I recently replaced the sea water pump impellers on both 496 Horizons. A Diacom recording on both engines showed normal sea pump pressure. I then pulled both raw water sea strainers and cleaned them out. Nothing to speak of was caught in the strainers. As soon as I replaced the strainers and ran more tests, the sea pump pressure was all over the place.

One week passed and I ran the tests again. Now both engines are reading 43.46 PSI and I'm getting error codes thrown for excessive sea pump pressure. There is never any deviation from 43.46...it just stays there...even if the engines aren't running.

I have disconnected both cables to the sea pump sensors and made sure there is a good connection. When I unplug them, I get the 2 beeps every minute and the Diacom readout says I'm down to 90% of full power available.

Could I have dislodged some junk that stopped up both sensors, are the sensors in any way slaved together, or is there something else that is making the readings inaccurate? I find it strange that BOTH engines are behaving the same exact way.

Both engines are exhausting plenty of water, both the manifold exhaust temps are around 110 F, and the overall coolant temp is at 158 F.

I guess my next step is to replace both sensors but I hoped that possibly someone else has run into a similiar problem.

Thanks,
Richard

It was late and I had been in a bilge all day when I wrote the original post and forgot to mention this.

The actual code was "Sea Pump CRT HI" and the PCM Manual says faulty wiring to the sensor or the sensor itself is bad. I checked the wiring for any obvious shorts or breaks and couldn't find any. It just seems strange that both sensors on both engines would throw the identical fault.

Turn on key, in software after you hit the pencil eraser button it gives you some options. Choose "erase trouble codes". Then, go to the history button and choose to erase trouble history and verify by reading afterwards that there are no guardian second counts.

It sounds like you actually never cleared the codes from when they were unplugged.

Regarding the fact that cleaning and replacing the strainers caused the eratic behavior.... try this first to see if codes come back and then pull off the inward hose on the ps cooler and see if there's any residual stuff clogged there. That's the first line of defense IF you have the honeycomb style cooler.

Do you also have an analog water pressure guage at the helm or do you go stricly by the SmartCraft Sensors?

Hi SDFever,

I erased both the codes and the history (twice). The Strb engine immediately starts the 2 beeps every minute when I turn on the ignition and throws a new code. The Port engine doesn't give me the two beeps unless I disconnect the sensor.

Both engines on the Diacom software give a steady reading of 43.46 psi and do not fluctuate regardless of engines not running, running, and any RPM up to 2500. I'll check the PS coolers but it's just weird that both read the exact same PSI. Both engines also show about the same coolant temp and manifold exhast temps.

Where are the sensors located? Back of the water pump - power steering cooler??

how long have they been in the water (salt)?

btw pressure sitting in the water (not on hose) should be around 2-3 at dead idle and will run up to 30+ wot. that's the normal range if located on the pump. pressure is pressure and it will be real close no matter where they are but if they've never been replaced it's possible you could have helped them along by messing with them but I wouldn't blame it on that yet. Pull them out and inspect.

Also, in Diacom on the ECM Data page, what is the value reading while engine is running for the "sensor power"? Should be 5 volts.

The sensors are located at the back of the pump. Yes...salt water. I think maybe you have a point with respect to fooling around with them. The Starb engine will throw a code, I'll clear it, unplug the sensor and reconnect it. Fire up the engine and no code. Turn the engine off and restart it again and get a code.

Sensor power from the Diacom is 4.99-5.00 on one engine. The other shows 5.04 all of the time.

When I replaced the impellers they were completely intact but there were small circular groves at both ends of the brass housing. I was told that the pumps should be replaced with ANY groves at all. Do you agree?

If the sensors were reading irratically I'd feel better about replacing both pumps and both sensors (a quick $1000) but I'm really curious as to why both would just show 43.46 PSI with ignition on, idle, 2500 RPM, or any other condition.

Well, first I'm not telling you to replace pumps because of scoring. Mine are pretty worn and still cool ok. You will have minor grooves just from use. You're not having any heat issues so save that money til you need them. Just my opinion based on what you've said.

Merc has gone back and forth on making some sensors plastic, brass, and back to plastic etc... Sometimes the other way around... Best I remember, Merc never changed that particular sensor to anything other than brass. I'd use a sensor test lead (Diacom) and test the sensors before just throwing parts at it but you may not have that ability.

I will say that both sensors on two separate motors failing at exactly the same time seems pretty rare to me so here is what I think ( but I don't know without seing it).

Your pressure reading is right around what the "programmed max" limit is in the ecm. So that means that you probably have a ground problem at the sensors which in turn is sending a full 5 volts back to the computer telling it that it's maxed out. Those are 3 wire sensors - ground, power, & data.

If the ground is broken or shorted just right, then the reading to the ecm will be the physical max.

Check and advise -

This is a tough one cause I can't see it. Those whether pack plugs sometimes come apart on you but you really can't see it without taking it further apart or putting a good light on it and noticing a difference on the wire insulation where it "used to be".

43.46 psi is the full scale reading for sea water pressure per ecu. Is has to be electrical - my 02.

Thanks very much SDFever & Rage.

SDFever. you nailed it. Apparentley Merc had two problems with the early sensors. The paint job on the pump that the sensor screws into was too good and the water hoses that connect to the pump had too high electric resistance, this caused static electricity to build up in the sensor and damage the sensor.

This all came from a very sharp marine mechanic in Mpls. Mn. who was having the same problem. Merc redesigned the sensor 2 years back and the mechanic said to be safe, it's a good idea to run a dedicated ground wire from a good spot on the pump to the block.

Rage, thanks for confirming the max pressure reading. With both sensors reading the identical value off of the Diacom, I had to believe something was up. Possibly when the pumps were remounted to the engine, the static electricity buildup trashed the older sensors.

In any event, it's great to come to a answer on this and I really appreciate all of the input from everyone. Forums are about the best thing the Internet ever provided.

Yeah that's the other starting point on elec problems. Merc uses soooo much paint that it causes lots of little issues from time to time. I'd love to know what they use to put that stuff on. It's amazing if you've ever tried to remove it and equally frustrating.

Cheers to a good season on your boat