YAMAHA 150 HPDI Intermitting Rough Running Problem

I’m a second owner of a YAMAHA 150 HPDI (Z150TXRZ) outboard engine that is giving intermittent trouble. The engine has about 10 hours on all new filters throughout the engine. On some days it runs good and on other days it runs very rough at idle and when throttled up it only ramps up to approximately 2,000 RPMs then cuts back and almost stalls. The engine has approximately 300 hours on it with approximately 170 being at or below 1,000 RPMs. I wire brushed the spark plugs that were heavily carboned. I brought it to a YAMAHA mechanic who cleaned the O2 sensor, electrically tested both ends of the main engine wire harnesses, inspected and tested the pin squeeze on the electrical connectors and the output from the various sensors, which were all acceptable. He also connected the computer to the engine and told me that all readings were within specification with the exception of the MAP sensor which at times had readings of zero while the engine was running rough which he thought was a symptom of the problem. He faxed the computer readings to YAHAMA which they came back with a suggestion to replace the engine’s main wire harness assemblies and the ECU. After discussing this with other YAMAHA mechanics I’ve been told that when the ECU fails the engine will always run rough or not at all. Based upon how the engine is running (good one day and bad another) I’m skeptical that the ECU and the main engine wire harnesses are bad. Have you seen anything like this? Do you have any suggestions on what I should check before spending approximately $2,700 on these items? I’m planning to obtain hard copies of the computer readings that the mechanic faxed to YAMAHA next week.

[rkmartin]

Make sure to run fresh fuel (Ethanol Free if possible) and replace the spark plugs with NEW NGK's. Spark plugs are too cheap to clean. Especially if you found them practically carbon fouled. I'm not trying to push a particular brand but NGK plugs are OEM and are hard to beat in two stroke applications. Make sure to gap them properly as well. Run the boat for a few hours, if it still runs bad pull the plugs and take a look at them. If they are black that indicates a fuel "rich" condition. If the plugs are bone white that indicates a "lean" condition. Your engine might have a more serious condition but changing and inspecting the plugs will give you a general idea of how the engine is running before you drop some serious cash for someone else to do the same thing.

[bigfish1229]

personally i like the sounds of the moin engine harness being the culprit. has a short maybe. check all your frounds too. i have a 300 hpdi that did the same thing it had a loose ground on the ecu. when there is a weak ground the ecu gos into protect mode so as not to harm anything. the vst does sound like a fair diagnosis too though.

I checked every wire and cleaned each mateable connector with connector cleaner that I could access and blew them dry with a small amount of compressed air. I found one of the two green wires that came off of the magnetic pick-up unit near the flywheel scrapped approximately ¼-inch in length exposing the aluminum wires. The scrapped wire was approximately 1-inch from the unit and ½-inch from the flywheel. I’m not sure if the scraped wire was close enough to the flywheel to affect the ECU. I removed the bad section of wire (to show the mechanic) and used a marine grade butt splice and heat shrink tubing to repair the wire. I also wrapped each wire individually with electrical tape.

New spark plugs (NGK BKR6EKU) are due in sometime today. Mechanic told me that the fuel pressure (vacuum) coming from the onboard tank into the motor was low 3 in lieu of 5 (I’m not sure of the units, MPa or in-Hg) and told me to replace the ¼-inch fuel line with 3/8-inch line, which I did yesterday.

Mechanic did replace a filter P/N 68F-13915-00-00, I’ll check to see if this is the VST filter.

Present plans are to change out the spark plugs, drop the boat in the water and see where I’m at.

I’ll keep you posted.

Thanks so much for your suggestions! They are very much appreciated.