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Author Topic:   Engine overheats while trolling!Help
lpaton posted 05-25-2001 09:41 PM ET (US)   Profile for lpaton   Send Email to lpaton  
Fishing opened today.Did 15 miles to get to good spot,4800 RPM ,all fine. Started trolling with 200 HP Yamaha(1994) and engine overheat alarm went on after about 5 minutes. Engine did not feel or smell hot and water was coming out of pisser. Shut down for 5 minutes or so and started trolling again, same thing happens.Did this a couple of times. When alarm was on tried to increase throttle, engine misfired. Revved it up a few times in neutral and alarm stopped. Went home,15 miles at 4500RPM,no problem.Tried trolling when I arrived and same thing happened.
Anybody got any ideas? Engine trolled fine last year.Seems that it needs the higher revs to cool. Could impeller not be doing its job at low revs?
I appreciate any input. As soon as I finish installing the keel rollers on the trailer will haul it out.
DIVE 1 posted 05-25-2001 09:59 PM ET (US)     Profile for DIVE 1    
I think you found your problem. I would suspect the water pump.
lhg posted 05-25-2001 10:20 PM ET (US)     Profile for lhg    
I'm sure no engine expert, but do you have an engine temp gauge? If so, it would be "floored" if heads are overheating at low rpm's. Generally, the cylinder head temp alarm doesn't activate until the gauge is really showing full "H". You'd also be getting a lot of steam, including a "steamy" telltale stream. What about the engine thermostat? Could it be stuck?

What confuses me is that you're saying the engine's not hot, and you still have a telltale. I've had similar problems (a bad thermostat was cause, and that's my guess for your problem) and believe me, when my heat alarm goes off, the engine IS hot, and it shows.

Are you sure it's not the intermittant oil injection alarm, or even a malfunctioning heat alarm sensor? This is where an Engine Cylinder head temp gauge really counts.

Whatever it is, it shouldn't be hard to diagnose and correct. Let us know what you find.

Tom Byrum posted 05-25-2001 10:25 PM ET (US)     Profile for Tom Byrum  Send Email to Tom Byrum     
Both of my Yamaha 70s did the exact same thing and new impellers fixed it. Has it been awhile since you changed the impeller?
lpaton posted 05-25-2001 10:38 PM ET (US)     Profile for lpaton  Send Email to lpaton     
Impeller has never been changed. It is on order (was backorder).It's definitely the temp alarm as there is a blinking arrow over thermometer symbol on guage when alarm is on.
Telltale water is cool and I can keep my hand on all cylinders when alarm is on. Why would engine missfire when this happens?
Boy you guys are fast with the answers.
lhg posted 05-25-2001 10:48 PM ET (US)     Profile for lhg    
You Yamaha experts are confusing me. If the engine is overheating due to inadequate water supply, why wouldn't the telltale be very hot, or non-existant, and the engine hot?

The very reason for the telltale is to show inadequate water flow.

Or, are we dealing with just a top cylinder getting hot and triggering the alarm, but not enough overall heat to make the engine seem hot?.

Dick posted 05-25-2001 10:57 PM ET (US)     Profile for Dick  Send Email to Dick     
The way I understand it, when the alarm goes off it puts the engine in limp mode. It will not run above an idle. I had this happen on my Merc 50 4 stroke. The alarm came on and when you advanced the throttle the engine just sputtered. Turned out it was a bad sensor. This seems like the most likely cause the way you describe the problem.
Tom W Clark posted 05-26-2001 01:32 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The telltale may well be cold on an overheated engine. I don't know about Yamahas, but OMC outboards have the telltale water coming from the intake side of the cooling system, it is not water that has been through the engine.

Is the water pump suspected because at displacement speed the pump must pull water to the powerhead where as at planing speed the ram water pressure is enough to force it up without benefit of the pump? If this were the case wouldn't the telltale stream be nonexistant?

Tom Byrum posted 05-26-2001 02:17 AM ET (US)     Profile for Tom Byrum  Send Email to Tom Byrum     
I dont know why but it is just like Dick said. When my buzzer went off the motor started missing badly. I thought I had lost a piston. There was no indication the motor was hot other than the alarm. After a call to my mechanic he explained it just like Dick said. It goes into limp mode to allow you to chug home at a much reduced rpm. This happened 15 miles offshore in rapidly deteriorating seas by the way. Also happened to me one other time when I let my two stroke oil tank get to low.
Tom Byrum posted 05-26-2001 02:22 AM ET (US)     Profile for Tom Byrum  Send Email to Tom Byrum     
I should add that my mechanic showed me how to disable the limp mode in an emegency. Also if anyone near San Rafael CA needs a good Yamaha mechanic email me.
lpaton posted 05-26-2001 07:47 AM ET (US)     Profile for lpaton  Send Email to lpaton     
The limp mode makes things clearer to me. Was wondering if there was some sort of protect switch.I will change impeller as it is due and see if mechanic can determine if sensor is good. Will post results for benefit of all. Thanks
triblet posted 05-26-2001 09:10 AM ET (US)     Profile for triblet  Send Email to triblet     
The OMC system's limp mode limits the revs
to 2000. It turns off the limp mode after
it's been shut off and cooled down for a
few minutes.

Testing the sensor is easy. Pull it, hook
it to a ohmmeter, and put it in water with
a thermometer. Heat the water, and see
what the temp is when the ohmmeter goes
from high ohms to low.


lhg posted 05-26-2001 10:28 PM ET (US)     Profile for lhg    
TWC - In the Merc engines, the telltale is the hottest of any water exiting the engine.
It's definitely been through the engine! You can scald yourself on this even when the engines running at the high side of normal temp.

When the engine is first started, it takes a couple of minutes for the telltale to even show up, as the water moves through the engine.

OMC's design, as you describe it, makes no good engineering sense to me, for if the telltale is to be a "telltale", it would have to show decreased flow from an upper cylinder, and also show if it's really getting hot.

Excessive steam rising right off the telltale flow is one of the first indications you've got an overheat problem on a Mercury engine.

I agree with the others, that if the engine is showing no signs of overheating, it's probably not doing so, and the problem is as simple as a partially bad temp sensor. Sounds like it's gotten too sensitive, and going off at a lower temp than it should, as the engine runs hotter at idle speeds, although is not overheating. Mercury's system won't let you restart the engine until the sensor indicates a cooled down head. But I've not experienced the "limp mode" indicated with OMC and Yamaha.

Dan posted 05-26-2001 10:44 PM ET (US)     Profile for Dan  Send Email to Dan     
Thanks for explaining the telltale delay on Mercs at start up. I was wondering why it takes a few seconds for the telltale to appear. I think at idle and very low revs my Merc 115 only runs on 2 cylinders -- it seems like it wants to stall and occasionally does. The idle rev is about 750 and I've yet to see my temp guage needle move. Had it checked out and the mechanic said everything was "cool."
Ed Stone posted 05-28-2001 09:54 PM ET (US)     Profile for Ed Stone  Send Email to Ed Stone     
I also have been having one motor overheat
while trolling below 1000 rpms.The alarm
sounds at 171 degrees.At that point a
gaurdian system kicks in and only allows
40% of power to that motor until it cools.

At higher rpms both motors run in the 130
degree range.
Ed Stone

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