BMC 1.8 Odd revs increase, cause unknown.

[Tracy D'arth]

Happens very occasionally but I am stumped.

 

Narrowboat presents with 1.8D BMC diesel with weight pad on flywheel, clear prop, no smoke at all from exhaust.

This engine I know will tick over all day at 550 rpm.  Big prop, hospital silencer. Skin tank cooled

Serviced 150 hours ago, fuel filter not changed but inspected. No sign ever of bug. Has been run for the past month daily.

No water in tank, fuel separator or filter.  Uses 1 to 1.4 litres of fuel an hour.

No fuel leaks and no air leaks found on any fuel pipes or filter.

 

Starts instantly hot or cold, without heater.

Runs beautifully as it warms up, tickover on dropping revs, 550 rpm out of gear.

When cruising, fully warmed up, not overheating, every 10 to 15 minutes the revs will rise from the 1200 rpm cruising speed to around 1500 rpm for 4 to 8 seconds. Then it settles back to normal. If it is dropped to tickover in or out of gear it is too high at around 800 rpm and will not slow down further.

The common cause of revs rising on the 1.8 is momentary fuel starvation but this engine will run hard at 1800 rpm without any hesitation for minutes or until the canal runs out of water. So I have ruled starvation out, the tank has plenty of fuel in.

 

Stopping the engine briefly and restarting it is the same but stop it for a couple of hours it is as normal on restart.

I have known this engine for some 20 odd years since I rebuilt it. It has done around 6000 hours since without any real problems. DPA CAV pump was rebuilt then and it has had a new lift pump around 10 years ago because the original leaked around the top.

It is driving the owner to drink and I don't know what is causing it.

I could simply change the injection pump but I would rather establish the cause first.

 

Advice required please before I lose my marbles.

[Tony Brooks]

I think all it can be is loss of pressure on the governor valve so the governor thinks the engine has slowed down and opens itself up by spring pressure, but why I have no idea apart from some temporary loss of delivery from the lift and transfer pump.

 

I think that I would have a look at the strainer under the injection pump inlet connection hexagon, make sure the regulating valve is free to  slide in its housing and have a look at the ball valve in the return union in the filter head. Not that I have any idea why any of those could cause it. I have heard of a 1.5 that revved itself up occasionally until the lift pump was changed but, again, I can't see how that would come about unless a lift pump valve was playing silly beggars.

 

If none of that reveals a clue then I am stumped as well.

[beerbeerbeerbeerbeer]

I’m usually of little help but have to ask the obvious, is the air filter clean?

🤷‍♀️🤷‍♀️🤷‍♀️
 

 

 

 

[Tracy D'arth]

Thanks Tony, I'll get her to check the pump inlet and return banjo.

 

Thanks Goliath, not checked as this engine draws its air from under the cabin floor but the fact that it will produce as much power as asked of it for as long as it does without smoking would tend to suggest that it is clean enough. Air filters on boats don't tend to be necessary, no dusty  roads!

I do know that it is a foam K&N filter, I fitted it.   I have however thought that the engine breather could by now have dropped some oil into the air cleaner which would cause some revving up or even a runaway if excessive but the fact that the tickover stays up and that the period of revving is brief would suggest that this is not the case.

[Tony Brooks]

8 minutes ago, Goliath said:

I’m usually of little help but have to ask the obvious, is the air filter clean?

🤷‍♀️🤷‍♀️🤷‍♀️

 

Unlikely because if it was clogged enough to do anything it would probably slow down with more exhaust smoke and Tracy would have noticed that. If it is a typical BMC wire mesh filter I can't see one blocking between services unless there are some very hairy dogs aboard. If it is a paper element filter with crankcase breather into the housing then it might block. However, that has set me off on anther train of thought.

 

Could it be gas finding its way into the engine bay. I did hear of a leaking aerosol can causing something similar, do they  tore any fast thinners with the engine?

[beerbeerbeerbeerbeer]

1 minute ago, Tony Brooks said:

 

Unlikely because if it was clogged enough to do anything it would probably slow down with more exhaust smoke and Tracy would have noticed that. If it is a typical BMC wire mesh filter I can't see one blocking between services unless there are some very hairy dogs aboard. If it is a paper element filter with crankcase breather into the housing then it might block. However, that has set me off on anther train of thought.

 

Could it be gas finding its way into the engine bay. I did hear of a leaking aerosol can causing something similar, do they  tore any fast thinners with the engine?

cheers, only way to learn is ask, 

thanks for explaining why it most likely isn’t the air filter. 
 

(and Tracy says it’s a K&N filter, something I thought only went on high performance petrol engines) 

[Tony Brooks]

3 minutes ago, Tracy D'arth said:

Thanks Tony, I'll get her to check the pump inlet and return banjo.

 

I am not on about the banjo on the filter, I am talking about the valve that screws into the "in" port on the filter head that is connected to the union closest to the engine block on the injection pump. It would be worth ensuring the 0.5mm hole in that banjo bolt is clear just in case air is building up in the top of the filter and randomly passing a bubble into the injection pump. That just might give such symptoms as the                air bubble passes through the pump.

 

Unless I was absolutely sure of their competence I would never advise a boater to take the big hexagon out of the pump inlet, too many small springs and the small regulating valve in there that might take flight. Also, it needs to be kept absolutely clean.

 

 

[Tracy D'arth]

5 minutes ago, Tony Brooks said:

 

Unlikely because if it was clogged enough to do anything it would probably slow down with more exhaust smoke and Tracy would have noticed that. If it is a typical BMC wire mesh filter I can't see one blocking between services unless there are some very hairy dogs aboard. If it is a paper element filter with crankcase breather into the housing then it might block. However, that has set me off on anther train of thought.

 

Could it be gas finding its way into the engine bay. I did hear of a leaking aerosol can causing something similar, do they  tore any fast thinners with the engine?

Pretty sure nothing in the engine hole but I will check that they have not spilled anything in the cabin which has gone through the floor as the air for combustion and cooling is from under the cabin sole.  The periodic timing is what is so weird and the continued increase in tickover.

[Tony Brooks]

2 minutes ago, Goliath said:

cheers, only way to learn is ask, 

thanks for explaining why it most likely isn’t the air filter. 
 

(and Tracy says it’s a K&N filter, something I thought only went on high performance petrol engines) 

 

3 minutes ago, Goliath said:

cheers, only way to learn is ask, 

thanks for explaining why it most likely isn’t the air filter. 
 

(and Tracy says it’s a K&N filter, something I thought only went on high performance petrol engines) 

 

You can fit them on anything, but I am not sure I would bother on a BMC. I expect it has the foam mesh version of the old wire mesh filters and the foam disintegrated over time.

[Tracy D'arth]

1 minute ago, Tony Brooks said:

 

I am not on about the banjo on the filter, I am talking about the valve that screws into the "in" port on the filter head that is connected to the union closest to the engine block on the injection pump. It would be worth ensuring the 0.5mm hole in that banjo bolt is clear just in case air is building up in the top of the filter and randomly passing a bubble into the injection pump. That just might give such symptoms as the                air bubble passes through the pump.

 

Unless I was absolutely sure of their competence I would never advise a boater to take the big hexagon out of the pump inlet, too many small springs and the small regulating valve in there that might take flight. Also, it needs to be kept absolutely clean.

 

 

Got you.  Looks like another visit to the boat is called for. Trouble is I will have to cruse it for an hour get it warm enough for it to play up.

Just now, Tony Brooks said:

 

 

You can fit them on anything, but I am not sure I would bother on a BMC. I expect it has the foam mesh version of the old wire mesh filters and the foam disintegrated over time.

If it did disintegrate I reckon the engine would have eaten it by now. Worth checking though, I'll take the inlet duct off the manifold. 

[Tony Brooks]

This topic is a brilliant example of the difficulties offering advice. I was more than happy to suggest to Tracy that she had a go at the pump inlet, but there are few others I would advise to do the same.

[Tracy D'arth]

I will heed the advice. I have never actually dismantled a DPA pump, I always give them to a diesel specialist as I have neither the spares or the clean conditions for assembly and test.

[Tony Brooks]

Just now, Tracy D'arth said:

I will heed the advice. I have never actually dismantled a DPA pump, I always give them to a diesel specialist as I have neither the spares or the clean conditions for assembly and test.

 

If you clean the area first and put the bits in a pot of clean diesel as you take them out and work from a digram of the regulating valve it is not difficult. Only use lint free cloth to avoid fibres getting inside the pump. There are no adjustments to worry about under that hexagon.

[StephenA]

When its at tick over is it really at tick over - is the throttle resting on the stop?  Basically is the increase in speed it actually speeding up to the proper speed with the rest of the time it's actually running slow?

[Tracy D'arth]

4 minutes ago, StephenA said:

When its at tick over is it really at tick over - is the throttle resting on the stop?  Basically is the increase in speed it actually speeding up to the proper speed with the rest of the time it's actually running slow?

I checked this. The throttle ( not really but we all call it that ) is closed onto the stop yet the tickover is raised. I have checked the cable, Morse control and the spring on the throttle, all as should be.  I can reduce the tickover speed by winding in the stop but it is then too slow with the engine cold.

The speed is increasing by some means not to do with the linkage. It does depend on temperature and is repeatable every time the engine gets to running temperature.

[Tony Brooks]

Here are some diagrams that may help:

 

https://www.fujiyachts.net/manuals/CAV DPA Pump Rebuild Manual.pdf  Page 2

 

and this which is my simplified diagram of how it works. I think the top spring actually sits on the valve itself.

I think I put a description in the FIE notes on my website - not the maintenance notes.

Edited August 20, 2022 by Tony Brooks

[Tracy D'arth]

3 minutes ago, Tony Brooks said:

Here are some digrams that may help:

 

https://www.fujiyachts.net/manuals/CAV DPA Pump Rebuild Manual.pdf  Page 2

 

and this which is my simplified diagram of how it works. I think the top spring actually sits on the valve itself.

So, if the valve is sticking the transfer pump pressure cannot bleed back to the lift pump,   BUT the lift pump valves will prevent flow back so the pressure bleeds into the transfer pump inlet.

Thanks for the diagram, I have the CAV manual somewhere and a reseal kit which I bought 30 years ago and never opened!

[Tony Brooks]

1 minute ago, Tracy D'arth said:

So, if the valve is sticking the transfer pump pressure cannot bleed back to the lift pump,   BUT the lift pump valves will prevent flow back so the pressure bleeds into the transfer pump inlet.

Thanks for the diagram, I have the CAV manual somewhere and a reseal kit which I bought 30 years ago and never opened!

 

Not quite what I was thinking. Transfer pump pressure will always be greater than lift pup pressure when the engine is running so my thought was that if the valve stuck up a little it would allow transfer pump pressure to leak back towards the lift pump and that may well hold the lift pump diaphragm down. The only "seal" you should need to worry about is the soft washer under the big hexagon. I think it is copper and will probably reseal OK, but f not I would just anneal it.

[David Schweizer]

2 hours ago, Tracy D'arth said:

When cruising, fully warmed up, not overheating, every 10 to 15 minutes the revs will rise from the 1200 rpm cruising speed to around 1500 rpm for 4 to 8 seconds. Then it settles back to normal. If it is dropped to tickover in or out of gear it is too high at around 800 rpm and will not slow down further.

 

The BMC 1.5 in Helvetia did exactly the same about 300/350 hours after the fuel filter had been changed. Changed filter and everything was back to normal.

 

 

Edited August 20, 2022 by David Schweizer

[Bod]

1 hour ago, Tracy D'arth said:

I checked this. The throttle ( not really but we all call it that ) is closed onto the stop yet the tickover is raised. I have checked the cable, Morse control and the spring on the throttle, all as should be.  I can reduce the tickover speed by winding in the stop but it is then too slow with the engine cold.

The speed is increasing by some means not to do with the linkage. It does depend on temperature and is repeatable every time the engine gets to running temperature.

Temperature dependant.  Has any work been done on the cooling system?

Pipes moved, cables moved against or away from hot pipes/exhaust.

Thermostat changed, increased speed for a short time, thermostat opens and shuts in about the same time scale.

Random thoughts barking up the wrong tree......

 

Bod

[Tony Brooks]

If anything external causes the engine to rev up the transfer pump should up the pressure on the governor valve and shut down a bit, reducing the fuel flow and slowing the engine. We do know that partial fuel starvation lowers the transfer pump pressure so the governor valve move to increase the fuel flow more than the fuel starvation so the engine revs up.  This is why I still think it may be (no more than may be) transfer pump pressure related, but how or why I have no real idea.

 

I suspect David's problem may well relate to the pore size in the filter so having caught some contaminates it could not pass enough fuel at times.

[David Schweizer]

A lot of talk about dismantling various components, why don't you just change the fuel filter and see if that solves the problem. I have met several people who have had the same issue and resolved it by changing the fuel filter. My only concern about that diagnosis is that Tony Brooks has not suggested it.

[Tony Brooks]

1 minute ago, David Schweizer said:

A lot of talk about dismantling various components, why don't you just change the fuel filter and see if that solves the problem. I have met several people who have had the same issue and resolved it by changing the fuel filter. My only concern about that diagnosis is that Tony Brooks has not suggested it.

 

It certainly would not hurt given the price of the filter. The reason I have not suggested it is that I would expect Tracy to have considered that.

[Bee]

I only have two thoughts. 1) change the filter because its easy and 2) these things sometimes speed up just before they run out of fuel so I wonder if it is sucking a tiny bit of air from the filter or bleed screw from time to time. That's as far as go with these things - pumps frighten me.

[Tracy D'arth]

1 hour ago, Bod said:

Temperature dependant.  Has any work been done on the cooling system?

Pipes moved, cables moved against or away from hot pipes/exhaust.

Thermostat changed, increased speed for a short time, thermostat opens and shuts in about the same time scale.

Random thoughts barking up the wrong tree......

 

Bod

Negative I am afraid, nothing changed.