Johnson water pump possible issue

I just recently in the last year have gotten my project boat ( 1995 cobia 20' with a 1990 Johnson 125 ) close to finished ( if they ever are ) and I was running it at full throttle and trimming it up to see what the top speed would get to and it hit 35 mph and then my overheat alarm went off , so I looked back and noticed the pee stream had quit , so I immediately shut it down.

Used my trolling motor to get up to a dock that was close by and pulled the cowling and everything appeared to be in order , so I cranked it up and it started peeing after about 5-6 seconds and the alarm went off after about 10 more seconds.

While I was trimming the motor up the bow never rose up and it never porpoised up and down , I am hoping I just accidentally trimmed it up too much and the intake screen came out of the water to quit pulling the water in ? But the prop never cavitated and it never porpoised , which is weird . I also have one more question , this is a brand new impeller ( installed it roughly 3 weeks ago ) , did I just ruin it ?

Could anything else cause this problem ? There are only two things that ALWAYS worry me about doing the water pumps on these motors and those are the little plastic drive key that turns the impeller and the little rubber grommet for that water tube that slides into the water pump housing when you put the lower unit back on , could either of these have an issue ?

I am wanting to eventually get a water pressure gauge and install it because I think I could keep an eye on it when I'm trimming it up and know when to stop , or is it as simple as watching the pee stream ?

Gee, when was the waterpump last rebuilt? Ditto on the thermostat.

Just so you know, when trimming motor, most props will slip before cooling water to the motor is interrupted.

Chris1956 said:

Gee, when was the waterpump last rebuilt? Ditto on the thermostat.

Just so you know, when trimming motor, most props will slip before cooling water to the motor is interrupted.

Click to expand...

Thermostats are less than 2 years old and as mentioned in the post the water pump is 3 weeks old , second time in the water but first time running wide open with this water pump .

Sorry, I obviously did not read your post properly. I am not sure what to suggest.

You might check the passage under the waterpump base for blockage.

Also, my '98 Johnny has a strainer that was nearly clogged, however, it only ran to the telltale/flush port. After 27 years I cleaned it. It made no difference.

Clogs in the system, left over pieces if prior impeller was coming apart, check co the sensor, but it sounds like something may be blocking your water passages somewhere.

If it’s working as it should now you may never know the cause. I once had this happen and just as I got to the motor I saw a plastic shopping bag peeling away from the lower. A few more seconds and I may not have seen it. It must have been covering the intake perfectly.
The only way to know it your impeller was burned up is to drop the lower and have a look.
For me if I’m doing that I’m replacing it anyway so I know I’m good.

Plastic and diapers are common in my area.

So I figured I would report back on the problem and let someone else know what it was.

Apparently when I put the water pump in and went to install the lower unit a few weeks ago , the water tube in the midsection caught the little rubber grommet that goes into the upper pump housing and moved it enough to block about 1/2 the flow of water coming from the pump , i replaced it and no more over heating.

cobia205 said:

So I figured I would report back on the problem and let someone else know what it was.

Apparently when I put the water pump in and went to install the lower unit a few weeks ago , the water tube in the midsection caught the little rubber grommet that goes into the upper pump housing and moved it enough to block about 1/2 the flow of water coming from the pump , i replaced it and no more over heating.

Click to expand...

Many thanks for letting us know your findings !!