2003 Yamaha F115 will not get over 3000 RPMS when in gear

Bamacrusader

Recruit
Joined
May 30, 2015
Messages
3
I just bought a 2003 Sea Pro with a Yamaha F115. The previous owner had everything tuned up before he sold it to me. However, the boat will not get over 3000 RPM's when in gear. It seems to idle fine and run well in neutral, but not under a load. All filters have been replaced within the last month. I personally drained all old fuel out and put in 15 gallons of pure gasoline. Fuel pressure is a steady 41 psi. All spark plugs are firing. However, the #4 spark plug looks brand new after running, while the other three are blackened a little, but not fouled. I also replaced all 4 fuel injectors. I used a simple, basic test light to see if all my injectors were getting a voltage. All four of the red/yellow wires feeding each injector light up the test light like it is getting a good 12V to them. The top three injector wires that are purple/striped, light up the test light, but the test light is dimmer than it was when probed on the red wires feeding the same injectors. When I test the probe on the light purple/green striped wire feeding the fourth injector, the test light does not come on at all.

So, I have come to the conclusion that my fourth injector is not getting the signal to open/close like it is supposed to, and no fuel is entering my fourth cylinder. And that is why the spark plug for my fourth cylinder looks brand new, even after running the engine for several hours. I traced the wire back to the ECU. The wire is not damaged and has continuity from the injector back to the ECU. Does all this mean my ECU is bad? Or could something else be causing the injector not work? Thanks in advance for any help.
 

Bamacrusader

Recruit
Joined
May 30, 2015
Messages
3
I am sure that the injector itself is not the issue. I just replaced all the injectors with rebuilt ones that came with a graph of how each injector tested out and guaranteeing they are working correctly. I also had the same exact problem before I changed the injectors out. The voltage going to this individual #4 injector is different from the other three injectors.

I also performed the following test, while the engine was running at idle:
I unplugged the electrical connector from each of the other injectors, one at a time. Whenever I did this for each of the first three cylinders, the engine immediately started to run very rough. When I plugged them back up, the engine smoothed back out. Whenever I unplugged the fourth injector, the engine RPM's actually picked up and seemed to run even a little smoother. So I am pretty sure I have narrowed it down to the voltage/signal coming into that #4 injector. I just need to know what would cause this (bad ECU? and what may have caused the ECU to go bad and do this, if ECU is bad at all).
 

schematic

Lieutenant Junior Grade
Joined
Jan 12, 2008
Messages
1,102
Sounds like the ecu is shorted to ground on #4. It is also possible that the light purple/green is shorted to ground somewhere in the harness.
From what you describe, the injector is being held open which usually results in burnt injector...check the injector for resistance compared to others....it may have been damaged.
Electronics can fail in pretty much any way....
 
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