Yanmar low RPM under load

[foggysail]

Originally Posted by MikeFergie

Carboned piston caused by poor or no combustion
Causes - leaking valves
- rings not sealing
- injector not putting in enough fuel or not atomized fuel

Also restricted air intake

[Compass790]

Originally Posted by jmurray109

I am taking to machine shop to have new values installed. Hopefully this was the cause of the original problem.

Are you able to get pistons out? As A64 pilot & Wotname said earlier some 3GM30's had piston problems I'd check the ring gaps too if you can but still think it's quite likely that the valves leaking caused your problems. If you can feel a noticeable ridge at top of cylinder bore after you have cleaned carbon off it isnt a good sign. However some with bad ridges still run ok.

[jmurray109]

Originally Posted by Compass790

Are you able to get pistons out? As A64 pilot & Wotname said earlier some 3GM30's had piston problems I'd check the ring gaps too if you can but still think it's quite likely that the valves leaking caused your problems. If you can feel a noticeable ridge at top of cylinder bore after you have cleaned carbon off it isnt a good sign. However some with bad ridges still run ok.

After head is returned from machine shop, I plan to bolt it back on and everything will be perfect. Just in case I purchased a compression tester. If low compression, then next is to remove engine and redo pistons and rings. I should not have to do this because everything is going to work after this.

[Compass790]

Originally Posted by jmurray109

After head is returned from machine shop, I plan to bolt it back on and everything will be perfect. Just in case I purchased a compression tester. If low compression, then next is to remove engine and redo pistons and rings. I should not have to do this because everything is going to work after this.

Hope the stars are aligned for you. Probly gotta good chance. I wish you extremely high compression & let us watchers know what happens.

[jmurray109]

The top of the number 1 piston worries me. This was were the intake valve was not sealing. I still want to put the head back on and see what I get. Any thoughts. BTW I am still cleaning the surfaces, so don’t worry I am not done.

[Compass790]

On our engine the piston looked like that from salt water getting in thru cooling exhaust elbow failure. This erodes the top of the piston. The bore is likely to have water damage too. I cannot say wether it will be ok or not without pulling piston . Ours still ran like that BUT not nearly as good as with a new piston & rings in & better still when we put a new liner in. Is their any scoring in that bore? Cant see as yr dodgy piston is at top? If bore is scored you may have a broken ring or rings.
Are you doing this in the boat? If the engine is out of boat I'd recommend taking piston out & checking but can understand you wanting to try just the head job.If your head surfaces are a bit dodgy I'd recommend using hylomar or Loctite 518 or Heldite on gasket. Works for me.

[Compass790]

P.s. Good idea to tape over the end of your high pressure fuel lines that go to injector. You don't want any dirt from your cleaning to get in one of those as that can cause grief. Just noticed that in your pic. Can you post a pic/ pics from the inside of the bore of the dodgy piston?

[Wotname]

Good advice from Compass790. In addition I would take the opportunity to check the liner protrusion while the head is off. Although it is 99.9% sure to be OK, right now it only takes minutes to check. The manual will give the correct protrusion but in absence of the book numbers, I reckon 4 thou would be reasonable. Measure each liner "around the clock" i.e. 3,6,9 and 12 o'clock positions. I mention this as I recently had a YSE that was way out and the most creditable explanation was that it was that way since manufacture.

However my concern still remains with the evidence in the OP (emphasised below)

Originally Posted by jmurray109

I have a Yanmar 3QM30H and recently while in gear it will only go up to 1900 RPM. It also kicks out extra black unburnt fuel from the exhaust. While in neutral the motor RPM goes up to it normal level.
I have pulled off the mixing elbow (2 years old) and it still is in good shape. I removed the injectors and had them professional cleaned. I visibly inspected the exhaust manifold, while still attached to the motor. I could not see anything that stood out. Also while under load, I manually disengaged the compression from each cylinder. There was no change in motor sound on the middle cylinder. The other 2 you could hear the change in the motor.
So that brings me to the next question. What is it. The only thing else I thing I could do is to remove the exhaust manifold and inspected it better. Other than than, is the middle cylinder done. Any assistance would be appreciated.

Nothing I have seen in this thread so far explains why the second cylinder doesn't appear to be firing.
Wishful thinking is that it is an aberration and all will be sweet when you run it up next.

[Compass790]

Very good point that Wotname makes about 2nd cylinder. Need to know why that wasn't firing.

[jmurray109]

I cannot explain the 2nd cylinder. Prior to taking the head off I was able to get some gunk out of the intake and the de-compression lever started to work. It still was not as good as the 1 and 3, but there was a change. I am hoping after removing the head and doing a proper cleaning of the intake that this was not the problem and it was the leaky number 1 valve.

[Q Xopa]

Originally Posted by foggysail

Also restricted air intake

Also Injectors 'dribbling' instead of spraying a nice 'atomised' spray. This usually produces white smoke when its running from excessive unburnt fuel.

[Compass790]

Originally Posted by jmurray109

I cannot explain the 2nd cylinder. Prior to taking the head off I was able to get some gunk out of the intake and the de-compression lever started to work. It still was not as good as the 1 and 3, but there was a change. I am hoping after removing the head and doing a proper cleaning of the intake that this was not the problem and it was the leaky number 1 valve.

That maybe your explanation, could be the No.2 decompression lever wasnt working. On the piston tops it looks like No.2 was going at a guess

[jmurray109]

Sorry for the delay, but the job gets in the way of volunteering. So I put it back together and the good news was it runs. The bad news is it has the same problem and goes up to 16ish hole in gear. In neutral it runs great. So I had ordered and received a compression tester, so hopeful tomorrow I can check the compression. I think the number 1 cylinder will have some issues. If this is the case the motor will come out and get rebuilt. Sorry for the delay and once again thanks you for your help. BTW if the motor comes out it is going to a professional this time to do the rebuild.

[jmurray109]

Originally Posted by jmurray109

Sorry for the delay, but the job gets in the way of volunteering. So I put it back together and the good news was it runs. The bad news is it has the same problem and goes up to 16ish while in gear. In neutral it runs great. Attachment 178602So I had ordered and received a compression tester, so hopeful tomorrow I can check the compression. I think the number 1 cylinder will have some issues. If this is the case the motor will come out and get rebuilt. Sorry for the delay and once again thanks you for your help. BTW if the motor comes out it is going to a professional this time for the rebuild.

.
Does anyone have any recommendations for a shop in Southern California.

[Mike Banks]

As a general rule--if it starts it should run OK. If it runs at an idle but has no power when loaded--suspect the timing is out or the fuel is simply not getting through.
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