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Old 27-08-2012, 10:43   #1
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Yanmar 3GM30F Rebuild Rookie - no compression

Greetings

I'm attempting to rebuild my Yanmar 3GM30F. I had the head/valves done at a local diesel shop with an excellent reputation. We have the engine mostly reassembled except for the valve cover, exhaust manifold, etc.

All of the timing marks line up as specified in the manual. I can put a socket on the front pully and turn the engine easily by hand, there doesn't seem to be any compression. On what I assume must be the exhaust stroke air is ejected through the exhaust port; however if I lightly wad up a piece of paper towel and insert it into the same cylinder's exhaust port the paper towel is sucked INTO the engine when it's turned over by hand.

I'm turning the engine clockwise when facing the front (non flywheel) side.

Any ideas? I know I'm probably missing something simple here.

Thanks

Gary
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Old 28-08-2012, 18:59   #2
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Re: Yanmar 3GM30F Rebuild Rookie - no compression

Injectors installed?
Valves adjusted?
Rings properly aligned at assembly?

Do you get "suction" on the intake stroke through the air intake?

You should be able to look up the exact valve timing in the manual but...

Compression Stroke - At or near BDC the intake valve closes and the piston moves up. Both valves are closed.
Power Stroke - Near and just before TDC the fuel is injected and ignites. The piston passes TDC and moves down. Both valves are closed.
Exhaust stroke - Near BDC the exhaust valve opens. The piston moves up and spent gas is expelled.
Intake stroke - Depending on the engine their may be an overlap where both the intake and exhaust valve are open near TDC. It has been explained to me that the moving "mass" of the spent exhaust helps evacuate the cylinder and draw the intake charge in monetarily.

So - if you valves have desigend overlap at the top of the exhaust stroke what you are seeing with the towel "could" be normal.

The correct tools to troubleshoot this - IMHO are...

- Differential compression tester. This puts a continuous charged of compresed air in the cylinder and differential compression in measured across two gauges with a fixed orifice.

- Piston position indicator - This is a guage that mounts on the injector hole with a pin that contacts the piston to measure TDC - then using a protractor on front pulley the valve opening angles can actually be measured.

Finally (back to simple) - I don't know how fast you are "hand turning" the front pulley but if not done with some gusto, any compressive charge will leak past the rings.

Without the proper tools I would rig up a battery to the starter with a proper remote starter swith (you can but at almost any auto store) and rotate the engine with the starter and feel the intake suction and exhaust ports.
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Old 29-08-2012, 13:05   #3
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Re: Yanmar 3GM30F Rebuild Rookie - no compression

Thanks for the reply.

We are hand turning very slowly, we'll get the starter hooked up and give that a go.

Thanks

Gary
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Old 29-08-2012, 13:15   #4
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Re: Yanmar 3GM30F Rebuild Rookie - no compression

There are compression release(s) on the head, I assume they are closed...
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Old 05-11-2012, 08:28   #5
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Re: Yanmar 3GM30F Rebuild Rookie - no compression

Quote:
Originally Posted by Ex-Calif View Post
Injectors installed?
Valves adjusted?
Rings properly aligned at assembly?

Do you get "suction" on the intake stroke through the air intake?

You should be able to look up the exact valve timing in the manual but...

Compression Stroke - At or near BDC the intake valve closes and the piston moves up. Both valves are closed.
Power Stroke - Near and just before TDC the fuel is injected and ignites. The piston passes TDC and moves down. Both valves are closed.
Exhaust stroke - Near BDC the exhaust valve opens. The piston moves up and spent gas is expelled.
Intake stroke - Depending on the engine their may be an overlap where both the intake and exhaust valve are open near TDC. It has been explained to me that the moving "mass" of the spent exhaust helps evacuate the cylinder and draw the intake charge in monetarily.

So - if you valves have desigend overlap at the top of the exhaust stroke what you are seeing with the towel "could" be normal.

The correct tools to troubleshoot this - IMHO are...

- Differential compression tester. This puts a continuous charged of compresed air in the cylinder and differential compression in measured across two gauges with a fixed orifice.

- Piston position indicator - This is a guage that mounts on the injector hole with a pin that contacts the piston to measure TDC - then using a protractor on front pulley the valve opening angles can actually be measured.

Finally (back to simple) - I don't know how fast you are "hand turning" the front pulley but if not done with some gusto, any compressive charge will leak past the rings.

Without the proper tools I would rig up a battery to the starter with a proper remote starter swith (you can but at almost any auto store) and rotate the engine with the starter and feel the intake suction and exhaust ports.
Thanks for the advice, turns out we weren't turning it fast enough to feel compression.
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Old 11-11-2012, 18:00   #6
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Re: Yanmar 3GM30F Rebuild Rookie - no compression

I ran in to this issue on the same model engine. Before you go any ferther check the exhaust manifold and riser. The engine I was working on had a clogged riser and exhaust manifold the carbon and gook was so thick the engine couldn't breath.
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