MEP-831A Fuel Pump

[loosegravel]

Today I’ve been working on this MEP-831A 3kw unit. I bought it home a couple of weeks ago. Initially I found that the Yanmar engine was missing one of the rocker arms and it had a bent push rod. I was able to get those parts and I installed them today. It’s also missing the lever on the inside of the valve cover for the compression release. The red lever is there on the outside, but nothing to make contact with the exhaust rocker on the inside. I’m not concerned about that right now, I’ll get back to that later. But I’m not getting any fuel at all out of the fuel pump. It looks like it’s been messed with, but without a comparison I have no idea if it’s missing some internal par(s). I can see with it installed that it’s engaging with the fuel control rack. With the fuel pump out, I can see the cam follower going up and down when I crank the engine over. Just one .010” mounting shim. Any thoughts or ideas? Thanks, Jeff

 

[loosegravel]

Today I’ve been working on this MEP-803A 3kw unit. I bought it home a couple of weeks ago. Initially I found that the Yanmar engine was missing one of the rocker arms and it had a bent push rod. I was able to get those parts and I installed them today. It’s also missing the lever on the inside of the valve cover for the compression release. The red lever is there on the outside, but nothing to make contact with the exhaust rocker on the inside. I’m not concerned about that right now, I’ll get back to that later. But I’m not getting any fuel at all out of the fuel pump. It looks like it’s been messed with, but without a comparison I have no idea if it’s missing some internal par(s). I can see with it installed that it’s engaging with the fuel control rack. With the fuel pump out, I can see the cam follower going up and down when I crank the engine over. Just one .010” mounting shim. Any thoughts or ideas? Thanks, JeffView attachment 889989View attachment 889991

I forgot to mention, I’ve got a steady steam of return fuel going back to the tank.

 

[loosegravel]

Well, as this forum has taught me, if all else fails then read the TM’s. I did find a break down of the fuel pump in the TM’s. The two copper washers (called gaskets) we’re in the same place when I took it apart. One is supposed to be on each side of metering valve. I reassembled it as per the drawings and imagine that…it works! Now I’m getting lots of white smoke while cranking it over because of the fuel present. But it’s not firing. I got it to light off with a little shot of either, but it would not stay running. I adjusted the valves to a tight .006” because I don’t have a .004” or .005” feeler gauge. It appears that there was a reason that this Yanmar engine was partially torn down and missing a few parts. I guess I’ll have to dive in deeper and take the head off to check the bore, valves etc. My shop is at 50 degrees when I’m working up there. No engine should need a squirt of either at that temperature. Going t switch gears for now and work on something that I can fix without parts!

 

[DieselAddict]

Yea, those engines are really easy starting when everything is working correctly. Sounds like you may not have quite enough compression.

Any way to pull the injector and do a compression test?

 

[loosegravel]

Yea, those engines are really easy starting when everything is working correctly. Sounds like you may not have quite enough compression.

Any way to pull the injector and do a compression test?

I have a nice Snap-on compression gauge, just need to find out if I can get a rubber tip attachment to push it onto the injector hole. It’s set up with thread in hoses and fittings for gas engine spark plug holes. I’ve got the engine out of the generator enclosure. It looks pretty simple to tear down.

 

[DieselAddict]

The compression is going to be pretty high if everything is OK. It may be more than you can do with a push-in style compression gauge.

Those engines are dead simple and parts are readily available.

 

[Mullaney]

I have a nice Snap-on compression gauge, just need to find out if I can get a rubber tip attachment to push it onto the injector hole. It’s set up with thread in hoses and fittings for gas engine spark plug holes. I’ve got the engine out of the generator enclosure. It looks pretty simple to tear down.

.
Yeah, like @DieselAddict mentioned - press in compression guage - not a good idea. Even a Snap-On. The measurement should be 0-1000.

 

[FarmingSmallKubota]

Make sure all the air is bled out of the system and that you take the injector nozzle apart and make sure its not clogged or not seated all of the way

 

[Ray70]

Hopefully you can get the compression gage rigged up to work for you, as that is a crucial piece of the puzzle.
IF you happen to find compression low, something I have seen on several different single cylinder Yanmar model MEP's is that a spun rod bearing could be the cause of a compression loss, and I have seen cases where the engine would start with ether and then run on diesel ( but not well, lot of smoke ) but then not be able to restart just on diesel. The small change in deck height due to the trashed bearing is more than enough to lower the compression, and the bad bearing is difficult to distinguish between diesel noise and rod bearing noise.
If you suspect this may be the case, gently rocking the motor back and forth should allow you to very lightly hear the knocking of the loose rod against the crank journal.

 

[loosegravel]

Hopefully you can get the compression gage rigged up to work for you, as that is a crucial piece of the puzzle.
IF you happen to find compression low, something I have seen on several different single cylinder Yanmar model MEP's is that a spun rod bearing could be the cause of a compression loss, and I have seen cases where the engine would start with ether and then run on diesel ( but not well, lot of smoke ) but then not be able to restart just on diesel. The small change in deck height due to the trashed bearing is more than enough to lower the compression, and the bad bearing is difficult to distinguish between diesel noise and rod bearing noise.
If you suspect this may be the case, gently rocking the motor back and forth should allow you to very lightly hear the knocking of the loose rod against the crank journal.

Good stuff, thank you Ray. I am hearing some noises along with the diesel knocking noise. My gut tells me that you’re right on. When I put a socket on the crankshaft and roll it around, it doesn’t sound good. Are these cranks and rods hard to find? Expensive? These look pretty simple to tear down, and the design is telling me that it’s probably worth repairing rather than replacing. I see that there’s a ball bearing at each end of the crankshaft. Also a separate balance shaft along with the camshaft. Probably bearings on both ends of them too. I’ve got a couple of other projects to deal with. After which I’ll get back on this Yanmar and tear it down. I don’t even think it’s worth my effort to try a compression test Thanks again everyone!

 

[loosegravel]

.
Yeah, like @DieselAddict mentioned - press in compression guage - not a good idea. Even a Snap-On. The measurement should be 0-1000.

View attachment 890081

That’s a nice set. Looks to be made for diesel engines.

 

[Guyfang]

If you can hold that tester on by hand, you are a better man then I.

 

[Ray70]

In the past I have purchased new crankshafts and rod bearings from Yanmar and rebuilt these motors, and yes they are very simple to work on.
The last one I ran across the customer choose not to rebuild it since it was out of his budget range.

 

[loosegravel]

In the past I have purchased new crankshafts and rod bearings from Yanmar and rebuilt these motors, and yes they are very simple to work on.
The last one I ran across the customer choose not to rebuild it since it was out of his budget range.

Very cool. Thanks for the great info!