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I have a 96 merc 115 hp. Motor runs smooth when ambient temp.

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I have a 96 merc...
I have a 96 merc 115 hp. Motor runs smooth when ambient temp. is under 85 degrees. As the temp gets above 85 the motor is hard to start, won't idle and misfires when accelerating. I also have a steady alarm that only goes off when these conditions are present. I have new plugs and impeller, fuel hose. The oier appears to be working fine. I checked for bad fuel and it seems to be good. ???????
Submitted: 8 years ago.Category: Boat
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Answered in 2 hours by:
6/27/2010
Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4,730
Experience: 42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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Welcome,
I will be happy to assist you. If you have a question or need clarification please post back at any time.
Your engine is overheating. If you have good water pressure, change your thermostat and gasket and also replace the poppet valve assembly, the complete kit. The poppet valve is next to the thermostat in the water jacket on the back of the cylinder head. If the engine runs too hot it will evaporate some of the fuel before it gets a chance to fire causing hard starting and a lean condition, which causes popping and misfiring.
Post back any questions.
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Customer reply replied 8 years ago
This condition occurs immediately on a cold start before the engine has time to warm up. I can run the boat all over the lake before noon, but when the outsode temp gets hot 85+ the motor will exibit the above characteristics after sitting for several hrs. Is that consistant with your original diag. The alarm is a constant buzzer when this condition occurs.
Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
Check the heat sensor if any shorts or if there is any continuity when cold. The only difference is the temperature so I would check the sensor and the cooling system. Put a IR thermometer on the cylinder head to see what kind of temperatures you are displaying.
The sender is next to the thermostat.
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Customer reply replied 8 years ago

If the sensor is bad, why would the engine misfire?

Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
The sensor wouldn't. That's why I want you to check the cooling system and the sensor. If the engine overheated enough it could melt the insulation on the sensor wires and cause the alarm to go off, or the sensor could be bad and the engine overheating at the same time. You just need to pull the sensor and check or replace it and go through the cooling system to ensure the poppet valve and thermostat are opening properly.
Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4,730
Experience: 42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
Verified
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Customer reply replied 8 years ago
Thanks I'll give it a try.
Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
Let me know when you pull it apart.
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Customer reply replied 8 years ago
I will install the t-stat kits on saturday and see how it goes. I was wondering if it might be a possibility that I have and ignition issue. Possibly the stator or power pack? Are these symptoms consistant with that?
Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
The stator nor the switchbox (powerpack) would activate the alarm. Only the overheat sensor, cooling system etc would do that. If it happened in cold waters I would suspect the oil module possibly but you say when it is cool it is OK, so we look at what is heat related, and sets off an alarm.
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Customer reply replied 8 years ago
I'll let you know how it goes.
Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
Yes please do it is an unusual problem. The only time I have seen it was in extreme south Florida during the summer in warn shallow waters.
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Customer reply replied 8 years ago

I installed the t-stat and the alarm continued to go off. The condition does not seem to coincide with the outside temp. anymore. I wiggled the wires for the temp. sensor and was able to make the buzzer go on and off. I replaced the ECT sensor and the buzzer still goes off all the time even when engine is cold/not started. What should I do next???

Marine Mechanic: Jeff G., Marine Mechanic replied 8 years ago
If you can make the alarm go off by moving the temp sensor I would replace the sensor and check all the wiring for any rubbed sport, shorts.
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