Have a Doosan GC25E-3, serial JZ-00180. Mitsubishi G424E engine with a woodward regulator and mixer. Original complaint, engine stalling and misfiring. Retrieve ECM codes. Found code 27. Checked vacuum at intake. Had no vacuum. Removed mixer. Found bottom of the plunger broken. Replaced mixer. Test and found engine still had misfire but not stalling anymore and vacuum restored. Did tune up, plugs, cap, wires, rotor. Engine running smoothly. Returned to service. Four days later, called back out. Engine stumble???? Check issue. Found engine to stumble intermittently at idle and through RPM range. Removed air intake from mixer. Found the plunger inside the mixer opening (plunger moving down) on its own, like someone is feathering the throttle pedal. Does this through RPM range as well. Checked manifold vacuum. Found that when the issue presents itself, the vacuum drops from 17 inches to around 6 inches but then catches itself... Never stalls out. I was certain that the throttle motor was bad, so I changed it. Still same issue. Swapped throttle motor back out. Checked for codes. No hard codes stored, however when the engine stumbles, the MIL light will flash 1 time. Unplugged foot TPS, O2 sensor and ECT sensor. Issue remains. Here is where it gets interesting. If I unplug the MAP sensor the issue goes away but I then have no throttle response. Renders the throttle motor operation. Any info or help would be greatly appreciated.
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Trim valve controls vacuum to lean or richen the fuel system. Usually either 1 or 2 of them located near the fuel regulator. They are plastic. Round in the middle with tubes on top and bottom that have vacuum lines attached to them. There is also a 2 wire plug connected at the round part. Electronically controlled. Google the part number A343009 for a picture. Hope this helps.
What is the trim valve and what does it look like
Replaced trim valve. Issue resolved. Hope this helps someone.
Replaced MAP Sensor, no change no go. Still troubleshooting.
Also checked regulator pressure. It has a solid 2, 2 and a half pounds of pressure all the time. Removed cover from regulator. Signs of minimal tar build up. Clean for the most part. Really stumped on this one. Also checked PCV valve, trim valve, vacuum hoses, checked for vacuum leaks, checked dry vapor hose etc....
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