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Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3372
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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Hey Jason, I have a problem with 02 50hp 4stroke efi overheat

Customer Question

hey Jason, I have a problem with 02 50hp 4stroke efi overheat alarm going off..Engine was operated in salt water environment in earlier life, but not many hours. Sat for a long time. Fuel system was terrible when I got it, required new lines, high pressure fuel pump, filters, etc...Now it starts, idles and runs great, except 3/4-WOT, the alarm goes off. As soon as I back off the alarm stops.
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.
Hello, Jason here, whom am I speaking with? When was the last time the engines cooling system was serviced with a new impeller and thermostat?
Customer: replied 1 year ago.
I am John in Queenstown Maryland. I bought the boat which has sat for some time. Impeller is new as of last week. Thermostat was removed and checked in boiling water yesterday. I ran the boat with and without thermostat and get the alarm at WOT both ways. As soon as I back off, alarm goes off...and it will run all day on 1/3-1/2 throttle w/ no alarm. Reason I think I may have blockage is that pisser is not solid, firm stream of water...
Expert:  Jason replied 1 year ago.
If you run it without a thermostat the engine will overheat internally. Install a new thermostat and let me know what happens.
Customer: replied 1 year ago.
can do that but can't get thermostat until tomorrow. If the thermostat opens in boiling water consistently between 125 and 140 degrees, isn't that an indication that it is ok? Without thermostat, shouldn't pisser stream be solid?
Customer: replied 1 year ago.
Excuse me, shouldn't pisser stream be solid, with or without thermostat installed?
Expert:  Jason replied 1 year ago.
It should be strong with the thermostat in.
Customer: replied 1 year ago.
New thermostat installed. Same problem. Head temperature 160-170 when alarm goes off (verified by infrared thermometer).
What next?
Expert:  Jason replied 1 year ago.
Is the alarm a series of beeps or a constant beep?
Customer: replied 1 year ago.
constant beep. It goes off as soon as I drop the throttle back....
Expert:  Jason replied 1 year ago.
170F isn't really overheating.
If you brought the engine to me. What I would do is pull the codes first and make sure the alarm is for overheat, and not something else. If it does come back overheat I would replace the engine temp sensor next, as it looks like it is acting up prematurely. Does that make sense?
Customer: replied 1 year ago.
I realize that but I can't pull the codes. I didn't think it was the thermostat, but spent the money on your recommendation. My thought would have been the temperature sensor...Can I isolate that and test it?
Customer: replied 1 year ago.
Did you get my last message?
Expert:  Charles replied 1 year ago.
Hello, my name is ***** ***** you till fighting this issue with the engine?
Customer: replied 1 year ago.
I still have the problem. Can u see the history of question and answers? Are u answering under previous request or will this cost additional? Thanks, John
Expert:  Charles replied 1 year ago.
Yes I can see the entire question. The fee that you posted inn the beginning, is still active.How is the oil in the engine?Can you get access to a gauge that can read over 70 PSI, and a 1/8 pipe nipple?
Customer: replied 1 year ago.
I can get a pressure gauge and pipe nipple (w/ standard 1/8"npt thread?). In the meantime, I'll also replace the temp sensor and see if that helps. Let me know where to plug in the nipple and how to test (I assume) water pressure. Thanks
Expert:  Charles replied 1 year ago.
Actually you will be testing the oil pressure on the engine. The pressure sender is near the thermostat and has just one wire on it. Remove that sender, then test run the engine while keeping an eye in the pressure.
Customer: replied 1 year ago.
test at idle with muffs ok?
Expert:  Charles replied 1 year ago.
Yes that is fine. That is how I test it in the shop.
Customer: replied 1 year ago.
K. When I started it cold oil pressure was about 43-45 at idle. Pressure rose as expected if I blipped the throttle. After 3-5 minutes w warm engine, pressure stabilized at 23-25psi....
Expert:  Charles replied 1 year ago.
How old is the oil?
What is the viscosity of the oil?
Customer: replied 1 year ago.
Don't know age or viscosity. I'll change it, retest pressure and get back to u. R u partial to any brands other than mercury oil? 3 quarts w filter, right?
Expert:  Charles replied 1 year ago.
I only use Merc oil. Out 25-40 in the engine. It will run on that better.
Customer: replied 1 year ago.
3 qts fresh Merc Quicksilver oil and new filter installed. Cold start up oil pressure now 52-53, 10 minutes of running on muffs and it's at solid 31, and it rises dramatically with blip of throttle. I can water test the boat tomorrow if you think that the lower pressure was what set off the alarm. BTW, the old oil was pretty black...
Expert:  Charles replied 1 year ago.
That is better pressure. Your oil pressure switch closes at 22 PSI, this is what sounds the alarm.
The new engines have an alarm set at 2 PSI.
Customer: replied 1 year ago.
Thanks Charles. I'll try to get it on the water for a test between the rain today and let you know.
Customer: replied 1 year ago.
Hey Charles, Just back from water test. After idling out of the marina for 5 minutes, I brought it up on plane and after 15 seconds, I got the alarm, and basically tried a few more times with same result. Where do we go next?
Expert:  Charles replied 1 year ago.
Solid or beeping alarm?
Customer: replied 1 year ago.
Solid alarm. When I back off throttle to no wake speed, alarm shuts off. When alarm sounds, boat starts running rough
Expert:  Charles replied 1 year ago.
Ok,
Can you drop the lower unit, then look up the water tube and see if you can see any black rubber up there?
Customer: replied 1 year ago.
I'll try although it's tough to see up there. Remember I replaced impeller. When I did, I ran compressed air everywhere I could see even though old impeller was intact. Looking up water tube as I remember it takes a sharp bend. Water pisser decent flow but not hard solid flow like on
My other OB engine.....
Expert:  Charles replied 1 year ago.
thats right, the tube bends, you cant see the top.
Try it this way. Hold your thumb over the water tube inlet, blow air into the water discharge. Does the air keep moving?
Customer: replied 1 year ago.
Charles, I removed the lower unit and covered intake water tube inlet. Blew air into discharge and experienced extreme air pressure against my thumb until I released it. I can't be sure air wasn't getting out elsewhere but it felt like 99% of the air was building up pressure against my thumb. I need to get out crabbing and am hoping the power head doesn't need to come off as I know that would be a large project....Wadya think Sensei?
Customer: replied 1 year ago.
r u there today Charles?
Expert:  Charles replied 1 year ago.
Yes I am here.
Customer: replied 1 year ago.
Charles, I removed the lower unit and covered intake water tube inlet. Blew air into discharge and experienced extreme air pressure against my thumb until I released it. I can't be sure air wasn't getting out elsewhere but it felt like 99% of the air was building up pressure against my thumb. What's next?
Expert:  Charles replied 1 year ago.
Can you run with the oil pressure gauge installed to rule out the oil.
Customer: replied 1 year ago.
I can probably do that but can we have a conversation first about likelihood of different options?
My top three in order are:
1) blocked water passage - what did we learn from the last test I performed? Should air have been able to escape elsewhere when I applied it to the pisser outlet and had the intake tube blocked by my finger (because it didn't escape)? I've heard that there are very small water passages at the bottom of the power head that can shrink up with salt water "corrosion"...
2) faulty temp sensor? I have a spare one that I could try I think...I may be able to track down a computer to hook up to the engine - does this help us get to the problem any easier?Thanks,
John2)
Customer: replied 1 year ago.
oops, I somehow overtyped my third guess which was supposed to be low oil pressure, but my thoughts are that this could be the third alternative at this point vs the first.....
Expert:  Charles replied 1 year ago.
Blocked water passages could be the issue if you boat in salt water.
The water goes out the discharge and the exhaust when the thermostat is opened.
If you have a spare temp sender go ahead and replace it.
If you can get a laptop to hook to the engine, we could read the code and know for sure which way to go.
Customer: replied 1 year ago.
Charles, I was able to hook up a computer to the engine and it appears that the alarm is going off for high water temp. My water temp at the time of the first alarm was 169. The second time I tried it was 170.... What temp should the alarm go off on this motor? 170 seems awful cool to kick an alarm....
Expert:  Charles replied 1 year ago.
the thermostat opens at 120 on this motor. Heat alarm would go off around 145-150. the computer is capturing the info on the screen at 170..
Pull your t-stat and make sure that is says 120* on the bottom of it.

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