200 EFI Merc Overheat at WOT

[bryceban]

1998 Mercury 200 HP EFI SN #OG606131

Hello, I’m having an overheat alarm at ¾-WOT. A steady BEEP. Motor has a new water impeller and housing aswell as new fuel pump installed by Mercury dealer last month before I boughtthe boat since it sat for 4-5 years without use. I have deleted the oilinjection and alarm module myself as I was having issues with it. I now run non ethanol fuel with 50:1 premix as well as Techron marine at 1oz per 10 gal.

I have installed new thermostats and poppet valve assembly myself and that has helped a bunch as the alarm was going off much sooner before. Also, my water pressure at idle with boat in the water is at 12-15 psi and quickly jumps to 30+ psi at anything over 1,500 RPM and has a great tell tail pee stream. I am able to get boat on plane and run at ½-3/4 throttle now and no alarm. Yesterday I ran it at WOT until alarm came on and shut it down and pulled the cowl off and took infrared thermometer readings and got 180°F on port side head and 165°F on starboard side right at temp sensor. I think It’s fair to say there is a blockage somewhere and need some ideas of least invasive first to start with please.

Also, I think I noticed one early morning while on plane at around ½ throttle, there looked like small puffs of smoke/exhaust coming out around the telltail or midsection of motor. Was only able to notice it because the sun light was just right.

So where should I start? Boat is 50 miles away in storage and would like some ideas on what to check and where to start if I have to bring it all the way home and take it back with no trailer brakes. Any ideas and help would be greatly appreciated!

[Savage]

Sounds like you might have compression leaking into the cooling system. Your at idle water pressure is WAY high, if the gauge is accurate.

[claypoolxx]

Sounds like you might have compression leaking into the cooling system. Your at idle water pressure is WAY high, if the gauge is accurate.

Thats exactly what happened to me. I didn't have an overheating issue but psi at idle was around 5-6 and 20-25 above 4000rpm. Also my telltale was strong but not a constant stream at idle, it sputtered. I now have a 2003 carb motor with my old lower unit and psi is now about 2 at idle and 12 at cruise.

[lpugh]

What is the water pressure at isle and when it starts to overheat
Could be a pressure loss due it motor to high of obstructions
Could be Savage is dead on as well

[bryceban]

Motor runs great until I back off due to alarm. Pressure is pegged at 35+psi at 1,500 rpm and above. 15ish psi at idle. I doubt gauge is 100% accurate but that's not the issue. I guess I could have an instrument technician at work look at gauge but I'm wondering where to start looking for an obstruction short of pulling power head, etc. Would like to start with easiest things first. I'm looking into pulling all hoses as well as back flushing I guess.

[bryceban]

Also, compression tested good at Merc dealer. I could do a leak down test before getting too deep into it.

[billybas]

Bryceban...I have an 03 XR6 150 Mercury with an overheating problem also...I have been flushing it for 3 weeks off and on trying all different kinds of things to get it cleaned out. I have a post on this forum as I have sucked up sand into my motor. I am going to pull the head this weekend with help from a mechanic. feel free to call me if you want as I may be able to save you some trial and error as I can tell you all the different ways I have flushed my motor. I work nights so you can text or call anytime from 3pm to 7am 917-807-3801

[Tx 201]

I am not a mechanic, but is sounds like you have a blockage somewhere in the system not letting cooling get to the motor and causing way too high pressure readings. At ilde pressure will barely register and 12-15 lbs @ WOT is in the correct ranges. How ling do you run it before it over heats? What does your temp gauge show with the key on but motor not running? When mine freezes during the winter it may show 15 lbs of pressure with the key off, and then moves up once the motor is running.

Don't play around long with the overheat issue, aluminum blocks don't like that!

[bryceban]

Thanks billybas for that. I may holler at you. It's crazy to think that flushing sand would be such a pain. Seems it would flush easy

TX 201 I don't have a temp gauge, wish I did and would love to install one at some point. It takes 20-30 seconds to get overheat alarm at WOT.

I need to find a good water flow illustration and go from there.

[bryceban]

UPDATE:

So after pulling lower unit to recheck water pump and get a look up in the midsection to look for any blockages and pulling all water hoses to check for blockages, etc. I pulled the exhaust divider plate to check water passages and found that someone had used the wrong gasket and half of all the water passages were about 50% blocked by the gasket. I just knew that may be it. But, after reinstalling everything and running it again I’m still at 15-17psi water pressure and noticed it is making my poppet valve leak pretty bad form the weep hole so I took it off to make sure it was installed correctly.

Here is pic of gaskets


Here is what I have done so far:

1. Rechecked new water impeller for missing pieces, old one was intact as well.
2. Checked everything in lower unit and midsection for any blockages, mud daubers, etc.
3. Replace thermostat and new poppet valve assembly
4. Taken off water lines to blow through checking for blockages
5. Inspect exhaust divider and replace wrong gasket
6. I have good amount of water coming from prop hub

So where to next?
I know the powerhead was replaced or rebuilt. Probably rebuilt by a non competent person so could they have gotten wrong rebuild kit and maybe used the wrong head gasket and/or exhaust PLATE since it had wrong exhaust divider gasket? I’m thinking pulling heads to check would be next and less invasive step vs pulling power head. Any opinions on where you would go next would be greatly appreciated!