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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 05-27-2013
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Bubbles in fuel return pipe. 2gm20. Normal?

Hi,

My 2gm20 almost didn't start on me the other day when about to enter the marina. It was certainly harder to get going when I left the mooring earlier. I decided to replace all filters, change oil, but it's still being a bit of a pig to start, even when warm. I've seen others start 'on the button' (I managed it once today), but this one now takes about 10 seconds and slowly splutters into life. It's making me nervous!

After changing both fuel filters I bled the air from both. Engine starts and runs, but as mentioned, not without effort to get started. I followed the instructions in the service manual and a few times have bled the filters, fuel return pipe, and injection pipes. I don't see any air, though after I last bled the return pipe, hit the starter, 1st crank it fired up. Thought I'd cracked it, but next time, back to normal.

I noticed looking at the fuel return hose, I see small bubbles entering it from the engine side and flowing up the pipe back to the tank. Sometimes a big bubble. Surely there should be no air in this pipe, or is this normal? Air must be getting in from somewhere and I cannot imagine/see where this would come from. It's not from the primary fuel filter which is the highest point in the fuel system. There are no fuel leaks I can see either.

Any suggestions? Thanks.
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Old 05-27-2013
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

Quote:
Originally Posted by pjsmith View Post
Hi,

My 2gm20 almost didn't start on me the other day when about to enter the marina. It was certainly harder to get going when I left the mooring earlier. I decided to replace all filters, change oil, but it's still being a bit of a pig to start, even when warm. I've seen others start 'on the button' (I managed it once today), but this one now takes about 10 seconds and slowly splutters into life. It's making me nervous!

After changing both fuel filters I bled the air from both. Engine starts and runs, but as mentioned, not without effort to get started. I followed the instructions in the service manual and a few times have bled the filters, fuel return pipe, and injection pipes. I don't see any air, though after I last bled the return pipe, hit the starter, 1st crank it fired up. Thought I'd cracked it, but next time, back to normal.

I noticed looking at the fuel return hose, I see small bubbles entering it from the engine side and flowing up the pipe back to the tank. Sometimes a big bubble. Surely there should be no air in this pipe, or is this normal? Air must be getting in from somewhere and I cannot imagine/see where this would come from. It's not from the primary fuel filter which is the highest point in the fuel system. There are no fuel leaks I can see either.

Any suggestions? Thanks.
Small air leak somewhere? Maybe restricted tank vent? Double check all connections, maybe replace hoses if they are old. Air leaks under vacuum can be hard to find. Maybe try something like this:

The Diesel Page, Electric Lift Pump and Racor Filter installation

Paul T
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Old 05-28-2013
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

Thanks. I was thinking about this a bit more last night. If air is getting into the fuel lines, it must be from the vacuum side? If it were a connection after the fuel pump on the pressure side, I'd expect it to squirt diesel, not suck air?

On the vacuum side I just have the primary fuel filter. I know that is not the issue because the filter is higher than the engine and has a clear pipe that connects to the engine. No air can be seen in that pipe as per the return pipe. The only other connections after that clear pipe are the short black fuel pipe connected to the engine itself (stock I think) and the lift pump itself.

I'm guessing here. Makes sense to me, but perhaps someone with some more knowledge can tell me if I'm barking up the wrong tree.
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Old 05-28-2013
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

On yanmars the most common air entry points are:

1: fuel hose onto engine nipple
2. on engine fuel filter housing. One of the two screws on top of the housing isn't tight.
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

Thanks. Screws are tight, but copper washers look pretty poor. Will replace washers and fuel hose and see if it solves the problem.

It is however considered completely abnormal to have air in the fuel return??

edit : I just found this quote on a yanmar engine faq "the fuel aerates under high pressure at the injectors and this aeration cannot escape unless it is dumped into the top of the fuel tank", via the return... Perhaps it is normal?

Last edited by pjsmith; 05-28-2013 at 08:53 AM.
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Old 05-28-2013
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

If the engine starts, why worry?
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

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If the engine starts, why worry?
That's the issue. It almost failed to start when at sea, appears to be getting worse and I'm looking for reasons why. I see videos on the net of the same engine where you hit the button and it just goes. I managed it once the other day after bleeding. Caught on first crank. Yanmar doc says abort if does not start in 5 seconds. At the moment it takes around 10 seconds cranking and perhaps a couple attempts to get it to catch.
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

[quote]
Quote:
Originally Posted by pjsmith View Post
Thanks. I was thinking about this a bit more last night. If air is getting into the fuel lines, it must be from the vacuum side? If it were a connection after the fuel pump on the pressure side, I'd expect it to squirt diesel, not suck air?
I would think so

Quote:
On the vacuum side I just have the primary fuel filter. I know that is not the issue because the filter is higher than the engine and has a clear pipe that connects to the engine. No air can be seen in that pipe as per the return pipe. The only other connections after that clear pipe are the short black fuel pipe connected to the engine itself (stock I think) and the lift pump itself.

I'm guessing here. Makes sense to me, but perhaps someone with some more knowledge can tell me if I'm barking up the wrong tree.
If the filter is higher than the engine and lower than the tank there may be some "drain back" somewhere, causing air to be sucked in from a fitting or perhaps the rubber hose that may be porous or have a very small crack in it.

If you pressurize the whole system a fuel leak may show. IIRC, the electric pump I had put out about 4 or 5 lbs of pressure.

Paul T
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

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Originally Posted by pjsmith View Post
Thanks. Screws are tight, but copper washers look pretty poor. Will replace washers and fuel hose and see if it solves the problem.

It is however considered completely abnormal to have air in the fuel return??

edit : I just found this quote on a yanmar engine faq "the fuel aerates under high pressure at the injectors and this aeration cannot escape unless it is dumped into the top of the fuel tank", via the return... Perhaps it is normal?
OK, if she's hard starting you either have no fuel going into the cylinders or an air leak. I agree that an air leak sounds more likely.

You mention the copper washers look a bit beat up. Where are these copper washers? Neither bolt in the fuel filter housing should have them. There should be a copper washer on the nipple fitting which your fuel line attaches, but I can't for the life of me figure why you would have removed this, so if we are talking about the bolts on top of the fuel filter assembly, they should have fiber or rubber washers to seal them.

There is a downloadable Yanmar GM series engine service manual on this page.

A diagram of what those bolts/screws on the housing should look like is on page 3-29

Lastly on the copper washers. If they have scratches, 400 grit sand paper will remove them. Then use a torch to anneal the washer and use it over again.
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Last edited by TropicCat; 05-28-2013 at 04:08 PM.
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Old 06-01-2013
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Re: Bubbles in fuel return pipe. 2gm20. Normal?

Quote:
Originally Posted by TropicCat View Post
OK, if she's hard starting you either have no fuel going into the cylinders or an air leak. I agree that an air leak sounds more likely.

You mention the copper washers look a bit beat up. Where are these copper washers? Neither bolt in the fuel filter housing should have them. There should be a copper washer on the nipple fitting which your fuel line attaches, but I can't for the life of me figure why you would have removed this, so if we are talking about the bolts on top of the fuel filter assembly, they should have fiber or rubber washers to seal them.

There is a downloadable Yanmar GM series engine service manual on this page.

A diagram of what those bolts/screws on the housing should look like is on page 3-29

Lastly on the copper washers. If they have scratches, 400 grit sand paper will remove them. Then use a torch to anneal the washer and use it over again.
The copper washers on the bleed screw on top of the spin-on filter are appropriate and should be there. It's what Yanmar uses to make the airtight seal.

I wouldn't think it would matter whether the return line has bubbles or not. It goes to the top of the tank and the bubbles join the rest of the air in the tank. Completely normal.

If you have air in the lines, the engine would seem to stumble and hiccup, eventually stalling. Air can get into the lines at either filter or any connection point, before or after the electric fuel pump. It could also be at the injectors. I'm wondering if your fuel pump (electric) is providing enough pressure. Do you hear constant clicking? It shouldn't stop as the fuel will recirculate back into the tank, never building up pressure needed to stop the on-demand pump. I'm thinking you are not getting fuel to the injectors. That could be because of air blockage or bad pump.

When you change the filters, do you top off the filters to eliminate air in the system? It should start right up. Puzzling, huh?

Good luck,

Tod
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