2QM15 Problems Solved

[RoyS]

After twelve years of effort and thousands of dollars spent, I have finally solved my problems with the Yanmar 2QM15 diesel engine. There are two likely problems with this poorly designed engine. The first problem is that after a few hours of operation the engine fuel lines may need to be bled. This problem is caused by the closed circuit in the injector piping that leads the injector return line back to the secondary filter instead of back to the fuel tank. This results, over time, in air build-up within the filter. Eventually this reaches a point that requires bleeding. Mysteriously, this only happens at critical moments, like when docking. This problem may be solved permanently by redirecting the return line back to the fuel tank. This necessary repair, however, leads to problem number two. That problem is hard cold starting. Un-intuitively, it has nothing to do with cold. This problem, solved by my local injector service company, is that the fuel siphons back through the input fuel lines overnight. This is partly because the air may enter the system via the new return line connection at the top of the fuel tank. Whatever check valves there are apparently leak. A $20.00 check valve must be installed after the injectors on the return line. Cold start in three seconds or less and no bleeding required.

 

[Dunlookin]

I have never had this problem with my 1978 2QM15. I find it suprising that someone would call an engine that has served owner after owner well for over thirty six years a," poorly designed engine."

Too bad the person who designed this "bullet-proof" engine did not work on ignition switches for GM.

 

[Stephen Trapani]

The 2qm15 is one of the best internal combustion engines ever made. Never had the problem you describe until I didn't tighten the line to the injector tight enough allowing air into the lines when it runs. Sounds like you've got a leak somewhere.

 

[weinie]

How could air get into the return line unless it was in the fuel system to begin with? You had an air leak somewhere.

 

[Benny17441]

Different engines may have different quirks so it does not help to generalize. I'm with the fellows and think that your problem may arise from an undetected air leak. Re-engineering the factory designed system may create other problems as you already have experienced. Check the banjo fittings and the filter seals.

 

[RoyS]

Thank you all for your input. With regard to the modification on the 2Qm15 of placing the diesel return into the fuel tank instead of the secondary filter: That is the more common arrangement. The QM was only produced for about two years. After that the GM series reverted back to the fuel tank return. Two diesel mechanics recommended this conversion. i do not know where the air in the fuel line comes from. The lines have been replaced and examined many times. The original owner's manual that came with the boat was full of hand written notes regarding bleeding the lines. The person I bought the boat from also reported having this problem. After that modification the problem was gone, five or more years now. Thanks again.

 

[Johnb]

I have found that after changing fuel filters and bleeding the system my 3qm30 will start readily and idle OK but stall under load. Re bleed and she is good to go for another year.

 

[rardiH36]

I already knew that my very well performing 2QM20 lacked the fuel return line common to most small boat diesel engines.

I have had two air-in-the-line failures during my seven year ownership of the 34 year-old engine. Both times were when the tank fuel level was <25% and while motoring in choopy conditions = air getting sucked into the fuel lead. In both cases, after sailing into calmer waters, bleeding restored normal performance. 99% a day sailor, with typical engine usage <30 minutes each outing, I now replenish the fuel level at 50%

 

[RoyS]

Regarding the air in fuel lines issue with the 2QM15. Yanmar abandoned this brief experiment with a closed circuit fuel line. The GM and YM series both had a return line to the fuel tank. In the YM maintenance manual that return line is referred to as an air removal device eliminating the need for bleeding the lines. As to where the air comes from; there is a paper from Parker (Racor) Filter Company that describes how air bubbles coalesce in diesel fuel when there is a pressure drop in either the lines or filter body. That short two page paper is worth the read. Find it at:http://www.parker.com/literature/Racor/Mobile_Air_Separation_In_Diesel_Fuel.pdf

 

[kwaai]

for my 2qm20 I added an electric pump inline before the filter, now if I need to bleed the system, just switch the electric pump on and open each bolt in sequence, could not be easier or faster