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RSRBOB
RSRBOB, Technician
Category: Boat
Satisfied Customers: 1078
Experience:  Former Yamaha Factory Service Rep, Dlr Line Tech, Serv Mgr, G/M
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Have 2004 yamaha fx ho, starts fine, when I accelerate,

Customer Question

Have 2004 yamaha fx ho, starts fine, when I accelerate, it stalls
Submitted: 1 year ago.
Category: Boat
Expert:  RSRBOB replied 1 year ago.
Hi and thank you for your question.I would like to know what the spark plugs looked like as far as coloration and deposits that you removed from the ski. On the surface it sounds like a fuel starvation problem. These skis are sensitive to oil contamination of the air filter but that should not cause it to stall, unless it is worse that you see. If you have more than 50 hrs on the ski, it is probably worth it to replace the A/F anyhow.The next test I would suggest is determining whether it is running out of gas or not as you throttle up. There are a few ways to test this. If the ski will exhibit the problem running it on a trailer and a flush kit, you can do that. If it doesn't exhibit the problem when the jet pump isnt under load, you can leave the ski securely strapped to the trailer and back it into the water. If you don't have a trailer, you can secure it to a sturdy dock post and pull against that. What you want to do is remove the top of the air box so you can look down at the injectors and watch the injectors spray as you rev the engine up. If you notice the spray diminishing as you rev the engine up, we have a fuel pump problem. The first thing I would do then is remove the fuel pump from the fuel tank and inspect the filter/screen on the fuel pick up for the fuel pump. You will have to remove it from the large plastic tube to access it. If the screen is not clogged or restricted, the actually pump could be failing. At that point, you will need to test the fuel pressure coming out of the pump going to the injectors.Let me know what you find and if you need further assistance.ThanksRSRBOB
Customer: replied 1 year ago.
Spark plugs from 1st and 3rd cylinder look fine, 2nd and 4th look wet and black. Engine oil smells like there is fuel in it. I can see fuel or oil residue in the lake water. Thank you
Expert:  RSRBOB replied 1 year ago.
Thank you for the additional information.
Based on that, have you checked the oil level recently?
I suspect it is going to show over filled.
If it smells like gas, change it no matter what. Make sure you don't over fill it. That can cause 2 problems, one is sucking oil in through the intake system and the other is it could cause the crank to be operating in oil, which it is not supposed to do. That would be similar to swinging your fist under water. It is hydraulic drag on the crank. The dark wet plugs are probably a result of the gas in the oil causing the probable high oil level.
So, game plan, replace the oil and filter, replace the spark plugs and test. I suspect that will resolve the issue. If not, the next step, or, if you are curious, prior to testing, is do a compression test, paying special attention to the test results from cylinders 2 and 4 since those plugs were wet and black.
Thanks, Customer
Customer: replied 1 year ago.
I did all that and it didn't fix the problem. I started it on the trailer today and heard very loud bang. It idles ok, when it stalls, it does take 4-5 tries to start again. Should I check oil level when engine is hot? It feels like engine is flooded. Thank you
Customer: replied 1 year ago.
I googled, and there is a guy who had same issue, turned out to be ECU.
Expert:  RSRBOB replied 1 year ago.
Thank you for the additional information.
I am curious if you did do the compression test? Since you haven't mentioned results, I suspect not. Although it certainly could be an ignition/electrical issue, the loud bang reminds me of a metallic type mechanically generated sound. A compression test is not intended to fix a problem, but to eliminate one of the 3 elements required to make an internal combustion engine run. If the compression turns out to be in spec on all 4 cylinders, then you have totally eliminated 33.3% of the possibilities by eliminating the mechanical condition of your engine. At a suggested retail price of about $1500 for a new ECU, I would imagine you would want to be pretty darn sure it is the ECU before you plunk down your hard earned cash on a new one. Bear in mind a couple more things. First, one person having what sounds like similar symptoms and resolving it with an ECU replacement is no guarantee that is wrong with yours. One incident certainly doesn't make it a common failure. If it is an electrical issue, it could be other components and not the ECU also. If you are relatively confident that is what is wrong with yours, your best move now would be to take it to a Yamaha dealer and let them connect it to their proprietary software to run a diagnosis on your ECU. Customeredited>42183.2144627083

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