Yanmar Engine Compression Test Values

[ArmyChief]

Kent not sure which comments lead you to that assumption. However, a quick Google search shows me the 4JHTE (could not fin the 4JH2)..I would assume (hope rightly so) the 4JHTE wold be similar enough for our hypothesis. My finding show 16.2:1 compression ratio.

Based off my posting above and these figures, you should be reading from 243 to 324 PSI..and I would feel better in the higher end.

Moreover, you posted the specs for compression (341-412)..so your reading of 241 would indicate a concern.

[four winds]

Originally Posted by kentheighton

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?

At first I thought you had no problem, because of the readings were consistent on all cylinders.

Now I think you have unusually even wear on all compression rings of the pistons. I didn't realize the spec would be 412psi for this engine. That's more than I would have thought.

Unfortunately, at 240psi, I thinking you have a problem.

[Bill Burgette]

I really can't speak of diesels but on a gasoline engine the compression numbers go up 10-20% depending on the throttle being wide open or closed. I would assume that diesels are similar. Also in a gasoline engine the test should be done while the engine is hot. When the engine is cold the pistons will be looser in the bore and will rock some. That motion will not allow the rings to set properly for this kind of test. Although the ring gap will close up some on a hot engine (raising compression) that is not the main reason. As stated the pistons rocking are the culprit on a cold engine.

I may not be a technician because I've built several engines, but I may be a technician because I have a Techtronics dual trace scope and I reprogram ECM's for best performance using my laptop, wideband O2's and other sensor readings.

Of course that's just IMHO.

[ArmyChief]

Originally Posted by Bill Burgette

I really can't speak of diesels but on a gasoline engine the compression numbers go up 10-20% depending on the throttle being wide open or closed. I would assume that diesels are similar.

Bill,

Very good point about throttle position and how it relates to testing. However, almost all diesels have no throttle body (I say this cause I know of a few that have throttle bodies to kill the motor if needed), diesels typically run WOT and control RPS by fuel/timing.

Again, I have very little knowledge of marine diesels if they are not also used in automotive applications.

Doug

[hellosailor]

"Manufacture specs for compression for this engine are 412psi with a lower limit of 341." And the surveyor says, excessive blow-by, which would not so co-incidentally show up as LOW COMPRESSION.

"Based on several folks comments, it sounds like a reading of 240psi might not indicate a problem? " Sure sounds like what the Magic8Ball says. You know, "all signs say yes!"

*All* the indications are that the engine has low compression, probably from wear. Since the compression is even, the engine is apparently qilling and able to keep working--but it probably is burning more fuel and putting out way less power than it is rated at. Can you keep running it that way? Probably for another five years or longer.

Do you *want* to keep running it that way? Possibly not. It might need new rings, or oversize pistons and rings, or cylinder liners. Depending on the engine and the nature of the problem there are all sorts of solutions. If your surveyor is familiar with the engine, ask for his idea. Your mechanic apparently is running on island time, but it might be worth asking him the same thing: What are your choices for stopping the blow-by and restoring the compression?

Although personally...I'd rather ask someone who wasn't quite as happy to see the glass was only half full, if you know what I mean.

[kentheighton]

Alledgely 2300hrs on the engine. The mechanic is Yanmar certified whatever that is worth. Quoted message from boat cartaker from yesterday:
All I can tell you is that today, and I presume because of Bob freeing the turbo, that she ran very well and with no smoke. I run it every month until it gets to operating temp, and then for about 10 to 15 minutes after that. all under load as it wont reach operating temp without it being in gear. I usually run it at 1800 revs with a few minutes at 2500 to clear any carbon.
And from 2 days ago:
I ran the engine for 30 minutes today and there was no smoke at all while running. I took here to 2400 revs and the turbo kicked in just fine. There was very little if any smoke upon start up this time. She ran very well. I hope that this helps you.

[doug86]

Kent, the most likely scenario is that the mechanic either has an inaccurate pressure gauge or he didn't really get the engine turning fast enough to get a full compression reading. Of course, his readings could be correct, but then I would expect the following symptoms:

- Very hard starting when cold.
- Lots of blue smoke when cold.
- Large sheen of fuel on the water at exhaust dump.
- Black smoke when the engine is warm and brought to full power under load.
- Reports that the engine has been using excessive amounts of oil.

If the compression is down 30-40% on all 4 cylinders, it would most likely be from worn rings, you would expect to see these kinds of things and overall poor performance.

I've never seen a Yanmar with only 2300 hrs that was just worn out from normal use. If the engine suffered some terrible abuse, like lack of oil or extreme overheating, I would expect that #4 cylinder would be the first to go, and now we're back seeing ONE cylinder with low compression, not all of them.

I don't know the answers, but if this is a boat you are considering purchasing, you probably should dig for more answers. Can you get a second opinion from another mechanic? Don't tell him anything other than a surveyor reported "excessive blow-by" and see what he comes up with.

What test did the surveyor do to conclude that the blow-by was excessive?

[never monday]

will the boat make full rated RPM and hull speed in gear?

[ArmyChief]

Have we gotten the full story yet?..I just got back..but I do not remember the customer concern yet?

Surveyor..ok..guess your looking to buy this boat.
Did the engine start?
Did it run well?
How long was it before it started? (can cause slightly low compression)
Were all four cylinders tested and consistent (240 psi)
Was the guage set used, tested on a known good and the readings normal?
I see its a turbo...turbo typically have lower compression ratios..hence lower compression numbers. However, you posted the specs..so I will not argue with that.

If I were looking at the boat..or just based off the provided info..I would not care or need to run engine in open water...but again..thats just me.

Doug (one of three)

[kentheighton]

Back in post #23 I relayed comments from the boat caretaker. Sounds like the only symptoms so far are blow by as evidenced from the oil filler cap and the compression test done by a mechanic. Interesting that the mechanic's opinion of the compression is that it was normal. Could that be? Manufacturer speced at 412psi and he measured 240 each cylinder.

[ArmyChief]

Only if he has an interest in selling the boat, his gauges are FUBAR or he is inexperienced

[hellosailor]

240, 412, ... Well unless he's dyslexic and his gauges really said 420 not 240...that's only a 60% difference. (58% but who's counting.) Or a 170-odd-% difference if you're looking at the ratio the other way.

I had a friend with an old Chrysler station wagon with the famous "Slant Six" engine in it. When we went to do the first tuneup and found two spark plugs literally chewed off and not firing...Gee, the engine ran just fine, but it ran even better when ALL SIX were running again.

412, 240...Heck, aren't the engines on USN vessels supposed to be rated at 3x their horsepower limit for combat operations? You know, a "2000hp" engine is really good for 6000 as long as you don't mind consuming it?

Sounds like your engine is "good enough for government work". Or your mechanic is. Just a little further out of compliance with civilian requirements though.<G>

When the numbers don't add up--gotta do the math again, and find out why. Something is wrong with that picture.

[never monday]

Originally Posted by never monday

will the boat make full rated RPM and hull speed in gear?

still waiting for this answer

[hellosailor]

"will the boat make full rated RPM and hull speed in gear?"

But with half an engine, it might very well do that. In flat water with a tailwind.

In 4' seas against 40 knots? Ah....might need the whole engine for that.<G>

[GordMay]

Originally Posted by hellosailor

... When the numbers don't add up--gotta do the math again, and find out why. Something is wrong with that picture.

Exactly.