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Why is my engine suddenly flagging?

2019 October 1

Dear Boat Doctor,

I am having a problem with my engine, a Yanmar 4JH2DTE.  It has always worked well, but recently I have not been able to get it to run over 2,200 rpm. I checked the Racor and the filter looks clean, so I am not sure what is going on.

Brian Kensington

Norfolk, Virginia

Dear Brian,

Since your engine was working, I suspect that you have a fuel issue or possibly an issue with excess drag. If you didn’t mention that the engine was working well, I may have suggested taking a look at the pitch of the prop. A prop with too much pitch puts excessive load on the engine, but that is likely not the case here.

In terms of drag, your boat may be just too hard to push through the water, which could be the result of a really dirty bottom. A little slime and hard growth isn’t an issue, but if you are growing an underwater garden, it may just be taxing your engine too much.

It is more likely that you have a fuel problem, and fuel filters are a great place to start. Your engine has two fuel filters, a primary filter on the engine and a secondary Racor filter. These filters are designed to provide staged filtration, the Racor pulls out the “big” dirt and any water, the on-engine filter gets the rest.  Try changing both filters and see if your performance improves.

I have heard people describe that they use a very fine Racor filter element to avoid having to change the on engine filter. This is a mistake. In doing this you are not implementing staged filtration, the Racor will clog more quickly and the fuel flow will be impacted. Secondly, the on-engine filter is not designed to last forever as the corrosion inhibitors will dissipate and the filter element itself will break down over time.  I would recommend that you use a 30-micron filter in your Racor and change them both once a year or more often if you run into dirty fuel situations

If you have a single Racor 500 housing, it is impossible to know if the is filter is clogging, even if you open it up. I think a vacuum gauge is a great idea. Racor has made this very easy with their T-handle vacuum gauge that replaces your existing T-handle. If the filter starts to clog, the needle will go up, letting you know you need to change the element.