Engine revving up on its own accord

[afright]

Hi ,I have a problem with my BMC 1.5, I have just replaced my old engine with a rebuilt BMC 1.5,on returning from the boat yard back to my moorings the engine was lightly revving up and down,once back at the mooring we checked all pipes for leakage which there is none so we have bled the engine roughly about 6 times,it settle down for a while ticking over ok,but now it revs high then just cuts out,we had the fuel tank fitted around 8 years ago it is only a 30 ltr tank,which is all we need,would it be possible that this has some crud in the tank,which is causing the problem or is it anything else that we can check,regards.

[MtB]

Despite your checking, I think the evidence suggests there IS an air leak in the diesel supply that you are not finding.

 

I don't know this engine well though so others who do will be along soon to confirm or contradict....

 

 

[Tony Brooks]

Those hydraulic DPA pumps will tend to rev up if the presumer inside the pump body falls. Often a sign of fuel starvation so apart from air leaks it could be something simple like you forgot to turn the fuel tap on or you have a tap on the return that is turned off but it could just as easily be partially blocked filters, pump pick up pipe, fuel bug or an air leak.

[afright]

Thank you ,All filters are new  if its a fuel bug ,is there a additive to clear this ?,I do believe  its just a elimination  process.thanks for the feedback ,Mike and Tony.

[Slim]

Identical problems to those I had some years ago with my BMC 1.8.There was absolutely no sign of a fuel leak but all the symptoms  of air in the system. I even had the pump and injectors overhauled. In the end I pressurized the drained fuel lines and spotted very small bubbles at a joint. Remade the joint with a new fitting and BINGO problem solved. Try it, all you need is a bike pump and a bit of rubber tube.

 

Frank

[David Mack]

I had the same problem on a BMC 1.5 years ago . Turned out to be fuel starvation as a result of crud in the filter on the inlet to the injector pump (i.e. beyond the main fuel filter).

[Boater Sam]

Stop control not fully off? Cable may have been fitted a bit tight.

Edited October 5, 2019 by Boater Sam

[Karen Lea Rainey]

Had exactly the same some years ago with mine. I removed the fuel filter, cleared out the crud in it because it can block the passageway through the filter. Refitted the existing filter because I hadn't got a replacement on the boat and it never did it again.

[john.k]

And clean the fine mesh plastic strainer in the pump inlet......the only known way to clean these is with a reverse jet of pressure water......or buy a couple of new ones.....PS ..dont drop any springs or screws into the bilge when removing the fitting.

[Benno 1688]

I had the same after renewing the fuel line clean out the diesel tank and overhauling the diesel pump. I found out it was a worn lift pump when it was running it was not giving the engine enough fuel. Every few hours it would need bleeding took me months to figure it out.

 

[Tony Brooks]

1 hour ago, Benno 1688 said:

I had the same after renewing the fuel line clean out the diesel tank and overhauling the diesel pump. I found out it was a worn lift pump when it was running it was not giving the engine enough fuel. Every few hours it would need bleeding took me months to figure it out.

 

Not saying you are wrong but what you describe sounds more like less than tight screws between the two pump housings or a slightly porous diaphragm

 

The DPA pumps have a vane type transfer pump inside them that takes the lift pump output and boosts its pressure. The similar vane pump on the Bosch rotary injector pump is often used to do all the fuel pumping from the tank so with most narrow boat tanks being higher than the injector pump giving a positive head plus the priming effect of the lift pump I suspect the transfer pump was sucking a very small amount of air into the system. However why this was not passed back to the tank via the return from injector pump to filter line and then the 0.5mm hole into the leak off pipe to the tank I have no idea.

[Benno 1688]

On 06/10/2019 at 15:37, Tony Brooks said:

Not saying you are wrong but what you describe sounds more like less than tight screws between the two pump housings or a slightly porous diaphragm

 

The DPA pumps have a vane type transfer pump inside them that takes the lift pump output and boosts its pressure. The similar vane pump on the Bosch rotary injector pump is often used to do all the fuel pumping from the tank so with most narrow boat tanks being higher than the injector pump giving a positive head plus the priming effect of the lift pump I suspect the transfer pump was sucking a very small amount of air into the system. However why this was not passed back to the tank via the return from injector pump to filter line and then the 0.5mm hole into the leak off pipe to the tank I have no idea.

I repacked the oft pump that’s all I know and I never had the problem again

[john.k]

The AC lift pump was originally a cheapie produced specially for the 1934 Hillman Wizard....the cheapest  car on the market  in the worst year of the depression.....The more expensive AC pumps had valve chambers that could be unscrewed and cleaned,without disturbing the mounting.......somehow ,this cheap pump became the standard AC pump.