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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10593
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Hello, I have a 2001 Formula 280ss with twin 350mag Horizon

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I have a 2001 Formula 280ss with twin 350mag Horizon motors. the port motor runs extremly rich when you first start it, quite a bit of black smoke at idle. Once the boat warms up the problem basically goes away. I have recently run a mixture of 50/50 SeaFoam and gas through the fuel filters two times. This only helped a little. The IAC valve has also been replaced.

Hi my name isXXXXX will assist you.


The engine is not heating up quick enough. The thermostat could be sticking or the ECT (engine control temp) sensor is bad and not telling the ECM the engine is warm. Put the engine on a computer and check the ECT if it check good check the thermostat for sticking open.

. Post back with engine serial number and questions.

Customer: replied 3 years ago.

Thank you for you quick response. will check the T-stat. I do not have a a scanner at my disposal. Can you recommend a good scanner to purchase online if the T-stat is not the issue. I did forget to mention that the engine surges at idle when cold as well. I will get the model and serial# XXXXX evening. Chris

Hi Chris, If the engine is running rich it is most likely the ECT. The thermostat and the ECT are allot cheaper than a computer. I would just replace both and be done with it. Now if the engine were to continue to run rich even after being up to temperature I would suspect the fuel pressure regulator. You can check fuel rail pressure to eliminate this as a cause. You can also check the engine temperature at the thermostat housing using an infra red temp gun. .

Post back with questions and concerns. I will need the engine serial number to get the correct engine specs.

Customer: replied 3 years ago.

I agree on replacing both if the scanner is expensive. I have an IR gun. I got my wife to run down to the boat house and get the serial # XXXXX

The ECT sensor 805218T, thermostat 99155 1 . The fuel rail pressure at 1800 rpms is 43 psi. If the fuel pressure exceeds 43 psi at any rpms the fuel pressure regulator is bad which will cause a rich fuel condition all the time. Long as the engine only run rich when it is cold it is the thermostat or the ECT.


Now if you want to double check the computer you can go by a dealer and they can run a check for about an hours labor and tell you if there are any codes.


Is there anything else I can help you with?

Customer: replied 3 years ago.
I think I am good. I will order the t-stat and ect and hopefully have it be fixed. Thank you, Chris
Your welcome Chris........Thank you.......Tan
Customer: replied 3 years ago.
If you are interested. I sell electrical test instruments and would be happy to and you one of our samples. Let me know. I have clamp-on and VOM meters. Chris
Hi Chris send me info at tanpartin @ gmail .com Run all those together.
Tan and other Boat Specialists are ready to help you

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