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Mysterious overheating motor. Really, really need help please!

Q

QOB

New member
Mercury 175EFI 2stroke '98 model. I got it for a customer because the power head looks brand new with a tag stated that is a remanufactured powerhead by Mercury. All parts numbers and date code are still there. Ran a compression check, spark tedt and gearcase inspection before purchased from an individual owner.
Installed a customer's boat, started engine in a barrel of water, motor sounds nice, rev up and quickly cool down. Customer ran a mile out alarms goes off. I asked customer to touch block. He said it's kinda hot. I changed new impeller, housing, gaskets, still running hot. I bypassed thermostats, still running hot. I reduced relief valve pressure, still running hot. I drill poppet valve passage thru the block, still runnimg hot, I poked thru exhaust at tail pipe, no change. I changed to another set of 2.5L heads and new head gaskets. Got it cool down and ran 1/4 mile farther but the alarm still goes off and the block feels pretty hot. Hotter than a normal temp. 24 years of fixing these things and I got this 2nd mysterious overheating situation. I am about to get the customer to approve me bypass oil injection system and mix his own gas next.
Has any of you here come across this situation? Please help share your advice and recommendation please please please!
I had the same Mariner motor with this situation and changimg the heads helped!
I need help please before I tear this baby apart and re rebuild it. How can Mercury's Remanufactured powerhead can be this bad???
Many thanks and regards and,
Best of wishes to yall!
 
First you need to repair your attempted repairs. 2nd is the midsection a 1998? What type of boat? Did you bleed oil system when installed? Kinda hot is normal for a 2.5.......
 
Thanks for the input. Good point. It's Merc dealer who sold and put this powerhead on! I can build these engines from scratches using variety of parts of all hp and models; interchanging between EFI and carbs. I've done so hundreds of times. Looks like it's beyond normal tech's capabilites. I will pull the powerhead today and redesign its water flow passage my way. I live and work off and outside the books that technical words in them scared the pockets out of people. Was hoping to nail down the specific one year model of Merc that exhaust hole must be redrilled thru.
Thanks again.
Take care all!
 
You need to find a Merc 2.5 water flow diagram and study it. A 2.5 wont cool correctly with t-stats bypassed or poppet coming off seat to early and the new passage you drilled wasn't needed...Good luck on redesigning a proven system.
 
Appreciated the follow up post. I am 100% sure that new poppet valve and thermostats won't fix the overheating problem. I fixed thousands of 2.5L including Optimax by bypassing Tstats and reduced relief valve pressure. Don't mean to be argumentative. But are you reading something Merc says? Where I live, water temp never drops below 15 degree F. I do not need Tstats in my motors.
I go by experience and what I had done to all outboards, not only Mercs.
If I can gun the motor down the river early morning after my repair for 2hrs straight, my customers can do the same on his bass tournament!
Thanks again.
 
Just a quick guess right while removing powerhead bolts: this motor is not dumping water out fast enough into the exhaust pipe. It is over pressurized and contains boiling water in the block. Tstats and new poppet valve will overheat it quicker! It is overheating while running fast and not idling, trolling or cruising! It has to be in the adaptor plate and mid section, exhaust etc...
 
No need. I touched it with my hands and those several times overheating blew a set of head gaskets. I changed them heads and new gaskets already. I won't have it overheat the 2nd time. I am 24 years experienced outboard spe******t. I have not lost my touch and feel. The adaptor plate is all clogged not allowimg water into the exhaust. I am at my shop changing the whole mid section with older model right now. It will fix it. Just gotta rig new tilt relays for 3 wires tilt.
Thanks.
 
Fixed it. Also drill the end cap to exhaust plate to release water from there. WOT 15min, cruised at various speed for an hour. Idiot light flashes at times but mo buzzer. Touched block when it flashes, real warm but does not get extensively hot anymore!
My 2nd complicated and mysteriously overheating have been resolved!
 
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