb 2 stroke engines were used in Scammels.

[meggersdog]

Is it the single lever type ?

If so sounds like it could be a gear selector issue.Dis-connect the gear lever/ cable at the gearbox end and see if it still does the same.

Also check the clamps securing both outer cables aren't loose.

[DSdriver]

No its the old style two lever seperate throttle and gearshift with nicely pitted chrome levers. As far as we can tell the problem seems to be with the engine, not the controls. We have tried disconnecting the throttle cable at the engine end which would presumably fulfill the same objective.

[Albert Ross]

Dependent on the system used, it may have slack in the governor levers/springs. Does it have a conventional injection pump or is there an odd Swedish system?

[DSdriver]

Aha, I wondered about the governor springs because the book does say that they are a service item but doesn't explain the symptoms of worn springs. I would have thought that the Sabb engineer (it seems there is only one in the UK and he is down in Devon) would have thought of that when I phoned him to explain the problem.

Not sure about the pump, will have to read up on it.

I think the governor does warrant further investigation, It was the only part of the engine that didn't have paint on the nuts when we bought the boat, so maybe previous owner was having the same problem.

 

Edit/Fuel pump is a Bosch so presumably full of Germanic quality.

[e287yba]

Put some clear (transparent) fuel line inline just before the injection pump and see if there are entrained air bubbles (especially when the engine is under load)?

 

This is, of course, assuming that the tank is lower than the injection pump and that there is a low pressure pump to feed the injection pump - a description of the set up would help . Assuming I'm right, I'm thinking air leak into the vacuum side of the fuel line (between low pressure pump and tank) allowing air in (and while the engine will rev without load using a minimal amount of fuel, its fuel supply is restricted by the entrained air in that fuel supply when under load and more fuel is needed to do 'work')....

[DSdriver]

Thinking it through that doesn't explain why the engine doesn't respond to reducing the throttle whilst in gear when the engine just stays at the same revs until you put it into neutral. 

I have had a lot of experience of air in the fuel and this is not like previous times but the clear tube sounds like a good idea anyway. Yes there is a lift pump, the tank is below the engine in the keel. Its a good old-fashioned long keel boat.

I thought I would just give the basics and that you guys would know the questions to ask otherwise I could just bore people to death with a lot of irrelevant twaddle such as:

What is neat about this engine is that you can take the injectors out and bolt them back on upside down (facing outwards) so that you can check they are spraying OK.

[PhilA]

Just a thought, because I have no idea about boats...

 

What if it's all out of adjustment on the direction linkage- in neutral it's meant to disengage the throttle so you presumably can't race the engine needlessly, and only bring the revs up when it has the propeller engaged?

 

Phil

[DSdriver]

Hi Phil

That sounds likely with the modern single lever systems but this is a much more basic system with separately operated throttle and gearshift. Looks a bit like this:

I can see the throttle lever moving the throttle on the engine, but with no effect when in gear.

 

[meggersdog]

If your bracket is like that one ^^^ with both cables bolted to it have you checked to see if the bracket is cracked ?

The strain of putting it in gear might open a crack which might shorten throttle control.

[DSdriver]

 

I can see the throttle lever moving the throttle on the engine, but with no effect when in gear.

Lets look at it from a car perspective, If you open the bonnet and twiddle the throttle at the engine end then it will change the speed. Now it would be hard to do this while the car is in motion but if you did you would expect the car to speed up and slow down. With this engine it doesn't. 

[holbeck]

You might try googling Peter Thompson at Marine Engine Services in Uxbridge. He's a Lister dealer but Lister and Sabb are associated (if you buy a Sabb these days, you get a Lister). 

[DSdriver]

Cheers Holbeck I will follow that up if I cant get it sorted, I'm going to have a poke around in the governor as Albert seems to be on my wavelength. And I have just found the SABB parts book which came with the boat and the governor page has various notes in pencil which leads me to believe that the guy who sold me the boat may have been withholding information. 

The boat was cheap enough that putting a new engine in it would still be economical but the thought of having a modern diesel going into limp home mode just as a supertanker looms over the horizon on a dead calm day fills me with dread. Mind you, this engine is not much better at the moment.

[PhilA]

ok so

 

The throttle shifts the diesel pump linkage no matter what, that's a good thing. Is the gear lever connected to anything other than the gears? By the sounds of it you're on the right track with the governor assembly

 

--Phil

[jbz2079]

If the governor is centrifugal it will probably have two springs fitted, if one was broken that just might explain the strange throttle response.

[alf892]

I don't see how it can be the governor.

 

If the throttle linkage/cable connects straight to the pump arm............some older industrial types don't............but assuming your does it sounds more like a problem with control cables?

 

The governor does not 'know' load conditions it just keeps revs at a given setting. If something pulls the revs down the governor will allow more fuel until set revs achieved and so I don't get why it would rev in neutral but not in gear.

 

Have you checked actual movement of the throttle arm (on pump) when operating the control while in gear?

[DSdriver]

Update

I took the governor off today. This is what it looks like.

governor2 by DSdriver, on Flickr

I couldn't see the springs - I assume they are inside under the cam followers at the bottom, both of which are well sprung and requiring equal amounts of pressure to push them in. Next I looked at the sliding bar thingy which was quite loose when pushed towards the "stop" end at the right but half way along becomes very stiff. Is this normal? I see it has a rack on the back which presumably connects to a pinnion which does magical things inside the governor. The pin on the front connects via a heavy duty forked bar to the throttle control so as you open the throttle it should push the bar to the left. However I suspect this is not happening, maybe part of the rack or the pinnion is borked, or what they connect to.

This means disassembly. Are things likely to ping out or need to be put back in a particular way? Should I have a go at this or leave it to an expert?

[Rocket88]

Give it to a man "who knows". Autoshite is littered with the corpses of people being bombarded with springs and small ball bearings after an ill-advised spot of dismantling............

[holbeck]

Err... that's the underside of the fuel pump we are looking at (looks to be similar to that fitted to a Petter AC2). The fuel supply is controlled by the slider (rack) seen on the front.

 

I would suspect the governor would be a series of weights and springs housed within the camshaft drive gear, ( I say suspect as I've no experience of Sabb's own engine).

If it's the same as a Petter, the pump rack attaches to a linkage which as engine speed rises the governor weights fly out through centrifugal force and reduce fuel supply to stop the engine overspeeding, the whole thing is sprung in both directions so if load rises  and the engine slows the weights come back in and the pump rack springs over to chuck more fuel in.

The throttle is attached to this linkage and just varies the amount of tension on the springs to achieve different speeds.

 

Hope this is of some help.

[Cheggers]

Is the prop OK & not too fouled or barnacle-encrusted?...Just wondering if it won't rev over 1500 because of excessive load in the drivetrain?

[DSdriver]

Ah, yep its the fuel pump, I is stoopid. Your information is very helpful. Found it on the exploded diagram in the parts book. It does seem to possibly be the source of the problem. with that sticking slider. Or are they all like that? What should be my next plan of action, is there anyone who services fuel pumps or do I buy a new or second hand replacement.

Hi Cheggers, sensible question but its been doing it since the boat was relaunched just after I bought it so very unlikely. I have thought about letting her dry out on the half tide scrubbing dock to make sure but the Nicholson 32 with most of its keel aft of the mast has a tendency to fall on its nose. Here's a pic of her being put in the water. I have prodded the prop with a long pointy thing and it seems to be clear.

Cleaver in the strops by DSdriver, on Flickr

[Cheggers]

Ah, fairy nuff...unless it's brutally "over-propped" it sound like it's an engine thing rather than a prop/ shaft thing.

 

Lovely old things, Nic 32's, I learned to sail on one all those years ago and still have a soft spot for them

[holbeck]

The fuel pump will be serviceable (If needs be). I'm not an expert on Bosch pumps but the equivalent CAV types are readily tested and refurb'd by fuel injection specialists.

[DSdriver]

Cheers, i will find a local Bosch injection specialist.