A reader's question about engines gets answered by one of PBO's friendly experts

QUESTION:

I’m an elderly retired marine engineer, and am helping my friend who has starting problems with his Yanmar 2GM.

If the engine is left for less than 24 hours since the last run there’s no problem in starting it, but if left for much longer than that it takes about six or more attempts to start – which takes a lot out of the battery.

I checked for diesel leaks and found none, so I then suspected a fault in the lift pump. I replaced it with a new one but that made no difference. Eventually I found a small leak on the fuel filter housing (between lift pump and injection pump) so that too was replaced, but again didn’t solve the problem.

I’ve also made some other checks and observations:

  • It seems to me that fuel is not getting to the injection pump until the starter has been operated for some time – but operating the lift pump manually prior to starting doesn’t help.
  • Fuel level in the tank is above the lift pump, so the suction pipe must be under slight pressure.
  • I have ruled out problems with the injection pump, injectors and compression because the engine, once started, runs well with little or no exhaust smoke.
  • All fuel filters are clean.

I am now running out of ideas, and wonder if you might have come across this problem before?

Robert Mackay
St Andrews, Fife

PAT MANLEY REPLIES:

The thought that it’s a fuel problem is logical and your process of elimination is sound. It is worth taking into consideration whether the problem suddenly occurred or developed over some period of time.

If using the lift pump manually before starting makes no difference, I’d suggest you explore this a little further. You could bleed the engine via the fuel filter bleed screw before the attempt to start from cold to check that there is indeed no air. An extension to this would be to crack the injector lines and bleed these as well. If there is fuel all the way to the injectors and the engine still doesn’t start within 10 to 15 seconds, then I think we can rule out air in the fuel line.

Next, I suggest you turn the engine over for the equivalent time that it normally takes to start, but with the decompressors in use. It’ll be easier for the batteries to crank and will ensure that fuel, if available, will be injected. Then try starting normally.

The fact that the engine runs normally once started is not a definite indication that the compression is OK.

The actual temperature of the cylinder head is important, and even after quite a few hours the head may be sufficiently warm to allow the engine to fire up.

The 2GM has a pre-combustion chamber under the fuel injector but no pre-heating, so having sufficient compression to achieve combustion is important. The compression can be checked by removing the injector and using a compression tester.

It’s worth noting that it is possible to bend the connecting rod(s) by having water in the combustion space, and bent rods will reduce the compression ratio of the engine. Yanmar uses con-rods that will bend so that no more serious damage occurs in the event of hydraulicing.

This can be checked by measuring the ‘top clearance’ between the piston at TDC (top dead centre) and the cylinder head.

Here’s how to do it:

  • Remove both the injector and the pre-combustion chamber.
  • Lower the piston.
  • Insert some 1.2mm diameter fuse wire into the space and then crush the wire by raising the piston to TDC by hand.
  • Lower the piston, remove the wire and measure the thickness of the crushed part. The top clearance should be 0.68-0.88mm.

The full procedure is given on page 2-21 in the Yanmar 1GM10 service manual, which is helpfully available online at www.yanmarmarine.co.uk

Finally, one last thought: how old is the fuel? Blended diesel fuel with a bio content ages quite rapidly and suffers from a lowered cetane number, which can cause starting problems from cold.

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