Air in fuel line?

[Burceh]

Yesterday we were running the Yanmar during a sea trial on a H-28.5 at 2000 rpms. Every 20 minutes or so the rpms would drop to 1800. The owner said the air had to be purged every 20 minutes from the fuel line. Eventually, bleeding the air did no good and the engine stopped. The screw ended up breaking so I made a temporary fix by installing a bolt and letting the air bleed around the threads and catching the overflow diesel in a can. At 1300 rpms with this fix it got us back to the marina with no problems.
The boat has both new tank and fuel filter. Any ideas what is going on and more importantly, anyone have a solid fix?

 

[Don S/V ILLusion]

Wild guess #1. - loose connection somewhere in fuel system. Good news - easily fixed.

 

[Johnb]

Replaced Fuel Pump

I had episodes of loss of power on my 3QM30 that went away after replacing the on engine mechanical fuel pump. That is not a hugely expensive part.

 

[Burceh]

I was hoping it was something simple. Will check both of these and see shat we get. Thanks

 

[s sauer]

Air Leaks in fuel lines

On the 2GM20F that bleed screw on the engine filter is notorious for stripping its threads, but there are two bleed screws, so you can effectively seal one up and still be able to bleed air out at the top of the filter. I'd start by replacing the metal fuel line banjo fitting copper washers; one at the manual lift pump, two on the engine filter inlet and outlet. If the one bleed screw is shot, you can clean the threads with alcohol and seal it up with a screw set in PC7. While you're at it, replace the 'O' ring and filter cartridge, then replace the small copper washer on the other bleed screw, bleed out the air using the lever on the manual lift pump.

 

[Steve Dion]

Do not forget to replace any banjo washers that have been accessed.

 

[Barnacle Bill]

No way should you have to re-bleed the engine every 20 minutes. Shouldn't have to do it all year, if not for several years if the system is tight.

 

[Nelsonhow]

Not a hard fix. My guess is its a ( in descending order of probability)
1) bad nylon washer on the bleed screw
2) bad seal on the secondary filter bowl on the engine mounted filter
3) has seal on the primary filter bowl
4) crack or leak in the fuel supply line
5) a bad copper washer in on one of the banjo bolts
6) a cracked banjo bolt
7) cracker metal fuel line
8$ bad lift pump

I'd start with the bleed screw and tighten all fittings and reseat the primary and secondary filter bowls and check all the fuel supply lines for a cracks and tight fitting.
If cracked replace otherwise see it the problem is cured. If not replace all the nylon and copper washers, rebelled and try again. I replace all the washers were 5 or 6 years and keep a supply of replacements on hand. Full set of all is only about $20. I also have a dew extra banjo bolts. On the 10gm there are 3 sizes. For about $15 I have an extra of each.
If that's not the cure you may have a bad lift bump or a cracked metal tube.

My bet is in the filter bowl gaskets or bleed screw or bleed screw washer.

 

[RichH]

8) check ALL compression fittings or 'retighten' as necessary ... especially ones that were recently loosened and re-tightened.

Compression fittings as found on fuel systems are really one-time tightening devices, and to insure sealing integrity after disconnection one, you really should 'trim back' the copper to 'new' and reinstall the brass ferrule inside the fitting over 'fresh' copper when retightening.