MEP-831A engine repair

[loosegravel]

I posted a thread a few days ago entitled “MEP-831A fuel pump.” I had recently acquired this machine from auction in non running condition. My first problem was no fuel out of the fuel pump. That problem has since been resolved. I figured that I should start a different thread now because I’m no longer dealing with fuel pump problems.

I received some great help in that thread which led me to know that I would need to tear into this L70 Yanmar engine much deeper at a later date. Well, today was the day for tearing it down. I knew that I had some issues in the rotating mass, as I could “feel” it and I could “hear” it. As Ray previously mentioned, a spun rod bearing could reduce the compression just enough so it wouldn’t fire on its own. It was getting plenty of fuel while cranking it over because of all of the white smoke, but it wouldn’t light off (or stay running) without a snort of either. I fully expected to find what Ray said after tearing it down. But what I actually found was a bent rod. There’s been no contact between the piston and the valves that I can see. What I was feeling and hearing when turning the engine over by hand was the bottom of the piston crown coming in contact with the counter weights on the crankshaft. Usually when I see a bent rod like this on a water cooled engine it’s because of water or coolant intrusion into the combustion chamber and the piston “hydraulics” on the compression stroke. So I’m really scratching my head on how this could happen on an air cooled engine

But, none the less this engine is very good otherwise. I know we should not rely on the hour meters on these machines, but this unit has about 350 hours and it shows very little wear. I’ve ordered a replacement rod with the bearings. I’ve also ordered a rebuild kit with the gaskets, roller bearings, and some other miscellaneous parts.

I’m also looking for some guidance on the compression release on this engine. The engine was partially torn apart when I got this generator, missing the exhaust rocker arm and push rod. I’ve got those parts now, but it appears that it’s missing something on the inside of the rocker cover that would make contact with the exhaust rocker arm when the red lever on the outside of the valve cover is pushed down. (The red lever and the shaft is there on the valve cover) In looking at the TM’s, there’s also a “valve cap” on the exhaust valve that’s not here on this engine. Thanks!

 

[Ray70]

It's funny ( not really ) that you mention the liquid / air cooled possibility of a bent rod. I had seen the same loss of compression on 1 cylinder of an 803 I had and the cause was a very minor bend in the connecting rod, assumingly from water / coolant in the cylinder.
I didn't even bother mentioning it before for the same reason you're scratching your head.... AIR COOLED, not liquid.
I don't think these sets are prone to water intrusion through the exhaust. My only other thought was possibly a lot of water getting in quickly while running such as it being flooded in a storm while running??

As for the decompression lever, I don't recall what's in there when you remove the valve cover, but I also don't recall taking anything off or apart other than pulling off the valve cover, so I suspect everything is supposed to be self contained. In the Yanmar service manual it looks like the lever shaft sticking through the valve cover would just press on the exhaust rocker tip when it's rotated to it's high point, and probably just barely clears when the flat is aligned with the valve. In the TM does it look like the valve cap fits over the tip of the exhaust valve between the valve and rocker arm? That's how the Onan DJ engines are.

I'm sure someone has been in the top end of one recently enough to recall.

 

[loosegravel]

I was thinking the same thing. Maybe a pressure wash that caused water intrusion? But it definitely had to be liquid of some kind to bend that connecting rod.

I see people on this forum send pictures of pages out of the TM’s with impunity. For the life of me, I can’t figure out how to do it. So I took a picture of the page with my phone The “valve cap” is #3 in the photo. What you said Ray makes sense. I had adjusted the exhaust valve without this piece in place. With it in place it would raise up the flat on top of the exhaust rocker enough to come in contact with the flat on the lever shaft inside the valve cover. I’ll see if I can get part #105010-11490. Thanks!

 

[Guyfang]

Just look in ebay. There are several

 

[loosegravel]

Just look in ebay. There are several

I did just that Guy. The person I bought the connecting rod from has them. Just ordered it.

 

[uniquify]

Assuming the yellow plastic in the 3rd picture from post #1 is the underside of the oil fill cap in the top of the valve cover, maybe the engine took on a little water when that fill cap was off.

 

[loosegravel]

Assuming the yellow plastic in the 3rd picture from post #1 is the underside of the oil fill cap in the top of the valve cover, maybe the engine took on a little water when that fill cap was off.

I suppose it's possible, but then I would think that the water would flood the crankcase and not the combustion chamber.

 

[87cr250r]

Too much fuel in a cylinder can bend a rod. Is the fuel tank located above the engine?

 

[Guyfang]

Yes. It is. If someone spun it over long enough, and it never started, that could happen. But not from just standing in a corner for a few years. The normal fuel problem with this gen set is that its a PITA to get air out of the fuel system, and the darn thing will not start.

 

[87cr250r]

In once case on a much larger engine we blew a fuel injector through a valve cover. The tip had broken on the injector and the fuel tank was above the engine. This allowed the cylinder to fill while the engine was not running and then hydraulic locked during the next start.

When diesel engines are in stop mode, the injectors are all live to the fuel supply.

From what I can tell from pictures, this may be moot because the fuel tank is under the engine. Unless, does this engine have an electric lift pump? If left running and in conjunction with a bad injector, that would also fill a cylinder with fuel.

 

[loosegravel]

In once case on a much larger engine we blew a fuel injector through a valve cover. The tip had broken on the injector and the fuel tank was above the engine. This allowed the cylinder to fill while the engine was not running and then hydraulic locked during the next start.

When diesel engines are in stop mode, the injectors are all live to the fuel supply.

From what I can tell from pictures, this may be moot because the fuel tank is under the engine. Unless, does this engine have an electric lift pump? If left running and in conjunction with a bad injector, that would also fill a cylinder with fuel.

Yes, they have electric lift pumps. So that’s something that I’ll look closely at for sure. Thanks!

 

[Guyfang]

When diesel engines are in stop mode, the injectors are all live to the fuel supply.

That's something to consider, but without the IP forcing fuel into the injector, I cant see that as much of a problem. The injectors should not pop off, without pressure on them. An injector with a tip broken off, or stuck open injector is a different matter.

 

[loosegravel]

When diesel engines are in stop mode, the injectors are all live to the fuel supply.

That's something to consider, but without the IP forcing fuel into the injector, I cant see that as much of a problem. The injectors should not pop off, without pressure on them. An injector with a tip broken off, or stuck open injector is a different matter.

When I removed the injector, I didn’t see anything wrong with the tip, it was in place and looked normal. But I will leave the injector out and hooked up to the high pressure line when I get this engine back together. Then I can crank the engine over and check the spray pattern of the injector. Kind of like using an injector test bench!

 

[87cr250r]

Here is an example of a jerk pump injection pump. Fuel enters through right port and leaves out top to injector. You can see that while the plunger is down, the injector pressurized by the lift pump. The left port is the spill port and ends injection. The engine stops when the slot is aligned with the spill port.

 

[Ray70]

So one thing to consider, this is a single cylinder engine.
There is no way that something got into the cylinder and then someone tried to start it and mangled the rod, It's just not going to happen on a 1 lunger wit the pull rope or the electric starter.
Something had to get in there quickly while it was running. Something big enough to take up all available space in the cylinder at TDC plus a good 1/4" more X the cylinder bore, so probably a good 8 ounces at least.

 

[87cr250r]

It doesn't require a full hydraulic lock and the diesel combustion chamber is certainly much smaller than that. Using the compression release may let the engine come up to a sufficient speed to bend a rod once the exhaust valve closes.

 

[loosegravel]

It doesn't require a full hydraulic lock and the diesel combustion chamber is certainly much smaller than that. Using the compression release may let the engine come up to a sufficient speed to bend a rod once the exhaust valve closes.

I was out on the Olympic peninsula for a couple of days with some friends. We were out of cell range so I’m just now getting around to reading the messages. It’s cool to see the interest and the input that this thread started.

I’m not really sure how the fluid got into the combustion chamber on this engine or what kind of fluid it actually was. But it appears that we can all agree that this rod was bent due to a hydraulic lock situation when the piston came up on the compression stroke. Because this is an air cooled engine, and because I saw no internal signs of water intrusion when I disassembled the engine, you can bet that I’m going to look very closely at the fuel system for possible causes of flooding the combustion chamber prior to an attempt to crank the engine over with the starter.

As previously indicated, I highly doubt that the rod could be bent at all by means of the pull starter. IMO the rope would break first, or the human shoulder on the person who was pulling it would separate. But, I have seen cases of bent rods that occurred during cranking with a 24vdc starter. A 12vdc starter just doesn’t have the torque to do sbut a 24vdc starter with fully charged batteries does. This just means that yes, the engine may not have actually been running to bend this rod. Again, this is just my opinion which is based on what I’ve personally seen in my 40+ years as a mechanic.

The input here is fantastic! Thanks for all of it

 

[loosegravel]

It doesn't require a full hydraulic lock and the diesel combustion chamber is certainly much smaller than that. Using the compression release may let the engine come up to a sufficient speed to bend a rod once the exhaust valve closes.

The one thing that I can add to using the compression release while cranking the engine over, is that any excessive fuel/fluid that may have been inside the combustion chamber would then more than likely be expelled through the exhaust valve (while it was held open with the compression release) so it then wouldn’t/couldn’t create a hydraulic lock situation when the compression release was let go.

 

[Dieselmeister]

Also check the crankshaft, crank bearings, and anything else that could have stretched or bent. I recall a time from my childhood, when my friends and I were trying to blow up an old lanwmower engine. We probably had the thing going at over 10K rpm, when we decided to stick a garden hose into the air intake...... a very effective way to stop the engine. The damage was much worse than in your picture. (Somehow we all did safely reach retirement age)!

 

[loosegravel]

I found a rebuild kit on eBay from “militarysurplusparts.” This kit was very complete. Not only did it include all of the seals, o-rings, gaskets, filters, needle bearings, ball bearings and much more, it also came with a new connecting rod, fuel pump and fuel injector! Genuine Yanmar parts and a great price at $137 after shipping and tax. Looks like I’ll be cleaning parts and reassembling this engine tomorrow.