Merc 250hp overheats at high speed

[fish_wish]

Hi, I have a '96 Merc 250HP EFI. I did an extended run with it today after several months of no use other than regularly running it at the dock. The pee hole has good pressure and the temperature initially goes up to 175 degrees at high speed as always, but after a few minutes at high speed, the temp begins to rise quickly until the alarm goes off. I can run it all day at up to about 3700-3800 rpm and the temp won't budge from 175 (I ran it about 25mi this way today). If I push it up to about 4000 rpm it behaves as I described above. I initially thought it might be the impeller starting to go, but I am now thinking it could be a problem with the thermostats. Any thoughts? Thanks,

 

[Perfidiajoe]

Re: Merc 250hp overheats at high speed

Check poppet valves, I believe these are designed to allow more water at high RPM's. I have a 1995 225 EFi & when a thermostat went bad it would get hot at lower speeds. If I got to the bay I could run all day at high speed. Good luck, Joe

 

[KCLOST]

Re: Merc 250hp overheats at high speed

Yep, if you have a poppet on that motor that is most likely the culprit assuming everything else is in order. It should open up above 3000 rpms to release pressure on the head and increase water flow through that area.. With it not opening correctly it will overheat the engine at higher rpms...<br /><br />Let us know...

 

[fish_wish]

Re: Merc 250hp overheats at high speed

Thanks guys. I will give it a try. I changed the poppet valve on the engine a couple of years ago because it was leaking water into the cowling, but I will give a try to see if that is the problem. The boat is in the water, so I prefer trying that first instead of hauling it out to change the impeller first. I will update with the results.<br />Thanks,

 

[Perfidiajoe]

Re: Merc 250hp overheats at high speed

Do you have a water presure gauge? You can tell if the impeller pump is putting out by the presure reading. Joe

 

[fish_wish]

Re: Merc 250hp overheats at high speed

I don't have a pressure gauge. I have seen mention of that on these pages. Do you have some more information on them? Do you install them permanently, or is it just a tool that you somehow connect while the engine is idling. It would seem that you would want to measure the pressure at high speed in this case.

 

[Perfidiajoe]

Re: Merc 250hp overheats at high speed

It's a gauge you mount on the dash, it has a small tube that runs to a fitting on the engine. I believe it's mure important than a water temp. gauge, especially since you have a temp. alarm, & if you have the same Merc. Tach. as I do you also have a temp. light. Water presure gauge can let you know ahead of time if the pump is starting to go bad, or if you have a blocked water intake. IMHO Joe

 

[coves]

Re: Merc 250hp overheats at high speed

Greetings all. I have a '96 225 acting this way. I had the water pump/impeller rebuilt and it made no difference. The wierd thing is it is intermittant. I can run all day at different RPM's and all is well. Yesterday, under 3k, no problem. Over 3k and the alarm would sound, but not always! Argh!! Also, stopping the engine for only 5-10 seconds I could restart, no alarm, and back on my way. If the engine is really overheated would the sensor reset that fast? The water was pretty cool yesterday so maybe that is why itis intermittant? So I am making the first assumption that the sensor is whacky. But now you guys have me looking for poppets and thermostats? Difficult or easy to change out? I need to fix this, my skiers are getting annoyed with me! (grin)

 

[Perfidiajoe]

Re: Merc 250hp overheats at high speed

Thermostats & Poppets are relitively simple jobs, if you have reasonable mechanical skill, a decent set of tools, a book (if you never did this before), & PATIENCE. Good Luck, Joe

 

[fish_wish]

Re: Merc 250hp overheats at high speed

Well, finally got to work on the engine this weekend, It has been a month since I came to this forum for help. I changed the poppet valve diaphram and both thermostats. The thermostats looked pretty bad, almost clogged with salt & sand. Afterwards I ran for about 8 miles at close to 5000 rpm and the temp did not budge above 175. Thanks for the help guys. I got away with $32 worth of parts and a bit of a sunburn.