1990 150 overheats

[gottafishmore]

Before rebuilding the carbs on my 1990 150 hp Yam I ran seafoam thru the engines, on the final run I forgot to turn the water back on to the earmuffs. The engine only ran for a minute or so be fore I shut it off. The impeller blades were worn but all in one piece.
Installed a new waterpump kit & housing, both t-stats open properly. I put the boat in the water & after a few minutes at the dock the temp alarm sounds.
I bought the boat about a year ago, I replaced the waterpump, removed both heads & cleaned out the water jackets, installed water pressure guage. Any help would be greatly appreciated.

[Hamfisted]

Are you showing any water pressure on the gauge? Did you install a complete Yamaha water pump kit?, or just the impeller itself? I have seen the plastic housing damaged during an overheat, possible warping or distortion. Are you getting a good tell-tale stream (pisser...)?


-Mike

[gottafishmore]

No pressure on the guage at idle. I installed a complete kit with housing. There is a telltale stream. I also replaced the poppet & spring last year & checked that it is not fouled today. When I remove the plug at the top of the head there is no water when idling.
Thanks for the advise

[jb123]

in the past, ive dropped lower unit and hooked water hose directly to the water tube and run my motor...if it still overheats,then the issue up.... if not then its down.
for the headaches a bad stat can cause I always changem out yearly.
Lastly....you ran your motor hot....I'd do a comp check and think about replacing impeller....again, due to the dry run.

[gottafishmore]

JB, I took your advise, kind-of, with the lower unit off I blew compressed air from the access on top of the heads. Almost nothing came out the water tube. If you plug the end of the tube you can hear it leaking in the upper casing. There appears to be a hole in the tube.
The manual I have does not even mention the upper casing or this tube. How hard is it to remove the upper casing? Can the casing be removed without removing the powerhead? it looks like a new adventure with this one.
Thanks again for the help

[Hamfisted]

Did you check the pressure relief valve for freedom of movement? That's at the rear of the lower powerhead, where that 1" hose connects. The cover has two 10mm bolts retaining it. There's a 25 psi ss spring inside the cover and a plastic valve that slides in and out of the powerhead relief opening to open a bypass passage so the powerhead doesn't get over pressurized (so the thermostats can open and close freely with no excessive pressure...). There is a newly redesigned pressure relief valve available too, its got a mushroom headed design to prevent clogging. Might as well install the new design if you're going to the trouble of opening it up anyway. Its about $5 at the dealer, get a new gasket and grommet seal too. The mushroom head faces in TOWARD the powerhead. The spring goes over the plastic shaft.
I sent you the oldest service manual I had to your email, should show up shortly.


-Mike

[gottafishmore]

Thanks Mark, I could not receive the tech manual, I fixed the memory problem with my email. Could you please send it again. Sorry about that.
The poppet assy all looks good. Since there is a new style available I'll get one.
After putting the beer away I thought of what JB said, my idea of checking the passageway with air was bad. I removed the lower unit and connected the water hose to the inlet tube and ran the motor. All works well, much stonger stream from the telltale.
I have installed a new water pump kit w/housing and even double checked that the o-rings are in place. Checked that there is no obstruction in the pick-up of the lower unit. Still runs hot. I'm starting to go bald from all my head sratching.
Some additional background: When I bought this boat about a year ago the owner told me the powerhead was four years old and it was always flushed. I had a temp problem at hi RPM, replaced t-stats,removed both heads and cleaned out the water passages and installed water pump. That took care of all the problems and we used the boat quite a lot. This is something new.
Thanks again for all your help