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My 2004 Carbed Merc 90 4-stroke finally died. What a fiasco that motor was, but that's another thread.

I replaced it with a 2015 EFI version of the same motor. To keep rigging costs down, I had the dealer use the existing gauge package and tunnel cabling, which Mercury provides an adapter harness for. Someday maybe I'll put in the Smartcraft package, but for now this setup seems to work fine. Minor issue is dealer says there is no hookup for the old water pressure gauge. I know it's not water temp, but it was always a good indication of cooling system performance.

But...on the accessories page for this engine in the Mercury parts catalog is part number 8M0088855 HOSE KIT WTR PRESS at a very reasonable price. The old engine's water pressure sensor was connected to the gauge via a long thin plastic tube, which is still there, disconnected. All I really need (I think) is the sensor for the engine (under the assumption that the new sensor will hook to the old tubing, and on the further assumption that the existing gauge is compatible with this sensor).

Anyone have any idea?

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Assuming the water pressure gauges on the Mercury 4-strokes work the same as those on the 2-strokes, there is not actually a sender. All that is required is a brass compression fitting for the plastic hose, that goes into a tapped hole, plugged at the factory, for the thin hose (about the same size as a cocktail swizzle stick).

Water pressure gauges work the same as a speedometer gauge. That is, increasing pressure of the water at the engine block compresses air in the tubing which activates the gauge.

However, if the new engine is Snartcraft enabled, I have to plead ignorance of how water pressure may work on one of these later engine information systems. But I would think there would still have to be a plugged hole somewhere on the block for the water take-off fitting. Generally they are on the top of the block, since you want to read water pressure as high up on the engine as possible to make sure the top cylinde is getting cooled.

I remember someone at Whaler telling me that when 4-strokes came out, Whaler switched from temp gauges to water pressure gauges as a more accurate measure of engine cooling.

As an aside, the old 2004 Mercury 4-stroke 90 was actually a compete Yamaha engine, bought lock, stock and barrel from Yamaha. Only the cowl, mid and lower sections were by Mercury. All of the engine manufactures quickly learned that for 4-strokes, EFI is the only way to build them.

I have been hearing that these new Mercury 90 and 115's are strong performers with their large displacement of 2.1 liters. It is the newest platform 90 on the market. I'm seeing a lot of them out here in the Midwest. Let us know how you like it and how she runs.

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Took the cowl off, and the flywheel cover, and indeed there is a plugged hole on top of the block. It appears to use an allen head to unscrew it.

I don't know about Smartcraft, but when my new engine was installed and hooked to the old analog gauge package, the pressure gauge tube was neatly cut off and coiled inside the cowl, with clearly enough length to reach the "sensor" which as you explain is indeed probably just an adapter to match to the tube (which explains why the "kit" doesn't cost much - when a Mercury-brand aftermarket part is cheap, you know there's not much to it).

So I ordered the kit as it looks like this will work. Only possible unlikely issue is that the water pressure range is much different from the old engine's gauge calibration, which seems unlikely.

I got used to the water pressure gauge and it told me things, like fouled intakes, well before the high temp alarm went off. Do you really have to overheat the engine to tell you the flow is constricted, that can't be good in the long term.

I love the motor, every time I put it in idle I think it stalled because it's so quiet! And smooth. And I get at least 1-2 knots more speed at the equivalent RPM as the old engine. And yes displacement equals torque - gun this engine and the passengers in the rear of the boat fly off the transom into the drink :-)

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Generally it has been my experience that the engine fittings for the water pressure gauge come as part of the Water Pressure analog Gauge package, which is why water pressure gauges tend to cost more. You have to buy a Mercury gauge to get the correct compression fittings for the small bore plastic tubing. Teleflex gauges use larger rubber speedometer type tubing that doesn't work as well.

In your case, you (or your trade-in dealer) should have taken the fittings off the old engine, and replugged the hole. Do you still have access to the 2004 engine? if so, rescue those, and soak them in "Limeaway" to clean them up before re-installing in the new engine.

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OK. But I'll wait to see if this "kit" I bought has the adapter/plug in a it. A new one would obviously be better if it fits. The gauge is already on the console and the tubing runs into the engine through the rigging hose and terminates in a loose coil on the side of the engine. Looks like all the hard work is done.

If not, I'll see if the repower dealer has the fitting. I sold him the old engine for parts.

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I wound up ordering the kit from Mercury, which should simply contain the adapter to screw into the block in place of the plug there now, and the fittings for the thin hose, which I already have.

So what does one use these days as a sealant for engine block fittings? I would think teflon or teflon tape, but maybe I'm thinking plumbing.

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Just to finish this thread. The kit from Mercury contains a "sender" that one replaces the existing blank with, and a length of hose - easily enough to reach the old pressure sensor hose, and various fittings to connect the hoses up (the new hose is exactly the same diameter. A few tie wraps and the job is done. The "sensor" threads come precoated with some sort of red stuff which is I assume is a sealant, hence I didn't use PTFE or anything else.

The pressure readings are about 50% higher across the RPM range than with the old engine. Probably due to both the Command Thrust lower unit, and also 11 years of corrosion and wear on the old engine.

Thanks for the help.