Runnin hot...bah!

TheTinMan

TheTinMan

God
Pullin my hair out here. My 98 Merc 200 EFI gets an overheat alarm over 4000 rpm's. I replaced the waterpump, thermostats and the dealer did the poppet but still getting alarm >:(

I replaced a 15 psi water pressure gauge with a 30 psi one, 15 is too low for my motor. I also installed a Faria cylinder head temp gauge and sender.

I launched the boat today to run the Seafoam process....good stuff, try it! I needed to run the boat pretty hard after the Seafoam. Water pressure is WAY too high. The 30 psi gauge gets pinned at only 3000 rpm!

I did get her up to 50mph @ 5600 rpm ;).....and an alarm too! :'( :'( Temp gauge was only reading 145-150F. I really need one of those infra red thermometers that you just point and get a read on all cylinders to see if I'm actually running hot or not. Could just be a temp sensor maybe???

The really high pressure kind of points to the poppet valve not working properly. I have that pulled out and will take to the dealer tomorrow. If that doesn't fix the pressure problem I have a blockage somewhere. I'm hoping that's not the case because I really don't feel like pulling the power head and lower unit off.

Any Merc heads have any ideas? How bout you BigShrimpin?

Thanks
 
if you want ot buy one of those temp guns, Harbour Freight has them for under $50. NAPA sells them for $75, They really are handy to have around. Other wise go to a welding shop and get some temp sticks, they look like a crayon, they melt at a prescribed temp, get one for 150, 175, 200, 225 temps, when your alarm goes off, stick it to the head and see which one melts. Your block pressure seems high, but not knowing the accuracy of your gauge( all instruments are subject to question), I would verify that you do have a heatiing problem first, tehn look at the poppit valve, theres not much in that block to restrict water flow other than the poppit valve. You may have a bad alarm module screwiing with you.
 
While the head gaskets are definately a possibilty, I don't think so. A compression test should tell me that, correct?

I'm thinking it's an outlet obstruction now. I have a bad feeling I'm going to be removing the powerhead


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tinman...i have the exact same problem.....so far..new thermostats & temp sensors,rebuilt carbs, tonight i'm changing to a complete new water pump...which i doubt will fix it since the one i pulled off was damn near brand new.....next is water circulation for me..

i can run it all day long at 35mph...once i hit 40mph/5000rpms...my overheat alarm comes on......when i touch the heads its cool i can hold my hands on both...water is pumping strong and steady at idle and high speed......think i'm gonna buy a infared thermometer next too........i really hate to start tearing into this thing....maybe i'll just go fishing and say hell with it .. just dont go over 35mph lol
 
if the heads are cool, i'd make sure that it is indeed overheating, the sensor for the temp is located in the head, while it is posible for it to overheat in a small area( around the sensor) its unlikely to happen, it could be cooling off before you get the boat off plane and get back to the powerhead to check it. Try this, disconect the sensor wire to the temp alarm( tan/blue stripe I think) run the engine again and see if the alarm goes off again, if it does, and your sure you disconected the over heat sensor, then you've got something else going on
 
BS, not yet, hopefully tomorrow (thur.) afternoon. At this point I'm hoping it's a head gasket :-/

If compression is good, before I remove the powerhead I am going to drop the lower unit, remove t-stats and poppet, hook a hose up to the water tube and see if I can flush it out.....sound like a good idea? I figured I could check the waterpump housing too, maybe it's cracked or has a leak and I'm sucking air??? That would cause a crazy psi reading, wouldn't it?
 
What if I was to remove the poppet completely? Will that cause low RPM overheat? I would think it would definately lower the pressure.

Actually, I think that would cause the motor NOT geting up to operating temp at low speed.
 
Compression is good, 120-125 psi all across.....all 6 plugs looks the same, a little black oily residue.....probably some carbon left over from Seafoam procedure.

Next step is to drop the lower unit (again) and have a look-see.

Pretty soon and MJ will have more hair than me! :o :-/
 
Thats good though right Tin, if the head gaskets or something else similar were going bad you would not get that good compression. Right ???
 
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