Overheating Mercury 150

[Bob Buffalo]

Help! I bought a used Ranger bass boat as a winter project. The engine, a 1996 Mercury 150 EFI (0G318937) was in pretty rough shape as it had overheated badly and melted a lot of parts. It was used in fresh water only and the water passages are clean with no deposits. 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. Increasing the engine RPM did nothing. 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!

[EuropeanAM]

Welcome to BBC!

Where precisely is the water pressure gauge hose attached to the engine (picture may be very helpful).

[Bob Buffalo]

The gauge hose comes out of a "T" fitting on the top of the block and heads to the right and ultimately to the gauge on the driver's console.IMG_0473.jpg
It's the small hose without a tie-wrap.

[EuropeanAM]

That is the correct location for water pressure on this engine.

Based on your results, I would drop the gearcase again, inspect the impeller/housing and replace ALL THREE (3) gaskets. Make sure the impeller is lubricated with DAWN DISHSOAP (do not use grease).

[Bob Buffalo]

I will do that tomorrow. I have replaced at least 10-15 outboard water pumps over the years without any problems. It looks like the water pump tube feeds directly to the top of the exhaust chamber where the gauge take-off is. I will also use a bore scope to check the water tube. Maybe an old piece of pump impeller up there? Thanks for your advice; appreciate it!

[blackboat]

I had the EXACT same situation on a 98 200EFI. Like you,replaced everything,no water pressure. The solution in this case...Have you noticed that it's a struggle to install the base,gaskets,plate and housing over the studs in the LU? Like the studs are too far apart for the holes? If true,the nuts may be tight but the housing might not be fully seated,like it binds up on the studs.It took some extra effort to get everything tapped into place but once done and nuts tightened,normal water pressure and no overheating.

[JOE54]

I will do that tomorrow. I have replaced at least 10-15 outboard water pumps over the years without any problems. It looks like the water pump tube feeds directly to the top of the exhaust chamber where the gauge take-off is. I will also use a bore scope to check the water tube. Maybe an old piece of pump impeller up there? Thanks for your advice; appreciate it!

most times they stop in adapter plate if its a big piece has it ever ran correct temps ????

[Bob Buffalo]

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!

[wareagle24]

Could the gauge be wrong?

[Bob Buffalo]

No, the gauge and the hose to it have been tested and are OK.

[JOE54]

you got w/press at fitting on top of block ?????

[EuropeanAM]

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!

Remove the larger hose from the top of the block, and install a piece of clear line to that fitting. Utilizing the gearcase and it's water pump, supply water and start the engine.

Do you have water at this hose (steady stream) within 20-30 seconds? Any air bubble in the hose?

If you DO have water, with no excessive air bubbles, check for obstructions in the ORIGINAL hose (there is sometimes a restrictor installed, THAT would be ok, but the hose/fitting should not be PLUGGED).

Note: The telltale activity, and the STEAM are NORMAL FUNCTIONS.

[Bob Buffalo]

I will try your suggestion in the next couple of days. I had surgery on my wrist last Monday so my wife did not let me near the garage! Thanks for all your help so far!

[Bob Buffalo]

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?

[EuropeanAM]

All of your above testing, despite the effort, does not test the system under it's normal operating parameters.

[Bob Buffalo]

What do you suggest?

[EuropeanAM]

Complete/Fresh Water pump, with ALL THREE (3) gaskets replaced using the appropriate OEM components.

Submerse the engine in the lake, or in a test tank to at least 3" above the gearcase mounting surface, START the engine, and run at idle speed to monitor water pressure and cooling system function.

Keep in mind that 143 degree thermostats will be passing VERY hot water... and water released into the exhaust (where temps easily exceed 900 degrees F) will produce steam exiting from all possible exit points of the midsection and gearcase.

[Bob Buffalo]

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!

[xcoach]

This has been an interesting read, you, like others on here, have benefitted from Joe’s and Don’s expertise.

[blackboat]

So...which one was the problem in your opinion? Water pump or tube? I'd like to see a picture of the tube.