Engine Surging, hesitating, not NPS

[Red550]

Hi, been having trouble with the YD25 for a couple years on and off, normally after sitting at lights, taking off would have a massive flat spot then all of a sudden kick in, just recently it's become a constant issue. The EGR is blanked, I've changed the fuel filter, changed the NPS, checked all the fuel line fittings. The boost builds constantly like its running normally, which leads me to believe its fuel related? At a complete loss as to where to look next. Any help would be much appreciated. Thanks

 

[Red550]

Ran a fuel purge exactly as @RustyNav09 posted. It seems to have made the hesitation far worse. Like its got no fuel at all but only under load. So now I'm thinking either the pressure relief valve or the suction control valve. Also I have noticed the primer won't prime, doesn't get hard to press even after like 20 presses or more.

 

[Old.Tony]

If the primer's not getting firm, is there any sign of diesel leaking while you're pumping it? It does sound like it's just not getting fuel through properly. This might be the primer bulb itself, not the connections on the filter. You may also have something blocking the fuel pickup? That's harder to deal with, but if you're handy it shouldn't cost the earth to remedy.

 

[Red550]

Do you mean inside the tank? Are there known issues with the primer bowl?

 

[Old.Tony]

There have been issues with the primer bulbs in the past. They crack under repeated use, the valve stops working properly ... troubles!

And yes, the fuel pickup inside the tank. If there's some algae or dirt blocking it, you'll have issues drawing fuel through it.

 

[Red550]

Okay thanks heaps for that, I'll have a look tomorrow and see. Didn't think of the primer bowl itself.

 

[Red550]

Thanks @Old.Tony the primer bowl housing was indeed cracked, I've now changed it out, I found that the GU patrol housing which seems to be a hardier unit, and $150 cheaper fits with some small adjustments to the forward fitting. I have now pulled the tank as well which appeared clean as a whistle. But unfortunately Im still having the same problem, its slowly getting worse having now dropped into limp mode twice making me have to pull over and shut it off and restart it. I'm seriously starting to think fuel pump failure. Any other advice before I drop a couple K on replacing it?

 

[Red550]

Seems to be solved!! Changed the suction control valve, which took all of half an hour, and hasn't hesitated or surged since.