[dabnis]

I tested the compression of each cylinders of my 2GM20F. Cylinder #1 270psig and Cylinder #2 250psig. These numbers seems way too low as expected is 370-450psig. I should have squit some oil into each cylinders before trying again to see if the pressure will come up. If it does I suppose it means my piston rings are wore. But I forgotten about this step. I'll schedule anither day for this test again.
Comments anyone?

Have not had this experience, but have read that it might fire if you squirt extra oil in the cylinder. Maybe warm the engine up a bit before taking the readings?

If it starts OK when cold, the compression may be OK? But if it is below spec it may be an indication that the rings are worn or the valves are leaking.

I have read that an oil analysis may show where a problem may be? It would be interesting to know what you find.

Paul T

 

[dabnis]

Not familiar with that engine, 20 HP? Maybe not enough power to swing a bigger prop? If you are running it close to max RPM's for long periods of time that may be why it is using some oil, or it may be a bit on the tired side? Was there any work recently done on the engine? Is the timing spot on? Injectors clean and up to spec? If you can run it that fast, it appears it is getting adequate fuel?

Any info on similar boat/motor combination, forums, owners clubs, etc?

Paul T

 

[dabnis]

Found the info on adding oil on a diesel compression test:

Unlike a gasoline engine, adding oil is not recommended due to the high compression in the cylinder. You risk having the compression tester be damaged or personal injury due to the cylinder firing. Remember, a diesel engine can run on oil and the peak compression reached from the combustion process is in the thousands, way beyond the range of the compression tester. In addition, if you do a compression test and the results are bad, you can do the test again. There would be no point in adding oil since you already know there is some problem either with the gauge or engine

.

How to do a compression test on a VW TDI or Audi TDI engine and results | VW TDI forum, Audi, Porsche, and Chevy Cruze diesel forum

Paul T

 

[dabnis]

18 rated

Not familiar with his boat, is 18 HP enough?

Paul T

 

[dabnis]

this is gold info...part of the reason I didnt recomend doing wet tests per se here...they are usually for gas engines wiuth lower compressions and ease of testing...doesnt mean you cant do it though

basically even compression tests wont be really iluminating for the op here...

if the engine starts, doesnt sputter, has good fuel pump and injectors you either live with it(yes it might be a bit underpowered) or you tear it down and rering, deck and cleanup up the valves and tolerances

usually though if it AINT broke no need to go there

I would look into repropping better for the meantime

again is it smoking bad, or just losing oil at high rpms? oil loss at high rpms is a given in most engines...especially low reving ones...

anywhoo

Agree, the OP says it starts OK, & can run up to 3,000 + RPM, so who knows? I too, would try a prop that bogs it down & then go lower on the pitch?

I have done many compression tests, but all on gas engines. I imagine that if the diesel fired it would damage the gauge, & maybe the person, when the whole mess blows out of the head?

Paul T

 

[dabnis]

I can't swing a larger or higher pitch prop as it would not reach 3200rpm. I forgot to mention I've a 2 blade 15"D x12"P Flex-O-Fold as well. Max it could make is shy of 3300rpm.
So far all checks and feels points to rings wear. Compression test seems to confirm that. Anyhow, I'll try the "Warm" and "Wet" test and post results here. Unfortunately, test analysis of crankcase oil isn't common or known here.

Sounds like you have the prop issue dialed in. If the engine can't pull more pitch it appears it is too small or "tired"? Any way to compare with similar size boat/engine combinations? Maybe boat manufacturer specs? Not that it matters, as it will only put out as much as it is, but about how many hours on the engine?

Paul T

 

[dabnis]

I just re-read this.

If you are getting 3300 rpm then there is nothing wrong with the engine. You may have too much reduction in the transmission. A 2GM20 with that big prop, with that much pitch, should push an 8000# boat at or near hull speed, IF you don't have too much reduction in the transmission (Yanmar put several different trannies on those engines). In other words, the engine may be at or near max rpm, but the prop may just be spinning too slow. Look on the tag on the transmission and see if you can find the reduction ratio. It should be 2.21, 2.62 or 3.22. If it's 3.22, at 3300rpm the prop would be spinning at 1025rpm. For a prop with 12" pitch that would be about 10kts (rpm x pitch x 60 then convert inches to knots). Since most props have about 45% slip, you can't reasonably expect more than about 5.5kts. On the other hand, if your reduction ratio is 2.62 then you should be able to get pretty close to hull speed (i.e., 7 knots or so).

I can't remember what the OP did on props, but short of changing the gear box ratio, perhaps fitting a prop with enough pitch to bog the engine down to a bit under 3,000 RPM or so, & then reduce pitch to achieve max RPM might work?

Paul T

 

[dabnis]

My experience with head gasket leaks is some/lots of white smoke at start up, then gradually clearing up. 1/4'' in 2 to 3 years doesn't seem like much, depending on how much you run the engine. I have read that an oil analysis can detect coolant in the oil. Not sure about how much, but that it can be detected.

I had a VW diesel that used to weep coolant at the head/block joint in extremely cold temperatures, but would stop as soon as the engine warmed up a bit.

Also, a leaky head gasket can force hot gasses into the cooling system, causing the expansion tank contents to raise or be blown out, but I think that would be a fairly severe leak.

Paul T

 

[dabnis]

On average I do 130 engine hours per year. Its total hours is 1550hours now. This isn't alot ...is it?

Generally speaking, I don't think that is a whole lot? There can be lots of variables like the frequency of oil changes, coolant in the oil, overheating, & so on to consider. Properly maintained diesels can run along time.

The commercial fishing boat we had, below, had many thousands of hours on it, as best as we could tell, and ran fine. It was a Detroit 2 stroke 3-71 diesel, aka "screaming Jimmy" Lots of variables to consider.

Maybe try a prop with enough pitch to "bog" it down and then re-pitch just enough to reach maximum RPM?

Paul T

 

[dabnis]

If you can still pull 3,300 RPM the engine must still have some beans left in it. What would it pull originally? Unless your oil pressure dropped significantly on the long run & the engine didn't develop a death rattle, it still may have some life left in it?

If it won't pull the same RPM as when newer, with the same prop, then it may be getting tired. Maybe try a higher pitch prop until you bog it down & then adjust the pitch accordingly?

Paul T

 

[dabnis]

you can learn a lot from how tired an engine is by simply looking at oil pressure at startup and then how it lowers when warmed up...if its drastically lower in normal range again you can experiment with stp, thicker grade oil and the like as it will tighten up tolerances a bit and thats that

a good way to prolong the rebuild...

I experienced that exact situation with my daughter's VW turbo diesel. After about 60,000 miles when doing a long hard climb the oil pressure would drop from about 60 lbs to about 20 lbs at speed. at idle it would go down to about 5 lbs, triggering the red light. It never rattled or knocked, just scared her. Tried heavier oil, no change.

It didn't burn any oil to speak of & ran fine. Could have been a worn pump, or a bad pressure relief valve, but probably was the main bearings slowly going away?

All that being said, I think it would be easier to do some prop swapping before ripping the engine out?

Paul T

 

[dabnis]

Prop I used are: stock 2RH 15"Dx10"P, CSP 3RH 14"Dx9"P, & 7"P, & 6"P, KiwiP 3RH 16"Dx20deg, & 18deg.
I've tried SAE40 and SAE15W40. I don't find any difference. I've tried adding STP Oil Treatment (Silicone base). It helps raise max rpm by 100-200rpm.

If you could pull 3,300 RPM with all of these props perhaps none of them had enough pitch or diameter to indicate you were/are "over propped"? The only way I can think of telling how much pitch/diameter is too much is to find a prop that will not let the engine pull to its maximum RPM ?

Paul T

 

[dabnis]

I think a major part of the problem may be the prop?

After about 68 years of working on many different types of engines, I finally learned to make sure there was fuel in the tank before ripping the engine out.

My wife has told me that I have a tendency to "over focus" on things, something about the "Forest & the trees"?

Now, I try to do the easiest/cheapest things first, surprising how often that works.

Paul T

 

[dabnis]

Maybe take the test after the engine is warm? Then, if the readings do not come up, or close to, spec try the oil?

Paul T

 

[dabnis]

Just a question on the low oil:
As long as the low oil pressure light did not come on, could the engine have been damaged by low sump oil?

Does the crank run or slosh in the oil bath created by the oil pan? If so, I could see where low oil level would be a problem.

I think most "modern" engines rely on oil under pressure, rather than the "splash" systems in older engines. However, that being said, it is probably always better to keep the oil level at the manufacturers spec.

If the low pressure light/alarm didn't come on, probably pressure was adequate? However, if it was down to about a quart or so, the oil may have heated enough to lose some of its viscosity, allowing the pressure to be somewhat reduced, but still in the "allowable" range. A full crankcase is a happy crankcase, but not overfull, which can cause foaming.

Paul T

 

[dabnis]

Yes, that's the plan.

It would be interesting to know what you find.

Paul T

 

[dabnis]

Yes sometime we introduce a new component and along came new problem. Mr. Murphy is always lurking somewhere uke

I think I have read about a system where a pressure gauge, & light/buzzer can all be operated by one sending unit. Maybe a search?

Paul T

 

[dabnis]

yeah I dont think thats such a hard issue to resolve...I think a buzzer can be rigged on the same line...

thinking caps on! jajaja

I have used steel "T"'s instead of brass on two VW diesels with no problems. From time to time I would yank on them but could not break them off.

Paul T

 

[dabnis]

The results are in!! I've warm up the engine and check the compression, then squirk some oil into the cylinder and test it again annnnd the result IS ...No Different!!? 260psi.
Same as when I first did the test. So what does that tells me?

Likely that the rings are OK, or evenly worn enough where the oil didn't make any difference. If the compression is low according to the specs, It may be marginal valves or a slight leak in the head gasket between the cylinders.

If it starts OK & can run up to max, or close, RPM's, doesn't smoke excessively,
or make bad noises, or burn excessive amounts of oil, maybe leave it alone?

Paul T

 

[dabnis]

dabnis, yes indeed that's my thought on leaving it be as it is about what you mentioned.

Life is good.

For many years I was the ultimate tinkerer, many/most times accomplishing nothing, & in some cases, making things worse.

A quote from my wife:

"Make sure there is fuel in the tank before ripping out the engine".

Paul T