Mercury overheating problems

[bobbuffalo43]

Help! I bought a used Ranger bass boat as a winter project. The engine, a 1996 Mercury 150 EFI (0G3xxxxx) was in pretty rough shape as it had overheated badly and melted a lot of parts. I started out with the engine running roughly and no telltale water from the p-hole, and the engine would overheat (with no alarm) and I would then shut it down. I replaced the alarm buzzer, the over-temp sensor and the water pump (although the old pump looked like new). No positive results and no water from the p-hole. I did a compression test and the number five cylinder was about 25 pounds below the other five cylinders. I replaced the bad head gasket on the starboard side (it was leaking into the water jacket) and the compression on all 6 cylinders was then between 112 and 119 pounds. The condition of the cylinder bores on the starboard side was excellent thankfully. I replaced both thermostats which were frozen shut and would not open in boiling water. I checked the pop-off valve which was OK. As of today, when I start the engine it will idle smoothly for a few minutes with no water coming out of the telltale hole. After a few minutes water would come weakly out of the telltale hole, the thermostats would open and a lot of steam would then come out of the propeller exhaust. The engine did not overheat but the water pressure gauge on the dash read zero with an occasional flicker of 1 to 2 pounds. When I disconnected the hose to the water pressure gauge (at the top of the block) very little water sputtered out. The gauge read fine when air pressure was applied to the end of the hose. I’m at wits end as to what the problem could be. I’m thinking of something around the exhaust divider plate? Can anybody out there give me some ideas? Thanks in advance for your help!

 

[Dukedog]

ya say ya did tha pump? a complete kit (housing, impeller, ride plate, base, gaskets) or jus impeller?

 

[bobbuffalo43]

ya say ya did tha pump? a complete kit (housing, impeller, ride plate, base, gaskets) or jus impeller?

Replaced the whole thing, but as I said, the old one looked like new. I'm more than reasonably sure that the pump is not the issue.

 

[Dukedog]

pumps can look pristine but tha stainless ride plate and cup inside housin' can be worn and ya never know it.. if pump is all good tha only thing between it and top of tha block where ya get a pressure reading would be powerhead gasket and adapter plates/gaskets.. checking divider plate and gaskets is a good idea also... didn't say if you got any higher pressure at higher rpm? ..ya might wanna revisit tha poppet also... have you got a cooling route diagram.. if not here's one below. ..

 

[Sea Rider]

To date that's a 24 year old motor, used in fresh or salt water ?

Happy Boating​

 

[bobbuffalo43]

To date that's a 24 year old motor, used in fresh or salt water ?

Happy Boating​

Fresh water only. The coolant passages around the starboard cylinders are pristine.

 

[bobbuffalo43]

pumps can look pristine but tha stainless ride plate and cup inside housin' can be worn and ya never know it.. if pump is all good tha only thing between it and top of tha block where ya get a pressure reading would be powerhead gasket and adapter plates/gaskets.. checking divider plate and gaskets is a good idea also... didn't say if you got any higher pressure at higher rpm? ..ya might wanna revisit tha poppet also... have you got a cooling route diagram.. if not here's one below. ..

Higher RPM does nothing for water flow...

 

[Dukedog]

first we're on a computer.. hard enough if ya standin' next to tha motor.. so dialogistic's here is basically a "guess".. i can only recall two motors (all 2.0, 2.4 or 2.5's.. only motors i used) like symptoms over tha years.. never found tha reason causin' tha problem by doin' one thing at a time.. we took everything apart that had water goin' thru it.. water pump, copper tube, grommets, adapter plates, divider plates, head gaskets, stats, poppet assembly, every gasket (all new).. cleaned and/or replaced basically everything.. good luck with it.. hope ya find it!

 

[Faztbullet]

Since it overheat I would replace all head gaskets, base gasket, water tube grommet's, divider and inspect block for impeller parts. My bet its under powerhead..

 

[bobbuffalo43]

Since it overheat I would replace all head gaskets, base gasket, water tube grommet's, divider and inspect block for impeller parts. My bet its under powerhead..

Well, today I dropped the lower unit again. I made an adapter with ½” copper pipe and connected it to my house water supply. I inserted it into the output of the water pump to test for leaks. Water pump was solid and no problem. Then I inserted the water supply pipe into the water tube rubber gasket at the bottom of the power head, basically replacing the water pump. No change in symptoms. Started the engine, and after 3 or 4 minutes of no water coming out anywhere, water started coming out of the p-hole, tons of steam came out of the exhaust tube, and a lot of water came out of the midsection channel where the driveshaft and water tube go up to the powerhead. Still no indication of overheating but the water pressure gauge remained at zero! I have no clue as to what is going on! Help!

 

[Faztbullet]

1) Check the plastic feed tube to gauge to see if its melted closed somewhere. 2) When you turned on water to power head you should have had water coming back down exhaust whether running or not.

 

[Sea Rider]

Did you remove the thermos if having more than one on the power head when inserted the water supply into the water tube rubber gasket at the bottom of the power head ? If not test with removed thermos and see what happens.

For me that motor needs a full water passages clean out, doesn't matter if been running on fresh water for the last 24 years. Lake, river waters contains other contaminants not found in salt water that will end sticking and reducing the correct heat disipation of the water circulating around the power head to cool it properly as it should.

Happy Boating​

 

[bobbuffalo43]

Did you remove the thermos if having more than one on the power head when inserted the water supply into the water tube rubber gasket at the bottom of the power head ? If not test with removed thermos and see what happens.

For me that motor needs a full water passages clean out, doesn't matter if been running on fresh water for the last 24 years. Lake, river waters contains other contaminants not found in salt water that will end sticking and reducing the correct heat disipation of the water circulating around the power head to cool it properly as it should.

Happy Boating​

Latest symptoms. I put house water under pressure into the water pump grommet fitting under the powerhead, in effect replacing the water pump. (The lower unit was removed). The water pressure gauge, with the engine not running, read about 3 pounds and water was coming out of the exhaust passage and the passage which houses the drive shaft and water tubes. There was also a very slight dribble of water coming out of the p-hole. When I disconnected the hose from the top of the powerhead elbow, water flowed out of the elbow quite rapidly and solidly. There were no plugs in the hoses. Then I reconnected all hoses and started the engine. During the warmup no water came out of the p-hole until the thermostats opened. Once the engine warmed up and as soon as the thermostats opened, water came that of the p-hole more briskly and clouds of steam immediately came out of the exhaust tube. Also at that time, the water pressure gauge dropped to zero and when I removed the hose again from the top of the block elbow a lot less water came out of it. I reconnected the hose and shut the engine off and the water from the p-hole continued to slowly diminish for about 3 minutes. During that time, the water gauge pressure returned slowly to 3 pounds. I believe the 3 minute period correlated with the thermostats closing. Any ideas?

 

[Faztbullet]

You cannot test it correctly this way. If your house pressure is over 25 psi it will open the poppet immediately. It a 2.5 and it will NOT pee till the t-stats open. I reread your first post(below) and does the alarm work if you ground it out? The 2.5 have low PSI at idle(2-5 psi) and about 11-15PSI at WOT. I think your chasing a ghost(guage/hose problem) and motor is cooling normally

I started out with the engine running roughly and no telltale water from the p-hole, and the engine would overheat (with no alarm) and I would then shut it down. I replaced the alarm buzzer, the over-temp sensor and the water pump

 

[Dukedog]

the engine would overheat (with no alarm) and I would then shut it down.

how did you determine it was "over heated"?

 

[bobbuffalo43]

how did you determine it was "over heated"?

Removed lower unit and water pump. Replaced all water pump gaskets. Removed water tube and found it to be distorted at upper end. Replaced water tube and reassembled lower unit to top. Started motor in tank and idle water pressure was 2-3 psi. After engine warmed up, water pressure was 25-30 psi when revved. This I assume is normal; no alarms or overheating. Problem solved!