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1989 Mariner 175 overheating

deh4170

deh4170

New member
First thank you for any help on this issue. I have a 1989 Mariner 175 (Non-EFI) and have an issue with overheating. Once I get above 2500 rpm the engine starts to run hot. Assuming the impelIer/housing/gaskets were the issue I started there - replaced the base gasket, impeller housing gaskets/faceplate, impeller, and impeller housing. Also, just to be certain I replaced both thermostats and the poppet valve. Took the boat to the lake and same issue after 5 mins above 2500 rpm. Per water pressure gauge at 3000 rpm it was @ 9 psi and at idle is 1.5 - 2 psi. I dropped the lower unit again...removed the thermostat/housing and slid a water hose up over the bottom of the water tube. Looking up into the midsection no water was leaking from the water tube or seal going into the powerhead, but before water started flowing out of the thermostat openings on the heads water was running back down into the water tube/drive shaft section. There was a small stream coming out of a small hole between the (2) rear cavitation bolts, but I'm fairly certain that is an exhaust relief/overlow and is normal? But I don't think the water coming back down into the water tube/drive shaft section should occur unless it is leaking somewhere between the driveshaft plate or exhaust plate prior to base of powerhead. Also, I did check compression on each cylinder and it was showing @ 100-110 lbs. Is my thinking on track that one of the gaskets is bad on one of those plates? Thanks
 
Cooling water has yo return to the lake somehow.-----But do a compression test.-----Leaking head gaskets can cause overheating.
 
Race,
I did a compression test and all cylinders were above 100. I understand water has to return to the lake, but wouldn't it take the path of the exhaust ports and not inside the chamber that houses the water tube and driveshaft? I did allow water to flow out of the thermostat openings and both had good flow while I had water hose hooked up to bottom of water tube. When in the lake, the telltale has a good stream at idle, but once underway and above 2500 rpm it starts to run above normal temp and and there is very little water coming out of telltale with steam.
 
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That is normal as it a cooling wall of water, sprayed down midsection to : 1)cool exhaust can. 2) keep water pump from melting, 3) cool hub in prop
 
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