What's wrong with my Yanmar 3GM30F engine?

[Cheechako]

These engines, especially in that era have a habit of breaking the piston down low on the skirt.

[model 10]

Originally Posted by Richard Kollmann

While cylinder head is removed it is a good time to flood valve cavities with gasoline to test for valve leakage.[/FONT][/COLOR]

As a former engine service provider I would try and leak test every cylinder before installing. Depending on the repairs and who did them you might choose a different fluid for the leak test. We would never use gas as it's too thin and stinky. It came down to either water or solvent. Water is the one to use on an old engine with a questionable valves as it is less likely to leak past the valve seat. Solvent is the fluid of choice for a good valve job. I suppose diesel would be about the same viscosity as solvent. A little wetness over 30sec is about the limit. Of course no seepage is the ideal for an expensive valve job.

[TampaBaySailor]

I REALLY appreciate all of the suggestions! Here's a quick update.

In retrospect, I realize I should have had a compression check done.

Several of you pointed out that one of the valves in the "bad" cylinder looked different in the picture. Indeed, I completely missed this when I initially looked at it, but upon further inspection there looks to be clear evidence that one of the valves was not seating correctly in this cylinder head.

The piston top on the "bad" cylinder shows a wear pattern that looks like an image of the valve bottoms on the head. The other two pistons don't show this effect. You can sort of see it in my picture and several of you pointed it out.

This piston is not rising as far as the other two - when I got some calipers and measured, the gap between the piston top and the cylinder-wall top is 0.04" for the other two pistons and 0.16" for the "bad" one. Based upon your comments, I suspect something is bent/broken between the crankshaft and the piston.

Careful inspection of the cylinder walls didn't show any excessive wear - at least each compared to the others.

Finally, I went ahead and poured a little clean diesel in each cylinder and let sit overnight. The "bad" cylinder completely drained. The other two were still filled in the morning. Looks like the rings are not tight on that piston.

SO... it looks like I have a problem with the valve, rings and perhaps rod (or bearings) in one cylinder. I am debating next steps ... perhaps continuing disassembly and take parts to machine shop...??? Or is it time to scrap for parts??

[GrahamHO]

Originally Posted by TampaBaySailor

I appreciate any help that might be offered.

This 1994-era engine was pulled from my boat and is in my shop. As an exercise, I am taking it apart and trying to figure out what is wrong. I have no experience with diesel engines but have time, space and the manual.

The symptoms when the engine was pulled are:
1) engine ran roughly with unburned fuel in the exhaust and much smoke.
2) I determined one cylinder was not firing using the decompression levers.
3) If I switched injectors on the non-working cylinder with another injector from a working cylinder, the original non-working cylinder continued not working and the other cylinder worked as before. Thus the injectors seem fine.

My first thought was maybe a problem with the valves associated with the rocker-arm assembly. Once I got the valve cover off, I manually spun the crankshaft and observed the valve movements. Everything looked identical on all three cylinders. No obvious problem here.

Next, I proceeded to removed the engine head. I have attached two pictures - one shows the bottom of the head and the other shows the pistons. The "bad" cylinder is marked in the photos.

My observations:
- both parts are covered in black carbon soot. Is this normal or a sign of the age of the engine.
- the "bad" piston has slightly different wear marks on the top from the other two but I don't know what this means.
- all three pistons move as expected when I rotate the crankshaft. However, the piston in the "bad" cylinder might not lift as high as the other two. The other two pistons essentially are flat with the top of the engine surface at their maximum height. The suspect piston might be a 1/16th or so below this height. However, any difference is not obvious to my eye.

Otherwise - all looks OK.

Does anyone have any advice what to look for next? I thought I would probably have located the problem by now...

Thanks!

If one piston is lower measured at TDC ( top dead center ) than the other, it's quite likely that a conrod is bent. This can happen when cooling water somehow gets in a cylinder. Water can't be compressed so the conrod bends. Also likely in that case that the piston rings are broken. New conrods are very expensive ( NZ$1500 ) but an experienced engine repairer can straighten them using a hydraulic press and skill.


Water can enter a cylinder if; the cylinder head face is distorted; the head gasket is blown; a faulty exhaust bend; or a weak starting battery or for other reason needing to crank the engine too long. In the latter case the water pump turning over fills the exhaust and then the cylinder with cooling water. There's usually a sticker on a Yanmar reading "avoid overcranking". You can get a situation when perhaps the yacht is heeled and cooling water siphons back


I write from having experienced this myself. A secondhand good conrod was found. My cylinder head needed planing -8 thou. The cylinders were over bored +60 thou. New pistons and rings. The crankshaft doesn't get damaged as the conrod bends first. At the same time new valves and the seats recut. Much better than a "valve grind".


Then I revisited the exhaust layout, installed a larger water lock and a Vetus anti siphon bleed. The extra compression from over boring and head planing caused the engine to stop with a thump on shutdown. Increasing the valve clearance from 8 thou to 9.5 thou fixed that and now it's better than a new engine at.....half the price.

[Cheechako]

Just get that piston out of there and you will know what it is. Cant take you more than a half hour at this point can it? My 3GM30 had a bad piston, parts are available and the cylinder wall was not scored... so it's not that bad a fix possibly.

[sailorchic34]

I of course had broken rings in the same cylinder on my 3gm30F, 2 years ago. Also a 94 engine. I rebuilt it in the cockpit. Parts at the time were $750 and machining to bore and sleeve my bad cylinder was was $300. If you caught it early then perhaps the bore only needs to be honed. First oversized piston is right at $100 with rings.

It's been two years since my summer of discontent and "Purple" (yes I named my engine) has 228 hours on it and runs like a champ.

The only gotya is you'll need to fabricate a 6" deep 36mm socket to remove the inner nut on the crankshaft. That was $86 for the socket and a welder to weld it up for me.


It's a easy engine to rebuild. Even crazy poor blonde sailors can do it.

[Wotname]

Yep, agree with the previous (Cheechako and others), get the crook piston out, replace and tidy up the other bits. The engine c will be good for a long time yet.

Do read Sailorchic's thread as advised early on in this thread. She has everything you need to know about getting your engine sorted out.

[Cheechako]

Originally Posted by sailorchic34

I of course had broken rings in the same cylinder on my 3gm30F, 2 years ago. Also a 94 engine. I rebuilt it in the cockpit. Parts at the time were $750 and machining to bore and sleeve my bad cylinder was was $300. If you caught it early then perhaps the bore only needs to be honed. First oversized piston is right at $100 with rings.

It's been two years since my summer of discontent and "Purple" (yes I named my engine) has 228 hours on it and runs like a champ.

The only gotya is you'll need to fabricate a 6" deep 36mm socket to remove the inner nut on the crankshaft. That was $86 for the socket and a welder to weld it up for me.


It's a easy engine to rebuild. Even crazy poor blonde sailors can do it.

Do you need to remove that crankshaft nut to just replace a rod and piston?
(if he's lucky)
My engine was a '92.
Both went bad less than 2500 hours.

[sailorchic34]

Originally Posted by Cheechako

Do you need to remove that crankshaft nut to just replace a rod and piston?
(if he's lucky)
My engine was a '92.
Both went bad less than 2500 hours.

The problem is the crankshaft intermediate carrier bearing housings, actually stick out into the cylinder openings. In theory, you could hone the cylinder, but I would be worried about getting dust into the bearings.

Unlike US engines where the crank drops from below, Yanmars cranks pulls out the back, complete with intermediate bearing housings and aft bearing housing.

The front and back bearings also need special tools to remove. But as everything else speced out within new clearance, I left the main crank bearings alone on my rebuild.

So there is not a clear space below the hole for honing or boring.

[GILow]

Originally Posted by sailorchic34

The problem is the crankshaft intermediate carrier bearing housings, actually stick out into the cylinder openings. In theory, you could hone the cylinder, but I would be worried about getting dust into the bearings.

Unlike US engines where the crank drops from below, Yanmars cranks pulls out the back, complete with intermediate bearing housings and aft bearing housing.

The front and back bearings also need special tools to remove. But as everything else speced out within new clearance, I left the main crank bearings alone on my rebuild.

So there is not a clear space below the hole for honing or boring.

After reading this I am going to go back to the boat and be extra-extra nice to my dear old Nissan ED33. These things sound like a bugger to work on.

Matt

[RaymondR]

I removed a 25 year old 3GM30 from my boat a few years ago because I thought it was getting a bit old and would start giving me problems (It also was underpowered for the 12 ton boat) A friend needed some piles drilled 6' into bedrock and we built a hydraulic machine to drill them in. We used the 3GM30 as the power supply for the hydraulics and ran it at full throttle under heavy load for three weeks. It did not miss a beat. I now believe that the motor is still in excellent condition even after a fairly long service life in the marine environment. I would have no problem installing the old motor in another boat.

The point of this little tale, these are very durable engines and provided the bore is not scored I would put a new piston, rod and bearings in and give the head an overhaul and put the motor back in the boat.

[GrahamHO]

Originally Posted by TampaBaySailor

I REALLY appreciate all of the suggestions! Here's a quick update.

In retrospect, I realize I should have had a compression check done.

This piston is not rising as far as the other two - when I got some calipers and measured, the gap between the piston top and the cylinder-wall top is 0.04" for the other two pistons and 0.16" for the "bad" one. Based upon your comments, I suspect something is bent/broken between the crankshaft and the piston.

Careful inspection of the cylinder walls didn't show any excessive wear - at least each compared to the others.

Finally, I went ahead and poured a little clean diesel in each cylinder and let sit overnight. The "bad" cylinder completely drained. The other two were still filled in the morning. Looks like the rings are not tight on that piston.

SO... it looks like I have a problem with the valve, rings and perhaps rod (or bearings) in one cylinder. I am debating next steps ... perhaps continuing disassembly and take parts to machine shop...??? Or is it time to scrap for parts??

This was EXACTLY my experience except on a 2 cylinder 1998 Yanmar 3 years ago. They are similar only different numbers of cylinders. I initially did a compression test with a borrowed gauge showing one cylinder way down and the other on the lower limit Then I took the head off and used (bulk) WD40 for a leak test. In the cylinder with very low compression it drained quickly.

That piston was also lower to the surface of the block. It's conrod was later found to be bent as I've already posted. Hard to notice without getting a specialised workshop to measure them all which I strongly recommend. My pistons and rings were both stuffed!! I did all the unskilled work myself removing the engine, taking the head off and stripping it. I had thought of doing all reconditioning myself until I found out the complication of retiming the injection pump which needed removing to do the rebore. So I got the local Yanmar workshop to do the whole recondition including bearing shells while they were at it. Later I delivered and reinstalled the motor in the boat saving all the out of workshop mechanics hours. They were happy with that.

If you put back a bent conrod even if it looks OK the engine will be out of balance dynamically. That might or will cause future problems.

I had considered a new motor but that would have cost 2X the recondition. Also a replacement Yanmar had things like exhaust and alternator in different places which would have meant more work and cost. I feel my reconditioned motor now runs better than new as it was carefully rebuilt by Rex, a skilled diesel mechanic. You could always replace with an outboard but not for me.

[model 10]

Connecting rods can be straightened in only in 2 directions. The bore of the rod journal can be bent to be parallel with the wrist pin bore or vise versa. You can also align the 2 bores along the length of the rod. Neither of these problems would have resulted in a lower deck height. If your piston is still in one piece and is lower than it is supposed to be, the rod is junk.

[GrahamHO]

Originally Posted by Guy

Connecting rods can be straightened in only in 2 directions. The bore of the rod journal can be bent to be parallel with the wrist pin bore or vise versa. You can also align the 2 bores along the length of the rod. Neither of these problems would have resulted in a lower deck height. If your piston is still in one piece and is lower than it is supposed to be, the rod is junk.

My experience with a bent Yanmar rod was that the workshop had a good second hand one from a "bits" motor that they kindly gave me. A new rod was NZ$1500. This very good workshop was going to straighten my rod but decided to save time and teach the apprentice to do that and keep mine as a spare. I don't believe the journals were out of alignment but rather the rod was slightly bent like a bow. This meant that the distance between journals was reduced slightly and that, that piston was definately slightly lower than the other. We are only talking thousandths of an inch but there was a measurement that was lower than it should be. I agree that a bent rod is junk unless it is correctly straightened.

[model 10]

Originally Posted by GrahamHO

I agree that a bent rod is junk unless it is correctly straightened.

That is just it, you can't take that kind of bend out of a rod and call it "correct". Just because a rod costs $1500 does justify the rationalization that it's ok to bend it straight again. Look at the structure of a rod, it's designed to not bend that way. If it does bend like a bow, it's toast.