Yanmar engine compression

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K9

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I am trying to determine the proper compression for my yanmar 40HP diesel engine. My Yanmar factory service manual says the compression is "18.0". I assume that is 18:1 ratio. I see someplaces on the internet that give different formulas for turning this in PSI. One of the simple to understand says to multiply a factor of 14.2 x ratio value. That comes out to 255 PSI compression on my engine. That seems almost too low of a value in itself. I seem to find some consistency that suggests that diesels need at least 300 psi to start. I paid someone to do a compression test on my engine and he said I have 190 psi. but that is a cold engine. I cannot comprehend if the data is meaningful, since the engine should be warm for a compression check so you can guesstimate the ring status. Question... can anyone confirm I have the correct formula for determining my compression value I am looking for. As we all know, if it is on the internet, it has to be true. Thanks Ken
 

Tazza

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Being diesel, the compression should be over 300. 300 is the lower end of what a diesel will still start with. Kubotas when new are meant to have 450-500 i believe.
190 is very low, when he did the test, he should have squirted oil in the cylinders to do a wet test to see if the rings were by-passing or not.
I don't believe it needs to be hot to do a test, as generally when you do a compression test it's because the engine is not starting.
 

SkidRoe

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Hi k9, you were close in your calculation of static pressure. Rule of thumb atmospheric pressure is 14.7 psi (depending on your altitude). You also need to add one atmosphere to your calculation, as the starting point of the calculation is one atmosphere. With that, you have: (18 x 14.7)+14.7= 279.3 psi. As I said, this is static pressure only, not including any piston ring losses (blow-by). The one thing that this formula does not take into account at all is any of the "supercharging" effect you get from the inertia of the air rushing into the cylinder as the engine is cranked at speed. This will further increase the pressure achieved during a compression test, but is a difficult value to calculate by hand. Valve overlap, valve size, port design, intake manifold design, etc. all affect this.
 

jerry

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Hi k9, you were close in your calculation of static pressure. Rule of thumb atmospheric pressure is 14.7 psi (depending on your altitude). You also need to add one atmosphere to your calculation, as the starting point of the calculation is one atmosphere. With that, you have: (18 x 14.7)+14.7= 279.3 psi. As I said, this is static pressure only, not including any piston ring losses (blow-by). The one thing that this formula does not take into account at all is any of the "supercharging" effect you get from the inertia of the air rushing into the cylinder as the engine is cranked at speed. This will further increase the pressure achieved during a compression test, but is a difficult value to calculate by hand. Valve overlap, valve size, port design, intake manifold design, etc. all affect this.
But you also have to use a multiplier of 1.4 for heat expansion according to the source I found which would make the 279 increase to 390 or so. Easier to figure that if you stick a gauge in the hole you need at least 300 to light diesel fuel.
 

SkidRoe

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But you also have to use a multiplier of 1.4 for heat expansion according to the source I found which would make the 279 increase to 390 or so. Easier to figure that if you stick a gauge in the hole you need at least 300 to light diesel fuel.
Yes, that is true, but you won't generate too much heat during a compression test on a cold engine. The compression multiplier only applies to a running engine, and the factor is an exponent, not a multiplier. In this case, you should be seeing cylinder pressures in excess of 856 psi when the engine is running.
 
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K9

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Yes, that is true, but you won't generate too much heat during a compression test on a cold engine. The compression multiplier only applies to a running engine, and the factor is an exponent, not a multiplier. In this case, you should be seeing cylinder pressures in excess of 856 psi when the engine is running.
Thanks to all. I found a reference for my engine and it seems to be designed for 457 PSI with 357 psi minimum. I took the engine out last week and found that the 1st compression ring looks nice and shiny, but the 2nd compression ring looks rusty, the first and 4th piston look the roughest. The cylinders walls were a little glazed, but cleaned up nicely with the hone. I am waiting for parts now. I know what I want for christmas.
 

Tazza

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Thanks to all. I found a reference for my engine and it seems to be designed for 457 PSI with 357 psi minimum. I took the engine out last week and found that the 1st compression ring looks nice and shiny, but the 2nd compression ring looks rusty, the first and 4th piston look the roughest. The cylinders walls were a little glazed, but cleaned up nicely with the hone. I am waiting for parts now. I know what I want for christmas.
Hopefully santa will bring the parts needed to get it up and running again :)
 
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K9

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Hopefully santa will bring the parts needed to get it up and running again :)
Okay. I am stumped. I pulled the engine ove rthe christmas period and put new rings in it, as well as new head gasket and associated necessities. Of course, I honed the cylinders. I now have 430 plus on compression across all cylinders. I was excited to get the engine back in, only to find there was no reason to be. Still will not start. I adjusted the timing on the fuel pump because it was out of whack, but i am near certain it is now correct. My Yanmar factory service manual says to adjust the pump timing based on the application of the engine. It is 16 degrees BTDC for some applications and 26 degrees for another. Does anyone know what "VM", "CH" or "CL" would be for apllication? My guess is VM is marine vessel, but no idea on the others and surprisingly, the book does not explain. I call Yanmar and get voice mail that does not get returned. Even more important. Where do I turn to get that last piece to make this thing burn some diesel fuel? I have fuel up to the nozzles, I have compression, and plenty of air for the intake, but none inside the fuel system that I can spot. I have bled the system the best I know how. It has a manual prime lever, unfortunately, it does not do much except prime the fuel bowl. I wish it went all the way to the injectors. I get a like action out of a short burst of ether, but as soon as it is burned off, it is back to zero progress.
 

Tazza

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Okay. I am stumped. I pulled the engine ove rthe christmas period and put new rings in it, as well as new head gasket and associated necessities. Of course, I honed the cylinders. I now have 430 plus on compression across all cylinders. I was excited to get the engine back in, only to find there was no reason to be. Still will not start. I adjusted the timing on the fuel pump because it was out of whack, but i am near certain it is now correct. My Yanmar factory service manual says to adjust the pump timing based on the application of the engine. It is 16 degrees BTDC for some applications and 26 degrees for another. Does anyone know what "VM", "CH" or "CL" would be for apllication? My guess is VM is marine vessel, but no idea on the others and surprisingly, the book does not explain. I call Yanmar and get voice mail that does not get returned. Even more important. Where do I turn to get that last piece to make this thing burn some diesel fuel? I have fuel up to the nozzles, I have compression, and plenty of air for the intake, but none inside the fuel system that I can spot. I have bled the system the best I know how. It has a manual prime lever, unfortunately, it does not do much except prime the fuel bowl. I wish it went all the way to the injectors. I get a like action out of a short burst of ether, but as soon as it is burned off, it is back to zero progress.
The injectors are not plugged up are they?
Where are you getting fuel up to? the injector its self or just to the pump?
What sort of fuel shut off does it have? if it is electrical, it may not be turning the fuel on to the pump so the injectors don't get any.
You are so very close, great compression, just need to get it squirting fuel in!
 
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K9

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The injectors are not plugged up are they?
Where are you getting fuel up to? the injector its self or just to the pump?
What sort of fuel shut off does it have? if it is electrical, it may not be turning the fuel on to the pump so the injectors don't get any.
You are so very close, great compression, just need to get it squirting fuel in!
The injectors are all new. I have installed the fuel pipe while holding the injectors outside the engine and I see the 4 quadrant spray pattern I would hope to get. With that observation, I can only assume the fuel is shooting into the piston cylinder as it is supposed to. Someone suggested maybe the engine is too cold. It has been hovering around 40-45 degrees highs and lows in western washington. This engine only has a air preheater, not glow plugs. Any thoughts about that? I would think 40 degrees is pretty sissy if that is the answer. Thanks a bunch for your insight
 

Tazza

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The injectors are all new. I have installed the fuel pipe while holding the injectors outside the engine and I see the 4 quadrant spray pattern I would hope to get. With that observation, I can only assume the fuel is shooting into the piston cylinder as it is supposed to. Someone suggested maybe the engine is too cold. It has been hovering around 40-45 degrees highs and lows in western washington. This engine only has a air preheater, not glow plugs. Any thoughts about that? I would think 40 degrees is pretty sissy if that is the answer. Thanks a bunch for your insight
Even with low temperatures, from what you have explained with the spray pattern with 4 quadrants means it is direct injection. They are very good at starting in the cold. Even with the ether you have used, it should provide the initial boost to get it going.
I'd like to lean towards the injector timing being off just enough to be your issue. If it's not going in at just the right time, it will not fire.
You did say what the injector timing was, but you had multiple figures and was unsure what one was just right. I'm sure my Kubota was about 11 before TDC, you may want to fiddle a little with it.
If you are getting fuel to the injectors and good compression, it has to be the timing
 
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K9

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Even with low temperatures, from what you have explained with the spray pattern with 4 quadrants means it is direct injection. They are very good at starting in the cold. Even with the ether you have used, it should provide the initial boost to get it going.
I'd like to lean towards the injector timing being off just enough to be your issue. If it's not going in at just the right time, it will not fire.
You did say what the injector timing was, but you had multiple figures and was unsure what one was just right. I'm sure my Kubota was about 11 before TDC, you may want to fiddle a little with it.
If you are getting fuel to the injectors and good compression, it has to be the timing
Good news: Out of frustration and forced admission of defeat, I took the machine to its second diesel mechanic. Be assured I took it to another shop than my previous selection and it went much better this time. After telling him everything I had done, and how I did it, he decided to take the timing gear cover off and verify everything was okay in there. I never had it off and would never had suspected something was wrong in there. As it turned out, the idler gear was off 5 teeth and the "A" "B" "C" marks were not lined up. As soon as that was corrected, the machine started without making a full revolution of the cooling fan. As shared before, when I bought it, it was as is and the previous owner tod me he had it running until the fuel solenoid made a burning smell and was shut down and never breathed again. It is apparent that I should not have placed so much faith in salesman character. At least it is burning diesel fuel again. Time to make up for lost time. Thanks to all for the advice and counseling.
 

Tazza

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Good news: Out of frustration and forced admission of defeat, I took the machine to its second diesel mechanic. Be assured I took it to another shop than my previous selection and it went much better this time. After telling him everything I had done, and how I did it, he decided to take the timing gear cover off and verify everything was okay in there. I never had it off and would never had suspected something was wrong in there. As it turned out, the idler gear was off 5 teeth and the "A" "B" "C" marks were not lined up. As soon as that was corrected, the machine started without making a full revolution of the cooling fan. As shared before, when I bought it, it was as is and the previous owner tod me he had it running until the fuel solenoid made a burning smell and was shut down and never breathed again. It is apparent that I should not have placed so much faith in salesman character. At least it is burning diesel fuel again. Time to make up for lost time. Thanks to all for the advice and counseling.
So simple..... But at least it's going once more.
The strange thing is, you had compression, and good compression, i don't understand why you had that with the timing being off.
I know how frustrating it must be, but at least it was a simple fix, you did all the hard work and know it will be a good enghine now. With that compression, you should have many years of life left in it.
 

buckledownbilly

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Nov 25, 2022
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So simple..... But at least it's going once more.
The strange thing is, you had compression, and good compression, i don't understand why you had that with the timing being off.
I know how frustrating it must be, but at least it was a simple fix, you did all the hard work and know it will be a good enghine now. With that compression, you should have many years of life left in it.
 

buckledownbilly

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I have a problem with 4 cylinder yanmar. will not start until 1 squirt of parts and brake cleaner then it runs great. bought as is, trying to fix. have checked all fuel connections, getting fuel to nozzles. any help appreciated, thanks Billy
 

brdgbldr

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I am not familiar with your machine but here are some of the things that can cause this problem in gasoline engines.

Some machines need to be primed. This means moving the accelerator back and forth several times so fuel pumps into the intake before starting. Check the starting procedure for your machine in the operators manual.

A manual choke needs to be used when starting a cold engine. If it is an automatic or electric choke it may have gone bad. In some older cars you had to push down the accelerator pedal before starting to set the mechanical choke.

Hope this helps.
 

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