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MC205 07-05-2013 12:43 PM

96 Prostar 350 TBI black smoke
Last two times out I have started to notice black smoke at idle. It seems to get worse as the engine warms up. In the past the engine has always had a little white smoke at startup, but nothing like this. Motor isn't using any oil. I have also noticed a significant amount of black soot build-up above the swim platform. Motor is the standard 350 TBI. I have always thought that it has ran a little rich since I have owned it just based on the smell of the exhaust? Any help would be appreciated.

gweaver 07-05-2013 02:18 PM

I'm not familiar with the EFI boats, but from an automobile perspective, I'd look at an O2 sensor or throttle position sensor. If one of those has gone bad (assuming the boat has them), that could cause a situation where the engine would run rich. O2 sensors are easy enough to replace on a car, it might be worth checking to find out if the boat has them.

mikeg205 07-05-2013 05:29 PM

Get a code reader for michigan motorz... will most likely show a code... $59ish bucks.

Mark rsa2au 07-05-2013 06:51 PM

Mine was doing exactly the same thing. Same motor! Check the engine temp sensor as this will tell the computer the motor is cold and run it rich. They are cheap to replace,or you can test their resistance etc. I also replaced the 2 injectors as they were filthy. I have also now had them professionally cleaned and keep them as a spare.

My money is on the temp sensor. It is the 2 wire unit out the top of the head near the throttle body sensor. Also when you replace it check inside the hole / head as mine also had a huge build up of muck arround the sensor which would not have helped either.

MC205 07-05-2013 08:36 PM

Thanks for all the replies guys. Will check for codes and also look at the engine temp sensor. Would this sensor cause the ECM to throw a code?

Mark rsa2au 07-05-2013 09:18 PM


Originally Posted by supturb89 (Post 954234)
Thanks for all the replies guys. Will check for codes and also look at the engine temp sensor. Would this sensor cause the ECM to throw a code?

It did not show any codes on mine, it was just reading about 15 degrees off when I tested it.

MC205 07-05-2013 09:31 PM


Can you point me to where the engine temp sensor is. Is it in the throttle body? I'm not finding a 2-wire unit in the throttle body? There is a 3-wire sensor which I believe is the TPS and a 4-wire unit that I believe is the IAC. Am I looking in the right place? Thanks.


mikeg205 07-05-2013 11:22 PM

should be in t-stat housing behind t-stat housing.... post a pic of the top of your engine without any covers on it.

Mark rsa2au 07-06-2013 12:04 AM


It is not on the throttle body / manifold but it is on the same side of the head as the TPS ie Left if you face the rear of the boat. I have a great photo but the site does not seem to be uploading photos so I will try to explain.

On the left of the motor, it is installed verticaly downward from the top of the head. It is in the area behind the water pump / under the electronic gear/ to the right of the oil filler cap and slightly ahead of it. It has 2 wires to it White and Black which end in a black clip that clips onto the sensor.

You will use a LOT of fuel if this is faulty. I was running 2 to 3 tanks a day untill I fixed the issue ( multiple issues ). My boat looked like a diesel... and yes 2 families sking hard, at least 2 tanks a day so it was a priority to find the fault :eek:

If you want to test the OHMS resistance there is a thread of mine called "temp sensor resistance" (I think) that compares the resistance of my old unit / new unit and the factory spec.

MC205 07-06-2013 08:37 AM


Thanks for taking the time to write the detailed description, however I still can't find anything like what you describe in the area you describe. The only thing that comes close is a sensor below the exhaust manifold on the left side of the motor. It sits just above parallel with the spark plugs and sticks straight in. It has a black and yellow wire going to it (pic).

Here are some other pics of the area you are referring.

pretty sure this isn't it, but what is it????

Thanks again!

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