Help/information needed for repair of old Mercedes OM312 diesel engine

[Chris Green]

We have a boat that lives on the canals in France. We visited it for the first time in 18 months last week (post Covid). Much to our surprise it was mostly in one piece, functional electrics (thanks to solar panels), not much water in the bilges, etc. Just needed a lot of cleaning and painting.

However, when I tried to start the engine it started OK but the governor appears not to be working and I had to shut it down immediately to avoid it over revving. I tried a few times with the same result every time.

It's a Mercedes OM312 'marinised' for boat use. It's a 4.5 litre, 6 cylinder, lorry engine from the 1950s or 1960s. I have a manual for it but it has no description or diagrams of the governor.

Does anyone here know about this sort of thing, or, more likely, know someone who does know about such things? I'm fairly OK with taking engines apart etc. (I took apart a Daimler v8 many years ago and have worked on lots of smaller engines since) but it would be nice to have some sort of idea how the governor works and where it is before I start taking things apart.

[David Wade]

Chris, Not sure if they have anything that far back or that big but perhaps

might help. Dave

[Brian Gaff]

Would the basic idea in lawnmowers not be used in this case? Its usually very mechanical and tied to the throttle. Its only got to detect speed and move a lever accordingly one supposes? Brian

[Tim Lamb]

In message [e-mail]>, Chris Green [e-mail] writes

The injector pump on agricultural diesel engines of that vintage sometimes include an *overfuel* button for Winter starting.

Normally can only be engaged with a high throttle setting and resets automatically when the engine starts.

>

[David]

We have a boat that lives on the canals in France. We visited it for the first time in 18 months last week (post Covid). Much to our surprise it was mostly in one piece, functional electrics (thanks to solar panels), not much water in the bilges, etc. Just needed a lot of cleaning and painting.

However, when I tried to start the engine it started OK but the governor appears not to be working and I had to shut it down immediately to avoid it over revving. I tried a few times with the same result every time.

It's a Mercedes OM312 'marinised' for boat use. It's a 4.5 litre, 6 cylinder, lorry engine from the 1950s or 1960s. I have a manual for it but it has no description or diagrams of the governor.

Does anyone here know about this sort of thing, or, more likely, know someone who does know about such things? I'm fairly OK with taking engines apart etc. (I took apart a Daimler v8 many years ago and have worked on lots of smaller engines since) but it would be nice to have some sort of idea how the governor works and where it is before I start taking things apart.

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This probably isn't going to help you very much but I have experience of Lucas CAV injection systems as used on diesel generating sets.

Firstly my guess is that your injection system is Bosch so what I say may not be relevant.

The governor on the Lucas CAV system is usually part of the injector pump and directly controls a rack which rotates the elements (as Lucas CAV call them), one per cylinder, which controls the amount of fuel delivered to the injector and the cylinder.

There may be an access plate for the governor which can be removed so you can see if there is any problem there. you may be able to push the control rack with your finger to see that it moves freely and returns to it's resting position which normally be for maximum fuel delivery - the fuel delivery is then decreased by the governor as the engine speeds up to it's nominal working speed - typically 1500RPM in the case of a DG set.

When a diesel engine is to be laid up for a period, it is usual to fill the tank with calibration fluid which helps to stop the injector pump from gumming up and run it for a few minutes to circulate it through the fuel system.

Now if the rack is stuck - stop. You will need to go to a Bosch service agent and get the pump repaired or get an exchange pump. Repair of fuel injection pumps requires specialist equipment.

I have experienced a stuck rack on one particular DG set - it was caused by incredibly fine rust particles in the order of microns from a rusty fuel tank getting through the paper fuel filters and coating the elements, which are machined to micron tolerances, the rust simply caused them to jam in the body of the pump rendering the governor ineffective.

Sorry for the possible bad news.

Good luck.

[Harry Bloomfield, Esq.]

Brian Gaff brought next idea :

I agree with Brian, it will be a fairly simple basic mechanism of bob weights flung out, mechanically linked to close the throttle/ limit the injector at a set speed. Not really much to go wrong - apart from at the injectors.

[David]

The injectors are probably OK - otherwise the engine wouldn't start.

Actually the injectors you can refurbish at home, all you need is a test pump capable of 175 bar (2500psi) to set the injector.

[Chris Green]

OP here. Yes, I think they probably are as it sounded fairly even when it started, just accelerated away without stopping!

:-)

[AJH]

If it's an inline pump, like a Simms or Minimec and the Bosch looks similar, there will be 4 ports, one for each injector. The governor controls a rack that rotates the pump pistons which have a helical spill off so the rotation controls the amount of fuel that is delivered, the rest of the stroke spilling off. The rack may be sticking and simply changing the lubricating oil may free it.

Yes this button when depressed allows the rack to go full travel, increasing the fueling for cold start, once the governor senses the over speed it moves the rack back and thus reduces the fueling. The button then pops out and stops the rack traveling past it until reset.

[David]

From what you described the injectors are OK.

If the engine is fitted with manual decompressors and you can hand crank it, with the stop/run lever in the run position and the decompressors enabled, you should be able to hear the injectors firing when hand cranked. When they fire it will sound like a squeek!

The type of pumps that were fitted to our engines were the Lucas CAV Minimec - such as

but there a dozens if not hundreds of different types within the model range.

In the photo you can see a cover with the words "Minimec CAV England" held on with two screws. The rack which I was referring to is underneath that cover which will move according to speed and load demand. That particular pump does not have a regulator which would normally be mounted on the right.

If you're interested in reading up on injector pumps, the following gives an excellent description of how the Minimec pump works

and

Remember the elements need to have micron finish and fit to delivery the required 2500psi to the injectors. From what I remember, there are no seals other than that provided by the precision fit.

You do have a test pump don't you?

I was curious to see what test pumps cost these days, and came up with this

Not bad price, and it goes up to 600 bar / 8000 psi. Now if you were forced through an injector nozzle at 8000 psi, you'd squeek!

[Chris Green]

Ah, OK, I hadn't realised that such a thing would be so cheap.