Motor losing RPM at WOT

[ohsoslow]

Hey guys, this one has got me a bit stumped, and I've been searching for answers, but seem to be drawing a blank in finding a solution!

Motor is a 1997 225 EFI salt water series, I believe it's an OX66 (as it has an o2 sensor)

Basically the motor runs brilliantly, asides from when you cruise along and then bury the throttle, it takes off, hits about 5000rpm, and then after a few seconds, will lose approx 400rpm. I can back off the throttle, and then put it down again, and it will do the same thing.

So far in my searching through the net, looking for information. I have replaced all the fuel filters, including the VST filter, I have replaced all fuel lines and made the run from fuel tank to motor as short as possible. I have also now just replaced all 3 low pressure fuel pumps. I've checked and cleaned the o2 sensor, all seems to be reading ok. And changed spark plugs a few months ago. Also checked compression, all even at 110psi.

The only oddity I noticed today while out on the water, was when trimming the motor up at the start of the day, it would lose 100rpm each time I pressed the trim button. Not sure if this is related or if I'm just imagining things.

Where should I be looking next?

Any help or advice would be much appreciated! Thanks!

[99yam40]

monitor fuel pressures both before and after VST to see if one or both are dropping when problem occurs should help pin point your trouble and tell you if it is fuel related

[jb123]

voltage/rpm drop sounds like a bad ground
varying rpms from my exp is either dirty VST or dirty 02, both which you say you checked
did your 02 test vary quickly on gauge ?

[ohsoslow]

Hey guys, thanks for your replies!

I'm going to reclean the o2 sensor and check it (I think the voltage was fluctuating reasonably quick, but I wouldn't say superfast, maybe once a second)

And I've brought a can of powertune to try to decarb any build up in the engine, while out on the water putting that through, will hook up a fuel pressure gauge and measure the readings.

Will try to get out on the water tomorrow after work, so will let you know how I get on!

Will be pretty gutted if it's the o2 sensor, those things aren't cheap!!

[Rookiemove]

fuel pump filter

You sure its not the filter in the fuel pump. I had a similar problem when I punched it or trim motor. My filter was clogged so the float would drop in the fuel pump housing unit because lack of fuel.

[ohsoslow]

Sorry I missed that last post, I have replaced the boat fuel/water seperater filter, and the motor fuel filter, as well as remove the VST filter and give it a clean (it wasn't clogged up at all in the first place) The problem isn't instant when going full throttle, it'll build revs, hold them for approx 5-10 seconds, then lose rpm.

The weather here hasn't been ideal for WOT runs unfortunately, but I have run a can of powertune through the motor, and recleaned and tested o2 sensor and housing. Voltage changes fast, and will fluctuate when I blip throttle (only been able to test in neutral)

I took the boat out for a run today, and ran fine, and idle seems to be improved. But haven't been able to try WOT to see if problem has gone away.

One thing I have noticed that seems a bit odd, and possibly the cause of the problem, the fuel filter on the side of the motor, the fuel level is only up to approx half way after taking it for a run?
Unsure of the cause of this. I can prime the line, bulb goes rock hard and stays hard, no signs of fuel leaking out anywhere, so I presume air can't be getting in if fuel can't get out?

[jb123]

it will never stay full
monitor fuel rail pressure at varied rpms

[ohsoslow]

So it's normal? Weird!

I'm in the process of sorting out getting it on the water for a run with a fuel pressure gauge on it. I figure giving it a rev in neutral won't really achieve anything. Will let you know how I get on.

[Tucker]

1) They made the test leads much longer for the 02 sensor, 'bout $45. Get someone to run the boat and check voltage
2) X2 on bad ground. Trim motor and RPM's drop?? Been reading about lots of weird stuff happening with bad grounds.
3) Sounds like classic LP fuel pump problem. Did you by OEM fuel pumps?
4) Get rid of the check valve between the filter and LP pumps. Ethanol seems to screw up the check ball.

[ohsoslow]

Hi, thanks for reply!

Will be taking it for a run this weekend to test fuel pressure, and will get o2 checked at WOT etc too. (at the moment water sprays up onto motor from transducer and speedo gauge device, so have been making a plate to deflect that before running with cowling off)

Yeah I agree with the bad ground, I noticed the gauges dim when it happens as well, motor starts fine, trims well etc, so will redo the wiring from battery to console and go from there.

Yeah got all three genuine yamaha LP pumps.

We don't have ethanol much in NZ, only one service station does it, and even then it's only E10. But I was thinking of running a new fuel line, from outboard fuel filter, straight to a tote tank, and taking that, to see if theres any problem with the existing lines and boat fuel filter.

[ohsoslow]

OK, bringing this back up, as I'm about ready to undo the bolts and drop the motor off the back into the ocean!

I'm at a loss as to what the hell is wrong! Now WOT just goes to 4800rpm, and stays there happily. (Has lost 300+rpm) I tried a mates prop, which is smaller, and in theory should of pulled 5700rpm, and only pulled 5300rpm. So it's almost like the motor has just plain lost power?! Also, start it up in the morning, will idle for say a minute then it'll do a random sneeze and stall, restarts straight back up like nothing happened. (unsure if that's related, but never used to do it)

I have now changed all fuel lines to motor, including removing that check valve, have taken injectors off and cleaned the filters in them (were pretty clean anyway) Have replaced the o2 sensor (and checked it's voltage, works mint) Have replaced all 6 spark plug caps, as well as the spark plugs themselves. And have done a compression test, 130psi across the board. Also have replaced the thermostats, checked TPS (0.5v thru to 4.0v at WOT) And checked fuel pressure throughout the revs, 36psi constant.

Where to next? Give up and take it to someone?

edit: I forgot to mention, I changed the reed valves to CCMS composite reeds, perhaps the other parts have fixed the initial problem, and now the reeds are causing the loss of power and random sneeze at idle? (in other words instead of improving performance they've lost me power?)

[Tucker]

Never heard of anyone replacing reeds with aftermarket and can't comment; but unless something was wrong with the stockers I wouldn've messed with them. Just throws something else into the troubleshooting process. Ohm out the spark plug caps, there's a resitor in there. This seems to have become a problem with the older motors. Been reading about guys have similar problems and discovering high resistance on the caps. Must be an age thing. I'm checking mine this weekend. They just unscrew and should ohm out at 5 ohms. Might also consider buying a winky blinky test light and check for codes. Don't get discouraged, got a lot going on under there and just need to find the offending item. Have you taken care of that bad ground?

[99yam40]

Only way I know to be sure is to put the factory reeds back in to see.

Might check with Yamaha shop to see if they have the test prop for your motor to run the motor in water to see if it is putting out the proper hp and hits proper RPM

If the rail pressure is not dropping when the problem is happening then the fuel supply side should be fine.

Maybe you need to send off the injectors to have them flow tested to be sure they are working properly

[jb123]

linc n sync ?
check lp pumps ?
clean screen behind pres reg ?
you may want to pull each inj lead off to see if one stands out from the rest

[ohsoslow]

Hey guys,

Bringing this up from the dead, as I replied in another forum and forgot to in this one.

After a LOT of checking this and that, replacing a whole lot of things, all of which I think probably needed doing anyway as general maintenance, the end result was tracked down to a faulty coil!

I'm sure a coil test or similar would of shown this up straight away, and saved me a whole lot of hassle and money, But coming from an Automotive background, I was sure I'd notice/feel a misfire or lack of spark on a cylinder!

I'm happy to admit I was wrong, and hope this helps someone in the future with a similar problem!