MacGregorSailors.com

You know that small stream of water that you check to verify the water pump is ok and coolant is circulating. The one that gets plugged once in awhile particularly if you operate in salt water? The same one that you use a piece of wire to clear the obstruction and there is no damage to the engine. Unfortunately the Tohatsu TLDI 50 and 90 are not so forgiving.

A plugged “telltale” water line can/will cause the air compressor to overheat and fail which in turn will cause the engine to stop running. Game over. Tohatsu offers no fix for this potential problem so my advice is to keep a closer eye on the telltale stream at the beginning of a days operation and frequently run a wire or something a bit softer through the discharge hole in the plastic fitting (nozzle) to prevent a gradual buildup of salt, sand or dirt deposits.

More detail if you’re interested: This recently happened to my 2001 50HP TLDI. Since a dealer was not convenient I ended up removing and disassembling the air compressor. The rod, wrist pin, piston, rings, cylinder and reed valves were ruined. The head of the air compressor got so hot it melted the plastic discharge elbow.

On my engine the plastic fitting at the exit of the compressor coolant discharge hose was plugged. When I examined it the flow area of the plastic fitting seemed much to small; it begs being plugged. To make matters worse there is no temperature sensor in the air compressor which could shut down the engine and prevent this failure if there is an obstruction in the cooling line. Only the block has a temperature sensor. Since this is a rare occurrence Tohatsu doesn’t plan to issue a service bulletin. It came of no surprise to learn that the 115 HP TLDI has a temperature sensor in the air compressor.

I didn;t know boat motors had an air compressor. What are you talking about?

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Here's a link to what he's refering to:

Tohatsu air compressor:

http://www.tohatsu.com/outboards/40tldi_features.html

Hmm, looks like that puppy is supercharged and the telltale water itself is used to either cool down the bearing or the intake (ie. intercooler) or both. I think later models of my bigfoot use the telltale water to cool the fuel pump too. I like the old fashioned kind where it does nothing but pee.

does that apply to the Nissan TDLI as well?

Yes, what effects Tohatsu outboards also applies to Nissan outboards.
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The Compressor is not a super charger. It compresses air which is used to whip the fuel into an atomized mist on its way into the combustion chamber. Since the fuel and air are better mixed it burns more efficiently and cleanly.
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The old way before the air compressor, the air passing through the carburator created a low pressure area which has a opening to let some fuel be sucked in. The fuel poured in as a liquid and mixed with the air as it swirled around into the combustion chamber. The mixing was not perfect so some of the fuel did not get time to burn and the exhaust was dirty with this partially burnt air fuel mix. On a cold engine the mixing is worse so a choke is needed to force the pressure to be even lower which caused more fuel to be sucked in so that enough would mix to make the engine run. When warmed up the choke must be off since too much fuel in the air also makes it not burn very efficiently or cleanly. The choke is a variable obstruction in the intake air path that would limit top speed performance if left on after warm.
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A super charger compresses all the air that goes into the engine, it is a higher volume compressor that runs at low pressure. Sort of like a leaf blower pressure vs tire filling pressure for the fuel injection air compressor.