I have a 1999 Yamaha 200 0X66 outboard that I bought last winter, with an unknown number of hours. Judging by the maintenance records, it looks like the previous owner had all the scheduled maintenance done, but I couldn't find any records for 2004. <br /><br />I have been using the boat since May, and probably put at least 200 hours on the engine. I have never had any problems with the engine.. at the beginning of the season I was able to get 5400 RPM WOT without any problems or hesitation. I typically run about 4k RPM and never go above 4.4k RPM in typical use.<br /><br />Last week I thought I would try another WOT run to see how the engine behaved and if the top speed changed any. When I got to around 5k RPM the engine started to surge a little bit up and down and couldn't hold RPM. After doing some research (and finding this forum) I believe the culprit is partially the 02 sensor.<br /><br />I jury rigged a test harness and measured the output voltage of the 02 density sensor. Instead of ranging between .4 and .7V, it started at less than .1V and slowly increased to over .7V while the motor was running for 5-10 minutes. I tried blocking the hole on the butterfly valve (I think thats what its called) for cylinder #1 and it made no difference.<br /><br />I removed the O2 density sensor and found that the draw tube was completely solid with carbon, as was the sensor itself. I slowly worked a drill bit (by hand) through the draw tube and have been slowly picking away at the carbon with a small screw driver. <br /><br />Now I am soaking the sensor, draw tube and cover in seafoam and hopefully will be able to clean them up quite a bit more.<br /><br />Two questions --<br /><br />What are the chances that the 02 sensor is still functional after all this carbon build up, and what are my chances for cleaning it back into a usable state? Will soaking in Seafoam for a few hours or overnight loosen things enough that they can be cleaned up?<br /><br />More importantly, what damage could I have caused by running the boat with this much build-up? Anything else I should check as a result?<br /><br />I've now 'educated' myself on the more common preventative maintenance that needs to be done to these engines.<br /><br />Any info is greatly appreciated, I have learned a lot by reading through the archives on this forum!<br /><br />Thanks,<br /><br />-- Tom
