2002 HPDI 200 HP---Need y'alls help.

[Wild Heethen]

Engine idles fine. Comes out of hole fine. But, when running on plane, regardless of rpm, about every 30 seconds or so the rpm will drop about 500 rpm for about 3 to 5 seconds then jump right back up to previous rpm without moving throttle. Does this repeatedly regardless of rpm running. It did this just a couple of times over a days time about a month ago. But I just figured must have been just a shot of bad gas at the time. Has been running good until now. Usually when I go fishing I put just 10 gallons of gas in only tank I use because don't want to leave bunch of gas sitting in tank going stale. I always run ratio of Ringfree at 1 ounce per 10 gallons and Marine Stabil at 1 ounce per 10 gallons. Yesterday I changed out fuel/water separater filter since it's been a while for that. Changed out bulb in fuel line earlier this Spring because original was getting 'hard'. Used Yamaha brand. Every 100 hours I take it to Hydro-Tech for them to go through for tune up, change all filters, water impeller, plugs, and whatever it needs. But, has been a while. Wide open for all advice and things to check. Thanks a lot.

[Wild Heethen]

Well, now I am kinda worried. None of you guys ever seen this problem before? Haven't had a chance to run it since the filter change. Hopefully that fixed it. Thanks anyhow.

[jdwo2212]

Do you actually feel the rpm drop, or is it just the gauge? The connections on back of gauge get corroded sometimes and need cleaning. If you are actually feeling the drop in throttle, i would start looking at the fuel system and maybe get compression checked. Especially if its surging.

[Wild Heethen]

Definitely feel it and see it on gauge. Acts just like bad fuel. But, like said, I only buy 10 gallons at a time and usually burn most of that up each trip. Changed fuel/water separator filter and haven't had a chance to try it out since. Hoping for the best and will find out next week. Up in Kansas now working on Daughter's house.

[SkeeterSD]

Had the same thing happen to me a few years ago. Ethanol fuel built up and clogged filters. Replaced all filters and it was good as new

[Wild Heethen]

SkeeterSD, thanks. Took it to lake today. Was some better after changing water separating filter. Wanted to burn up about 8 gallons of fuel in tank. Spent a lot of time aggressively zig zagging to mix fuel up good to burn through. Really only dropped RPMs occasionally when at 5000 or above. Finally got to where bottom bar on fuel gauge was blinking so went home and siphoned out about 3 gallons to empty tank. Will put fresh gas in next time and give it a shot. If still doing it will be heading to Hydro-tech. Will start another thread with new problem found .

[Wild Heethen]

Well, since changing fuel/water separater filter, have changed vst filter, gas filter in the clear bowl, and pulled oxygen sensor assembly out and cleaned all of that up. Sensor itself was pretty clean. Sensor housing was absolutely full of carbon. Sniffer tube that goes into block was almost 1 solid piece of carbon. Really surprised me since I ALWAYS use Ringfree and do a Blast Treatment with it every Spring. Was a real headache getting all this clean. I soaked housing and tube in Seafoam for 12 hours. But really don't think it was hardly any help. Question--do any of you guys know of ANYTHING that will melt this stuff next time? Haven't ran it yet. Will do in next couple of days with cooler weather arriving. Sure hope this fixes problem. Will let you know. Sorry for being so long winded.

[geodebasser]

Just curious. I have been thinking about putting my housing and sensor in an ultrasonic cleaner next time it carbons up. I recently cleaned up two hopeless motorcycle carburetors using my ultrasonic cleaner. They were a real mess with ethanol tar like deposits. I soaked them in carburetor cleaner and ran them through the ultrasonic cleaner. They came out like new.
Is there any reason that this would damage the O2 sensor? You can get an ultrasonic cleaner for $70. at Harbor Freight.

[Wild Heethen]

Geodebasser, that might work. I tried the Seafoam, brake cleaner, and carbcleaner. NOTHING would soften or melt that stuff. I have several different types of gun cleaners for cleaning carbon that I wish I would have tried. I've been wanting to get one of the Hornady parts cleaners that use heat and vibration with their solvent. Think I'll get one now. Next time I'm up at Hydro-Tech buying bulk Yamalube, I'm going to find out what they use.

[Wild Heethen]

Well took it for a run. Have now changed fuel/water separater filter, vst filter, clear bowl filter, and cleaned 02 sensor and housing. Filters all looked clean and inside vst tank was spotless. Will only 'stumble' above 5000 rpm for about 3 seconds about every 3 to 5 minutes. Now am going to replace both low pressure fuel pumps. Have learned Yamaha recommends this every 300 hours. News to me. I have about 450 hours according to my Yamaha gauge. If this doesn't fix it I'll probably just baby it until Winter and take her to Hydro-tech where I'll have them service the high pressure fuel pump, injectors, tune up, and check and set EVERYTHING. I'm going to try to change all my fuel lines from tanks to engine with new Ethanol-proof line just in case.

[Wild Heethen]

Took her for a run after changing low pressure fuel pumps. Still has 'stumble' above 5000 rpm. About only thing left I will dare do is change plugs. Thought about doing high pressure strainer filter above vst, but, will need special clamp with tool for installing (which I don't have) and costs 100 bucks! If plugs don't do it I'll just stop there and take it to Hydro-tech this Winter for 100 hour check, injector cleaning, let them change all the 'secret' filters in high pressure fuel pump and injectors, and whatever they reccomend. Guess I can't gripe too much cause this engine has been flawless for 18 years. Probably something simple but beyond me.

[MCIPinkie]

I ran that engine from October 2002 until January 2020. Ran it hard, put it away wet.

I found out, at least for my motor, that it was plug sensitive. Almost every time I had a problem, new spark plugs solved it. I carried 6 in the boat. Have put in new ones at the ramp.

You've spent a lot time not to have put $30 or so in plugs in it.

[Wild Heethen]

I ran that engine from October 2002 until January 2020. Ran it hard, put it away wet.

I found out, at least for my motor, that it was plug sensitive. Almost every time I had a problem, new spark plugs solved it. I carried 6 in the boat. Have put in new ones at the ramp.

You've spent a lot time not to have put $30 or so in plugs in it.

Yes Sir, you are correct. But, it just seemed like it was starving for fuel. But, plugs are next. You may be right because all replaced filters were clean and diaphragms in low pressure fuel pumps only looked slightly 'stretched' to me. At least I learned new to me skills doing all this. Thanks for your input. I'm always open to everyone's input. Will be a couple of weeks before I get to change plugs and test out because leaving next week for black powder deer camp. Thanks again.

[techman]

Change the 02 sensor. Did everything you did and still had the prob. Changed sensor and all hood now.

[Wild Heethen]

Change the 02 sensor. Did everything you did and still had the prob. Changed sensor and all hood now.

Going to change the plugs, after I get back from deer camp next week, and see if that fixes it. Have no idea how check to see if 02 sensor is even bad. I think it's probably working be cause was pretty clean when I pulled all that off earlier for cleaning. If plugs don't do it, will just use until this Winter when I was going to take up to Hydro-tech anyhow for a 100 hour checkup, injector cleaning, 'secret' filters changing, and let them check out and set EVERYTHING. Thanks for reply.