Mercruiser 5.0 Won’t Reach Operating Temp

[dubs283]

This is the one you’re talking about, right?

Yes

 

[dremison]

Might be the position of your muffs I found if mine are not exactly right it will overheat even though water is moving through it

Ok, weird weekend with temperatures - after replacing the thermostat correctly, it got up to temperature and maintained it on muffs and in the water. However, over the course of the day out yesterday, it started dropping in temp again. Got up to 160 at cruising speed and held, but was down in the 120s by the time we were cruising back in.
Is there any reason you can think of as to why this would be happening? I don't know if a worn out impeller can lower temperature or not, but I assume it will need to be replaced soon, seeing how I was unknowingly burning it up for a while. I check the temp at the thermostat housing and it was accurate, as well, so it isn't a sensor issue.
Any ideas?

 

[alldodge]

Pieces from the old impeller can get caught in the check balls and in the Stat.

 

[Scott06]

Pieces from the old impeller can get caught in the check balls and in the Stat.

this or the gauge is inaccurate. If you have a suspect impeller replace it now as its only $50 of parts

 

[dremison]

this or the gauge is inaccurate. If you have a suspect impeller replace it now as its only $50 of parts

It will be here Wednesday - can't find anything in stores around here.

 

[Lou C]

Don't have a Merc engine but have had that same problem (low temp like 120*) on occasion. It seems to be caused by either a piece of rust, or sand getting between the stat and its seat. Doesn't let it close all the way. If it sticks part open it won't reach normal temp unless you're up on plane. Cast iron engines will shed a lot of rust particles if they are raw water cooled.
Believe it or not, I cured this once by doing the following:
ran engine up on plane for a while to flush it out
let boat drop off plane, shifted to neutral...
took a small hammer and gently tapped all round the 'stat housing to vibrate loose any rust/sand etc that might have gotten stuck in there....worked!

 

[dremison]

this or the gauge is inaccurate. If you have a suspect impeller replace it now as its only $50 of parts

Well it wasn’t the impeller (pictured). I know the plate was worn down so I replaced it with the plastic Mercruiser housing, wear plate, and put in a new impeller anyway.
I fired it up no problem after the install (had the belt on wrong initially like an idiot but fixed it quickly) and now it won’t kick water out of the exhaust. It seems to not be getting past the outdrive like it’s not getting enough pressure. I went back in and pulled the plugs at the pump and it was definitely getting water - drained out a lot of water.
Not sure what happened or if I put something back wrong but I ran it for a little bit and shut it down quickly when I realized the issue.
Is there something I’m supposed to do to prime it or should I try another troubleshooting technique? I might just put the old impeller back in and see if that fixes it tomorrow but I’m stuck on this one.

 

[Fun Times]

The hoses are installed in the same correct location as the old…?

Sometimes in my experience bravo 3’s have a tendency to develop an seemingly air bubble of sorts between sterndrive and the impeller housing to where it may not pump water out the exhaust… when this happened to me a number of times it usually would seem to help if I raised and lowered the drive 2 or 3 times, ensure the muffs are on tight and start the engine then immediately go back and feel the back side (between the water hoses area) impeller housing and see if you feel heat or hopefully cool water temperatures… Heat is not good and your not getting good water flow… If raising and lowering the drive doesn’t help within a few tries then you may have to remove the hose and primer it with water the best you can with no air.
I’d be leaving the new impeller in at this point.

 

[dremison]

The hoses are installed in the same correct location as the old…?

Sometimes in my experience bravo 3’s have a tendency to develop an seemingly air bubble of sorts between sterndrive and the impeller housing to where it may not pump water out the exhaust… when this happened to me a number of times it usually would seem to help if I raised and lowered the drive 2 or 3 times, ensure the muffs are on tight and start the engine then immediately go back and feel the back side (between the water hoses area) impeller housing and see if you feel heat or hopefully cool water temperatures… Heat is not good and your not getting good water flow… If raising and lowering the drive doesn’t help within a few tries then you may have to remove the hose and primer it with water the best you can with no air.
I’d be leaving the new impeller in at this point.

I really hope it’s that simple. I’ll give it a shot in the morning and let you know how it goes. Thank you.

 

[dremison]

Update

I really hope it’s that simple. I’ll give it a shot in the morning and let you know how it goes. Thank you.

Update: solved it! My pump had deterioration on the other side, as well. I had the right part for a good pump, but once they get beat up a little bit you need a separate impeller kit. The part number is 47-8M0137219 and it comes with a wear plate for the inside of the pump, as well.
Also, my belt did NOT have proper tension in it. Once I figured those two things out, it primed and ran like a champ even on the hose.
Thank you to everyone who chimed in and helped out. I really appreciate the kindness and time - I will certainly be back but hopefully not too soon.

Mercruiser 47-8M0137219 Heavy Duty Sea Water Pump Impeller Kit Includes shorter heavy duty impeller and overmolded inner (front) wear plate for use when the front of the seawater pump impeller housing is worn due to debris.
Fits 2000 & newer MCM (sterndrive) V‑6 & V‑8 (305, 350, 496 & 502 cid) engines with the brass sea water pump excluding Mercury 4.5L & 6.2L models
Kit contains:
1 Formed o-ring
1 0.030 inch shorter heavy duty impeller
1 Overmolded 0.030 inch inner wear plate.
Installing instructions,

https://www.mercruiserparts.com/Attachment/DownloadFile?downloadId=63

Mercury-Mercruiser 47-8M0137219 Heavy Duty Sea Water Pump Impeller Kit

Mercury-Mercruiser 47-8M0137219 Heavy Duty Sea Water Pump Impeller Kit genuine factory part not aftermarket. Fast shipping - Click here to see live inventory status

www.mercruiserparts.com