When cold the port engine will start and run through all RPM ranges up to WOT normally. It will run at 4200-ish for long periods of time (tested up to an hour) with no issues. The issues occurs after 3-5 minutes of running at WOT. The engine will begin to stumble and surge, dropping RPMs down to 3000-4000 and bouncing around. Once it starts happening, you cannot get engine back to WOT. If you go back to idle, you can slowly raise RPMs back to about 4200, where the engine seems to run great. It will sometimes stumble in the 3000 range of RPMs, but once it reached 4200, the stumbling ceases. If you then try to go higher, it starts stumbling again. Pumping bulb does not help and VST appears to be filling properly. There are no codes thrown.
History:
Took out last fall, engine started running rough with 10 minutes. Took to mechanic and figured it was fuel related.
The next steps were to try to isolate the issue by swapping parts between engines. The injectors and high-pressure pumps were swapped, but issue remained on port engine. At this point everything external to the engine that was a likely problem had checked out and the mechanic was not able to take the troubleshooting any further. Mechanic suspects a faulty sensor that is causing issues.
I decided to test it myself and rule out anything else external to the engine. I did the following.
History:
Took out last fall, engine started running rough with 10 minutes. Took to mechanic and figured it was fuel related.
- drained fuel
- replaced hoses, water separators for both engines
- replaced VST filter
The next steps were to try to isolate the issue by swapping parts between engines. The injectors and high-pressure pumps were swapped, but issue remained on port engine. At this point everything external to the engine that was a likely problem had checked out and the mechanic was not able to take the troubleshooting any further. Mechanic suspects a faulty sensor that is causing issues.
I decided to test it myself and rule out anything else external to the engine. I did the following.
- Checked for air obstructions on intake, none found, screen was clean.
- Removed fuel line from the water separator on the engine side and connected to a hose/bulb that were straight to gas can to eliminate the tank/separator. No change.
- I removed the fuel delivery from the VST to the low pressure pump (VST --> hose --> Low pressure pump) and swapped these between engines. Port engine was connected directly from gas can to VST. I also swapped the plugs between engines. No change, so it doesn't appear to be VST, plugs, or low pressure pump or any fuel hoses before the engine.