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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3403
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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I have a boat with dual 250 hp Mercury outboards. The port

Customer Question

I have a boat with dual 250 hp Mercury outboards. The port engine is giving a steady state alarm tone as soon as the key turns on. The starboard will activate the steady state alarm after an indeterminate amount of time. As little as thirty seconds or as long as 10 minutes.
Both engines are 2005 with 800 hours on them. Port engine serial #1B106441 Starboard engine serial # ***** I have replaced the engine mounted oil tank, as well as the oil pump itself on the port engine, and there is no change in behavior. I am sure you will have questions as well so let's start there.
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hi Craig, my name is Jason.

On the cylinder heads you will see a tan wire. This is for the temp sensors. Disconnect them and let me know if the alarm still goes off or not.

Customer: replied 1 year ago.
we have disconnected that and the alarm is unphased by it.
Expert:  Jason replied 1 year ago.

Your going to have to get the engines scanned for codes, this isn't something I will be able to help you with.

Customer: replied 1 year ago.
Jason we will be going back to the boat on Saturday, and I will reverify that again at that time. Please advise any further tests that would be of benefit to you, and I will try to conduct them.
Customer: replied 1 year ago.
I have had a guy scan it, on both engines, and will upload their report, probably tomorrow. Our regular marine engineer had that done prior to replacing the oil pump and then we just had it done again but I have not received the second report as of yet.We are in Oman and there are not a lot of options for repair shops, and all of them are taking the easy way out and pointing to repower and offering to sell me a pair of new engines for a lot of money. The compression on these is within factory specs, and within about 3-5% of each cylinder, so I don't think the engine is excessively worn. The guy with the scanning computer had to come down from Dubai with it in order to get the right software to communicate with the engines.
Expert:  Charles replied 1 year ago.

Hello, my name is ***** ***** tech here on the site. I will assist you with these.

Your solid horn could mean a few ither things besides just overheat. Start with testing the batteries to see what the voltage is on them. Any higher than 15, there is an issue with the charging system If it is lower than 9.5, it will not start the engine. Start with this check and let me know.

I am only online in the eve, CST in the USA weekdays, and most all day on the weekends.

Customer: replied 1 year ago.
Ok, thank you, ***** ***** check the voltage
Expert:  Charles replied 1 year ago.

Sounds good, please keep me informed.

Customer: replied 1 year ago.
Charles,The boat has a dual battery setup. Prior to starting the engines we get 12.4 and 12.8 volts. With engine at idle I get 13.0 to 13.2.The alarm issue first appeared while we were on the water one day cruising at about 2300 RPM on smooth water. The port engine gave the alarm. Upon inspection it was found that the oil tank at the top of the engine had the float switch malfunction. Replacing that did not get rid of the alarm. Then the Starboard engine fuel pump went out. We replaced that. Then the alarm was present on either engine. Also, the oil pump on the Starboard engine has failed as well, and we have replaced that too.Prior to replacing the oil pump, we had the codes scanned, and the results of that were :
Port engin
Oil level sensor circuit high
Fuel level 1 circuit high
Starboard head temp circuit high
Water temp circuit high
Trim sensor circuit low.
After 50-60 seconds of operation, it would then report oil pump failure.Starboard engine:
Oil level sensor circuit high
Fuel level circuit 1 high
Water temp circuit high
Trim sensor circuit lowThey tested the horns on the alarm unit and all functioned as they should.This problem was previously asked on Just Answer. An Internet search using the engine serial numbers pulled that up.Htp://www.just was about a year ago, which would be just before I bought the boat.
Expert:  Charles replied 1 year ago.

The stbd engine is normal codes that come up without SmartCraft gauges.

The port looks to have a failed stbd head sensor.

I am unable to open that link. Can you verify it?

Customer: replied 1 year ago. is the actual link copied and pasted. Looking at it there is a good chance that you may have been the one that answered this, unless there are multiple ***** *****dling boat engine questions.
Expert:  Charles replied 1 year ago.

Yes, that was me. I tackle most of the harder questions on Mercs

Looking at your coils. Can you read the numbers that are embossed into them?

Customer: replied 1 year ago.
Charles,I will have to go down to the boat again to try and get that information for you. Give me a few days to arrange that please. Also, advise how many coils I am looking for and where to find them on the engine.
Expert:  Charles replied 1 year ago.

There are 6 coils on the engine, one for each cylinder. They are located on the rear of the engine, black in color with a single grat plug wire coming from it and a 5 wire plug-in on it as well.

Customer: replied 1 year ago.
Armed with that information, hopefully I shouldn't have a hard time finding that out for you. The boat is out of the water. Just needing a break in the weather to get down there.
Expert:  Charles replied 1 year ago.

No worries. There are no time limits here.

Customer: replied 1 year ago.
Charles,I have confirmed again the battery voltage with engines off is 12.3-12.4V DC on both batteries. With them running at idle they showed 13.7-13.8 and bringing the engine up to about 2000 RPM gave me a steady 14.3V on each.Coils on port engine are all the same:
879984 Rev-eStarboard engine had 1 of the above fitted and 5 of these:
879984I would guess that the 6 on the port engine are newer, and one of the 6 on the starboard have been replaced with new.
Customer: replied 1 year ago.
Also of note this time, I was able to power on the key, on both engines, and got the initial horn sound of about 1 second, and then no alarms after that. When the engine started, It was about 15-30 seconds later that the alarm solid tone started.
Expert:  Charles replied 1 year ago.

On the engine that gave you the horn, inspect the temp senders on the cylinder heads. Make sure that there are no melted wires from the sensor to the main harness. 14, and 42 are the two types of temp senders.

Customer: replied 1 year ago.
Given the cost of those two sensors, I could replace all 4 for less than the shipping of the parts would cost. Should I just order them and change them out? Also, to be clear both engines are sounding the horn. The only variable appears to be when they elect to do that after the key is turned on.
Expert:  Charles replied 1 year ago.

Are you using just one battery or two?

Customer: replied 1 year ago.
Two batteries.
Expert:  Charles replied 1 year ago.


Are all the posts clean and tight?

Before ordering the sensors, I would check to make sure that there is nothing melted in the harness anywhere.

Customer: replied 1 year ago.
Thanks. I will try and pull all that out and inspect it. The transom of this well craft is not access friendly, so it will take some time.
Expert:  Charles replied 1 year ago.

On the engines, not in the boat.

Expert:  Charles replied 1 year ago.

When was the last water pumps installed?

Customer: replied 12 months ago.
Charles water pumps were done about 1-1/2 years ago. We have had the battery side electrical looked into as you requested. The cables were old and the insulation was brittle. We replaced from the battery to the isolator switch, from the bilge pump to the isolator switch, and from the isolator to the engine, with all new cables. The problem is still pesent. Sometimes as soon as the key is turned on, and sometimes it takes 30 seconds then gives a steady tone alarm.
Expert:  Charles replied 12 months ago.


Lest move some parts between the engines and see if the problem follows. Start with the Stbd cylinder head sensor. swap from the port to the Stbd engine. Turn the key on ans see if the Stbd engine now has the horn and not the port. If still the same, move the port head sensor and repeat.

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