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Old 14-03-2011, 22:41   #1
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Yanmar Engine Compression Test Values

I have a boat with the measured compression of all 4 cylinders is 240psi which the mechanic is considering "normal". When I challenged him and suggested that seemed a bit low he responded by saying:
"Due to the differences in gauges, procedures and other factor in a compression test on a diesel engine the
most important thing that any mechanic looks for is the differences between cylinders. The fact is that
all the cylinders were the same. To get the top compression one has to turn the engine over hot at
a very high rpm. This is just not possible. As I said the readings were normal which meant the
rings are good."


I'm curious what others think of this comment?

Kent
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Old 15-03-2011, 03:43   #2
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Re: Yanmar Engine Compression Test Values

which Yanmar?
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Old 15-03-2011, 06:15   #3
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Re: Yanmar Engine Compression Test Values

I would agree w the mechanic. As pistons heat up they expand, forming a better seal, more compression etc. I would be more concerned with cylinder variation.
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Old 15-03-2011, 06:17   #4
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Re: Yanmar Engine Compression Test Values

I work for a Yanmar dealer in the summer. The guy is correct, in the field, the most important finding of a compression test is to compare the readings between the cylinders of the tested engine, rather than compare the numbers to some engineer's benchmark. The theoretical compression is produced from a running motor; how are to you propose to measure that in the field standard mechanic's tools?

On what did you base your belief that 240psi is "a bit low"?
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Old 15-03-2011, 09:13   #5
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Re: Yanmar Engine Compression Test Values

Kent,

I teach advanced automotive diagnostics. As you know..many of the technologies from the automotive field eventually transgress to teh boating industry. Moreover, testing a gas/diesel motor are the same.

I prefer to work smarter, not harder. The easiest, quickest test for compression testing is using a lab scope with a high amp current probe. This allows you to look at ALL cylinders at once and determine any variations. Add more channels to sync from the TDC source (Crank Sensor) and you can now determine mechanical timing. !

However, doing the math would be:

p{TDC} = 1 bar X CR^{1.4}

p=compression pressure @ TDC
CR=Compression Ratio of Motor Under Test

15:1 = 44.3 bar/642 PSI
18:1 = 66.3 bar/961 PSI
22:1 = 75.8 bar/1099 PSI

However, the above does not take into consideration losses due to cam profiling,
and mostly heat generated and absorbed by motor metal and cooling system.

Good rule of thumb is 15 - 20 times compression ratio.

So, the above would translate to:

15:1 = 225-300 PSI
18:1 = 270-360 PSI
22:1 = 330-440 PSI

Lastly, remember gauge accuracy is critical..which is why cylinder variation should be considered.

Doug
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Old 15-03-2011, 09:22   #6
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Re: Yanmar Engine Compression Test Values

Quote:
Originally Posted by ArmyChief View Post
The easiest, quickest test for compression testing is using a lab scope with a high amp current probe.
Where exactly does the high amp current probe attach to a diesel? Is it measuring electrical current?
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Old 15-03-2011, 10:03   #7
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Re: Yanmar Engine Compression Test Values

Pressure changes in the cylinder relate to different amperage draws on the starter. So, as the piston is traveling up during the compression stroke, the starter has to work harder..drawing more current. Using a scope to graph these transistions quickly analyzes all cylinders.
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Old 15-03-2011, 10:25   #8
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Re: Yanmar Engine Compression Test Values

Quote:
Originally Posted by ArmyChief View Post
Pressure changes in the cylinder relate to different amperage draws on the starter. So, as the piston is traveling up during the compression stroke, the starter has to work harder..drawing more current. Using a scope to graph these transistions quickly analyzes all cylinders.
Ah, that makes sense. But, isn't it really just comparing the readings between all cylinders, rather than giving an empirical compression value? I mean, how would you account for a weak battery or poor cable connections to the starter?

I understand your test, but how does it solve the OP issue that 240PSI "sounds a little low" to him? What would you say to him if the Amp probe test shows all cylinders at 240psi, and he says "that sounds a bit low"?

Not trying to be augmentative, but I'm not seeing that your test provides a different outcome than using a mechanical compression gauge in each cylinder, and analyzing the results by comparing all the readings to each other.

I do see how much faster and easier your test method would be. But, How many diesel mechanics carry a lab scope in their box?
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Old 15-03-2011, 10:38   #9
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Re: Yanmar Engine Compression Test Values

Quote:
Originally Posted by doug86 View Post
Ah, that makes sense. But, isn't it really just comparing the readings between all cylinders, rather than giving an empirical compression value? I mean, how would you account for a weak battery or poor cable connections to the starter?
Doug, not taking your questions as augmentative...it's what I do !
Anyway, you are correct in the fact that you are not getting a PSI number. That number..usually on a compression gauge is what techs/mechanics (I will not get into what the difference is here..lol) are used too. Weak battery is always a possibility, however, no testing should be done without a good battery or jump box attached. Moreover, there are two main figures (not counting the actual signature of the pattern) a tech looks at. 1. average current draw and 2. most importantly the value from the lowest to the highest..PER cylinder of current draw. It's this reading that will absolve the battery cable/battery condition. Thais reading tells the tech the amount of current each cylinder drew ... which relates to individual cylinder compression. Kinda like air conditioning theory, in the fact that as temperature of the refrigerant rises, so does pressure of gas.

Quote:
Originally Posted by doug86 View Post
I understand your test, but how does it solve the OP issue that 240PSI "sounds a little low" to him? What would you say to him if the Amp probe test shows all cylinders at 240psi, and he says "that sounds a bit low"?
Again, you cannot exactly convert amperage to PSI..but what you can do is look at the scope pattern signature to determine individual cylinder differences. Also, look at the delta from BDC to TDC current draw per cylinder and depending on what the customer concern is..for example:

1. Rough Running (misfire) - you disable fuel, scope-> current probe-> crank..you see one cylinder compression (current draw) significantly lower than the others..your done..it has internal engine concerns...took you 10 minutes. Removal of glow plugs is not fun..especially on some engines.

Not trying to be augmentative, but I'm not seeing that your test provides a different outcome than using a mechanical compression gauge in each cylinder, and analyzing the results by comparing all the readings to each other.[/quote]

Quote:
Originally Posted by doug86 View Post
I do see how much faster and easier your test method would be. But, How many diesel mechanics carry a lab scope in their box?
Thanks..been using and teaching this technique for years. You should see the techs faces light up. Not many mechanics have a scope...but more technicians are

Thanks.

Doug
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Old 15-03-2011, 10:53   #10
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Re: Yanmar Engine Compression Test Values

Doug, I'm with ya now. It all makes sense (except perhaps the part where I invest $$ for a scope ).

So, what is your response to the original question posed by Kent?
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Old 15-03-2011, 10:57   #11
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Re: Yanmar Engine Compression Test Values

Doug,

Since I've never seen a Yanmar..and another member asked which Yanmar...and no reply. I cannot give a definitive answer.

There are a few unanswered questions:

1. Why is compression testing being performed? (The condition was not stated...No Start, Low Power, Rough Running..etc)
2. What is the compression ratio of the Yanmar being tested? (With that..using the calculations I provided..you could get a decent first approximation)

Lastly, depending on the reason for test..the scope test would be done.
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Old 15-03-2011, 11:22   #12
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Re: Yanmar Engine Compression Test Values

Kent-
He's right on about the compression being even across all the cylinders. That's important and it shows there is no problem, no valve or ring issue, nothing "wrong" in any one particular cylinder. No head gasket issues.
As to the absolute pressure reading, you'd need to ask the maker what is normal for your engine. A low-compression gasoline engine (around 9:1) might be showing 175 psi at the cylinders so one might expect a diesel to be showing twice that (350psi) but again, the engine might not be running at that compression, the guages could be reading low...Your best reality check is with the maker, or a shop manual. Never having thrown a compression gauge onto a diesel, I could only say it does sound low for an engine that, in theory, should be squeezing much harder.
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Old 15-03-2011, 11:26   #13
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Re: Yanmar Engine Compression Test Values

HS,

It is definitely low..if the gauges are accurate..see my calcs and expected readings based on compression ratio.

As a mentor thou..I try to teach to fish...then just feed

Good to see you my friend
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Old 15-03-2011, 11:53   #14
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Re: Yanmar Engine Compression Test Values

The engine is a Yanmar 4JH2-TE 1993 vintage. The surveyor did a simple test and determined there was excessive blow by. His findings started my inquiry. Manufacture specs for compression for this engine are 412psi with a lower limit of 341.

Based on several folks comments, it sounds like a reading of 240psi might not indicate a problem?
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Old 15-03-2011, 12:12   #15
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Re: Yanmar Engine Compression Test Values

how many hours on this engine? We haven't ruled out a ton of stuff yet, including poor testing methods, bad pressure gauge, etc.

All we know so far is that all 4 cylinders appear to be producing the same compression, which generally rules out:

-Bent connecting rod
-broken ring(s) in one cylinder
-stuck or damaged valve
-warped or cracked head
-blown head gasket

Any of those above would usually show one cylinder with significantly less compression than the other 3.

So, now we are wondering: are all four cylinders producing such poor compression? OR, was the test method just unable to record the full compression that the engine is actually putting out?

Chief? What else?
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