Yamaha 70hp 2str Limp Mode

[davepros]

I'm having some issues with a 2000 Yamaha 70hp 2 Stroke.

It is 'spontaneously' going into to limp mode.

It's been to the marine mechanic 4 times in the past month for him to only guess at what the problems is.

He has replaced: thermostat, temp sender, oil sensor and most recently the impeller thinking it may have been an water pump issue.

I used the term spontaneously because it will run fine 5200rpm for a period say 15mins, then once switched off for a 5mins and back on for a another run, it will enter limp mode running at 2100rpm. By sitting in forward propulsion at idle speed for 3-5mins, I can then get it back up to 5200rpm. The engine temp after being worked for a while is only getting to 72 degrees Celsius and the tell tale is putting out cold-warm water, 30-40 odd degrees Celsius.

In the early stages it was getting oily spark plugs, I believe that problem is rectified, but could double check

My thoughts were electrical, but could be wrong. What are your thoughts?

[99yam40]

Does the alarm buzzer work?
And is it sounding when the problem shows up?

Might check spark on all plugs and the timing when the RPMs are limited(problem shows up) so you have an idea of what is going on

[davepros]

I took it for a run today, after talking to the mechanic again, I tried connecting and disconnecting the pink wire from the computer to the temp sender. The temp sender is new and functional. While disconnected it ran fine, when connected I had issues. No signs of overheating what so ever.

Replace computer?

[rodbolt17]

with the pink wire disconnected from the ECU it runs fine.
replace the ECU?
flawed logic.
what would Mr spock do?

you should be getting an audible alarm when and if its actually a fault.

if no audible then chase corroded connectors on the pink and gray leads.

this circuit is like a light switch.
its on,grounded or off,open.

if you buy a new ECU then you will most likely have a spare ECU.

[99yam40]

I tried connecting and disconnecting the pink wire from the computer to the temp sender.
While disconnected it ran fine, when connected I had issues. No signs of overheating what so ever.

Something is telling the computer there is a problem if it happens with the pink hooked up but not with it unhooked, so why would you replace computer?

Not sure how you figured out there is no sign of overheat, but measuring the temperature where the switch is located would tell you if it was seeing a elevated temp. Which it probably is if that is new also

Is there a reason you did not answer my questions about the buzzer?

[Aquasportfisher]

I may be missing something, but I don't see where the OP said anything about replacing the ECU, yet you're ripping him pretty good for it- "flawed logic" and all. I don't see where that is .... logical.

The problem is somewhere along that pink wire. If the sensors are good, and disconnecting the pink wire from the ECU turns off limp mode then it's not a connection corroded or a break in the circuit. I agree that he may have a faulty buzzer. He may be getting a warning and not know it. Test the buzzer- it should self-test when you turn the key on.
If you are getting the overheat warning, could be a bad thermostat or blocked cooling passages. Hope this helps.

[99yam40]

I took it for a run today, after talking to the mechanic again, I tried connecting and disconnecting the pink wire from the computer to the temp sender. The temp sender is new and functional. While disconnected it ran fine, when connected I had issues. No signs of overheating what so ever.

Replace computer?

no but he asked if he should

[rodbolt17]

reading comprehension. a lost art.
I have trouble with speeling sumtymes.
butt I kans reed abit.

[boscoe99]

I may be missing something, but I don't see where the OP said anything about replacing the ECU, yet you're ripping him pretty good for it- "flawed logic" and all. I don't see where that is .... logical.

The problem is somewhere along that pink wire. If the sensors are good, and disconnecting the pink wire from the ECU turns off limp mode then it's not a connection corroded or a break in the circuit. I agree that he may have a faulty buzzer. He may be getting a warning and not know it. Test the buzzer- it should self-test when you turn the key on.
If you are getting the overheat warning, could be a bad thermostat or blocked cooling passages. Hope this helps.

You are/were. Here, let me help you.

And, most would consider it illogical to replace a very expensive CDI without having verified that the CDI is the problem.

Many do just that. Yamaha appreciates their business.

Telling someone to actually verify the problem before taking any further action might be more logical than telling someone to not shotgun the problem by throwing expensive parts at it, hoping that something will stick, and solve the issue.

[boscoe99]

I took it for a run today, after talking to the mechanic again, I tried connecting and disconnecting the pink wire from the computer to the temp sender. The temp sender is new and functional. While disconnected it ran fine, when connected I had issues. No signs of overheating what so ever.

Replace computer?

Have you solved your problem yet, or not?

What gauges do you have? Are they analog with warning lights or multi-function type with warning features? If so, what are you seeing on the gauge with the motor goes into RPM reduction mode?

At what point was the pink wire disconnected? At the CDI, at the temp sender or maybe at the oil sensor?

Does the buzzer sound when the motor is RPM reduction mode?

Answers, we need answers man.

[throrope]

Had similar issue. May not be limp mode at all. Could be running on two of the three cylinders. Had my nephews out for fun on sunny Saturday with our TLR 90. After bidding them farewell at the dock, I went for a joy ride and lost a cylinder. Pulled the carbs apart for what I thought was a good cleaning and went for another go the following weekend. Ran great for twenty minutes and once again lost a cylinder. Giving up, I schlepped it to Rudy Marine in Wilmington DE. After a couple of weeks they rebuilt the carburetors. Best explanation is the fuel was contaminated and without clearing the entire system, I'd have a repeat performance with every trip. For me it was cheaper to replace the entire fuel system - tank, bulb, filter and fuel lines.

Since then I clear the boat of all fuel before putting away for more than a week. I empty the bowls and pump all fuel out and into the car. No fuel, no fuel to go bad. I rigged a $26 automotive electric in-line fuel pump to a 12 volt plug with fuel hose and Yamaha fuel connector. I also drain the water separating fuel filter and turn our portable 12 gallon tank upside down with lid off so it evaporates bone dry.

Good luck and keep us posted.

[Herman Melville]

davepros did you ever figure it out? I have a very similar problem