I've got twin 250s and during a long offshore trip last week the starboard motor started running rough. I had them set at 4300rpms and was cruising in when the starboard engine would lose rpms and shake and run rough. It got worse as some time went on and would sort of stumble for a sec (you could feel the boat slow) then would pick up again, and so on. We stopped at one point and upon restarting it was running very rough and couldn't even bring the RPMS up, so we ended up going the last 20 miles on one engine.
I started trying to troubleshoot it today. Fuel rail pressure is at a constant 35psi, regardless of load and even when it stumbles and runs rough. Compression is 110 to 115 on all cylinders. I cleaned out the VST tank and filter, replaced the spin on filter and when I put that tank back together, it ran better at idle. I drained the tank first, no water or sediment in the fuel. But upon testing, it's still stumbling above 4000rpms. Then i noticed a slight fuel sheen when the engine is just left idling. I thought it may have been the 3 diaghram fuel pumps, so I pulled all three of those. I tried to blow in them backwards and each of them basically does not allow much air to move backwards, however, there is a little bit of air that could get through. I opened up 2 of them and the rubber gasket thing looked to be intact. I also checked the TPS using a DVM and checking the resistance. The resistance changed as you opened the throttle, i think it went from 5.4k ohms at idle to 1.6k ohms wide open. I've also got the O2 sensors from both motors soaking in carb cleaner right now. The one from the problem engine was a lot darker than the other with a lot of soot caked up on the tube, but not blocking the tube. These are pretty much the symptoms I have so far and what I've done. I will replace the 3 fuel pumps tomorrow and reinstall the cleaned O2 sensor, but I'm not convinced this is my problem. Rodbolt or anyone else...do you have any suggestions for me? I'm really trying to avoid the dealership. I'm not overly impressed with their service and want to learn these engines for myself. Any input is greatly appreciated. Thank you.
Marshall
I started trying to troubleshoot it today. Fuel rail pressure is at a constant 35psi, regardless of load and even when it stumbles and runs rough. Compression is 110 to 115 on all cylinders. I cleaned out the VST tank and filter, replaced the spin on filter and when I put that tank back together, it ran better at idle. I drained the tank first, no water or sediment in the fuel. But upon testing, it's still stumbling above 4000rpms. Then i noticed a slight fuel sheen when the engine is just left idling. I thought it may have been the 3 diaghram fuel pumps, so I pulled all three of those. I tried to blow in them backwards and each of them basically does not allow much air to move backwards, however, there is a little bit of air that could get through. I opened up 2 of them and the rubber gasket thing looked to be intact. I also checked the TPS using a DVM and checking the resistance. The resistance changed as you opened the throttle, i think it went from 5.4k ohms at idle to 1.6k ohms wide open. I've also got the O2 sensors from both motors soaking in carb cleaner right now. The one from the problem engine was a lot darker than the other with a lot of soot caked up on the tube, but not blocking the tube. These are pretty much the symptoms I have so far and what I've done. I will replace the 3 fuel pumps tomorrow and reinstall the cleaned O2 sensor, but I'm not convinced this is my problem. Rodbolt or anyone else...do you have any suggestions for me? I'm really trying to avoid the dealership. I'm not overly impressed with their service and want to learn these engines for myself. Any input is greatly appreciated. Thank you.
Marshall