Priming - not Bleeding - Yanmar 3GM

[Slowboat35]

I've just rebuilt a 3GM after cylinder head and am preparing it for first start.
I haven't touched the metering unit or injectors but of course the delivery fuel lines between metering unit and injectors are empty of fuel. I have bled air as far as the metering unit using the lift pump.

Will the lift-pump produce enough pressure to fill the lines and purge the air at the injectors or must I now motor the engine to use the HP pump to do the job?

 

[RichardS]

Why not turn it over on the starter for a few bursts and see if it self-bleeds?

Richard

 

[black mercury]

Or turn it over on the starter with the injector pipes slackened at the injectors until you get diesel. Should only take a few seconds if you have diesel up to the pump. Once you see diesel, tighten unions and away she should go.

 

[lw395]

Yanmars are supposed to self - bleed if you get fuel to the injector pump.
Spin it over decompressed, good to get some oil around it?

 

[vas]

Yanmars are supposed to self - bleed if you get fuel to the injector pump.
Spin it over decompressed, good to get some oil around it?

just check your seawater supply not to flood back the cylhead if you do a lot of decompressed cranking. Better turn seacock off, or even better slacken the seawater pump pulley.
having spent the last couple of months bleeding a 2gm daily due to the crappy engine filter leaking air in (tank lower than filter) and an even worse CAV pre filter leaking generally, I can confirm that once it reaches the high pressure pump, you're good to go (with a few extra cranking sessions....)

V.

 

[Slowboat35]

Thanks. Got fuel to the injectors now by cranking decompressed. Engine is on the bench so no worries about flooding the exhaust.

 

[Fmurchu]

As a matter of interest, how old and hours on your GM before carrying out the work.

 

[Slowboat35]

Unknown age and hours I'm afraid but I'd say well used. Boat is '92, broker claimed engine had 1500hrs but that's just a number the owner told him. To my surprise therre was no wear detectable in the bores whatsoever, just well glazed so maybe the 1500hrs wasn't far out. Neither was there any wear on valve stems or anywhere else. Bores were glaze-busted, new rings of course, and valves reground.

But I've got a problem. I've bled the damn thing seven or eight times now but there's not a peep on trying to start it. I'm wondering if there's somethng fundamental I've missed but I've followed all the instructions to the letter. I'm not at all convinced there is any fuel leaving the injectors so may have to pull one this afternoon to see if it's spraying. There's no strong diesel smell at the exhaust elbow, just a little vapour after a starting attempt which again dosn't smell strongly of fuel.
Can the injector itself get drained of fuel and thus full of air when the head is off and upside down on the bench? If so do they self-purge? If a tiny bit of (compressible) air in the lines prevents the injector cracking then an injector body full of air will presumably do the same. Is this likely/possible?

 

[black mercury]

Once you have diesel up the pipe to the injector it should bleed through the injector without a problem.
You can put an ear to the injector or lightly touch the injector as you turn the engine over and you should hear or feel it squeak. If you do then it's firing.
An engine that has had major work can be difficult to start on the first attempt until things settle and bed in. Don't loose heart.

 

[Slowboat35]

I've pulled an injector and it's cracking just fine.
I can't understand how an engine that started, albeit slowly, now refuses to fire at all after a valve regrind and new rings. The only think I can think of is lack of compression but how? I haven't disturbed anything else that could affect starting - like injector timing for instance - what else is there?
Could the non-Yanmar head gasket be so much thicker that it prevents the engine from firing?

 

[black mercury]

Does it smoke from the exhaust at all when you are trying to start?
Have used non genuine head gaskets before on yanmars with no problems.
I still think you have low compression which will sort itself out once you get it running. But valve clearances all correct?
A little oil added to each cylinder and engine turned over by hand after to make sure it doesn't hydraulic may help to get it going. Or a blow lamp down the air intake.

 

[vas]

cannot really help other than when the mechanic who did the rebuilt on mine (v.experienced guy) tried to start the thing, took him some time. That was a total dismantle and rebuilt though...
I guess you removed the conrod bolts from the sump and pulled the piston/conrod assembly out to replace rings. So I guess no way to alter timings.

 

[Slowboat35]

Does it smoke from the exhaust at all when you are trying to start?
Have used non genuine head gaskets before on yanmars with no problems.
I still think you have low compression which will sort itself out once you get it running. But valve clearances all correct?
A little oil added to each cylinder and engine turned over by hand after to make sure it doesn't hydraulic may help to get it going. Or a blow lamp down the air intake.

There's vapour at the exhaust elbow and the injector I pulled was cracking just fine.
The pistons and new rings were flooded with oil on assembly so hard to see there's a loss of compression there. It's not as if its stumbling or missing, there's no sign of life at all, even on jump leads from a running car engine to ensure max voltage.

Someone suggested it could be injector timing out. How may degrees out does it have to be to stop an engine firing? What's the tolerance in degrees between an engine to run well or run badly?
is it feasable that a head-off could affect injection timig - I can't see how.

Any diesel engine mechanics able to comment?

 

[black mercury]

You only had the cylinder head off. The pump is cam driven and geared to the crankshaft, so it can't have slipped. You are getting diesel injected, so the only other thing is compression. If you are certain valve clearances have been adjusted correctly, you may have to do a compression test. If you turn it by hand with a socket on the crankshaft or by starting handle can you compare with how it used to be, although if you never tried this before it would be difficult to judge. You may have to resort to a liitle easy start sprayed on a cloth and the cloth held up to the air intake.

 

[vas]

along the same lines, could it be that something got mixed up on reassembly re decompression levers???
No idea if it's even possible!
Are they all down , lift one at a time, turn by hand and feel the pressure, swap, check again compare?

 

[black mercury]

along the same lines, could it be that something got mixed up on reassembly re decompression levers???
No idea if it's even possible!
Are they all down , lift one at a time, turn by hand and feel the pressure, swap, check again compare?

Yes, a quick way to check for this is to try it to start it with the rocker cover removed. That would be one part ruled out.

 

[Slowboat35]

Decomp levers on a Yannie just sit over the reocker as you drop the cover on - they work fine and you can hear the valve pop shut when you release them and feel the slack in the lever.
Feeling the compression is hard as with new rings and a honed bore the friction is quite different.

Compression testing today and getting a can of easystart.

 

[Fmurchu]

I see that you've found the problem. Good for you. If I ever get work done on my 3gm30 I'll try and remember that!!!!!

 

[JumbleDuck]

Will the lift-pump produce enough pressure to fill the lines and purge the air at the injectors or must I now motor the engine to use the HP pump to do the job?