mercruiser 5.0 mpi slowly overheating

woodguy81

Petty Officer 3rd Class
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Mar 30, 2019
Messages
98
Got my boat out for a test run today and it runs great except that after about 30 minutes of running about 75% throttle it overheated. Shut it off for 3 minutes and restart. immediately cooled down. Ran a bit longer and the temp was slowly climbing again and I stopped but didnt shut it down. I noticed at idle it looked like steam coming from behind the boat. It seems there is not enough flow through the motor But I cannot trace it. Heres what I did and a possible issue. Worth noting that the only change from this season to last is a new upper unit on the alpha one gen 2 drive.
First I disconnected the water lines from the power steering cooler and flushed it backwards. No restriction.
Next I connected the hose to the line there going to the motor and turned the hose on to look for leaks in the boat. (im getting water in the bilge and havent found the leak yet) No leaks, Water flowing through both exhaust ports on the back.
Next I connected the hose to the line going back to the drive and turned it on. Ive got water coming from the water pickup holes as well as from the front of the drive. My question here is whether it should all flow back through the water pickup holes or is it normal to leak out through the drive somewhere.
There are no flappers in the exhaust so that is not the problem.
No overheat issues last season and I ran this boat 40 miles offshore non stop.
Any ideas?
 

achris

More fish than mountain goat
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May 19, 2004
Messages
27,468
...Next I connected the hose to the line going back to the drive and turned it on. Ive got water coming from the water pickup holes as well as from the front of the drive. My question here is whether it should all flow back through the water pickup holes or is it normal to leak out through the drive somewhere....

That's a problem. You need to pull the drive, split it and service/replace the impeller. Make sure to use the housing alignment pins and the V-seal setting tool when putting it back together.

Chris.....
 

woodguy81

Petty Officer 3rd Class
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Mar 30, 2019
Messages
98
what is normal in this situation? Water only from the the line of holes on either side of the drive I assume. Where it would normally pickup from the lake. Keeping in mind i was blowing the water backwards through the drive from inside the boat. I gotta change the drive oil anyway so Ill pull the lower unit off and check it all out when I drain it tomorrow.
 
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Rick Stephens

Admiral
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Aug 13, 2013
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6,118
what is normal in this situation? Water only from the the line of holes on either side of the drive I assume. Where it would normally pickup from the lake. Keeping in mind i was blowing the water backwards through the drive from inside the boat. I gotta change the drive oil anyway so Ill pull the lower unit off and check it all out when I drain it tomorrow.

If you apply pressure on the engine side a good impeller will not allow water through to the intake ports.

if you hook up muffs and put pressure to the intake ports without the motor running and powering the impeller, water will not flow to the motor.

your impeller is shot.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
what is normal in this situation? Water only from the the line of holes on either side of the drive I assume. Where it would normally pickup from the lake. Keeping in mind i was blowing the water backwards through the drive from inside the boat. I gotta change the drive oil anyway so Ill pull the lower unit off and check it all out when I drain it tomorrow.

If you attempt to push water back into an impeller pump when it's not turning, you SHOULD be deadheading the pump. If you have water flowing out from the intake side of the pump, it's passing the impeller, which if the impeller is in good condition, should never happen... If the water doesn't come out of the inputs, but still 'goes somewhere', you have a leak between the hose and the pump, possibly one of the water pipe (between the pump and the bottom of the top housing) 'o' rings.

Chris....
 

QBhoy

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Mar 10, 2016
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I’m not sure I agree with the idea that water won’t flow back out the intakes when putting water through from the other end.
when winterising, and filling the manifolds with antifreeze using the two thin pipes off the t stat to manifolds...if you keep pouring AF in it will eventually run out the drive through the ports, I’m sure. It’s a vertical situated impeller housing
 

QBhoy

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Also true that after such a set up has been running and then the boat is taken out the water on trialer with drive up. Afterwards, if you lower the drive, you’ll get a load of water out the intakes running out and draining.
 

achris

More fish than mountain goat
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May 19, 2004
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I’m not sure I agree with the idea that water won’t flow back out the intakes when putting water through from the other end.

The facts don't really care whether you agree with them or not. They just are.

when winterising, and filling the manifolds with antifreeze using the two thin pipes off the t stat to manifolds...if you keep pouring AF in it will eventually run out the drive through the ports, I’m sure. It’s a vertical situated impeller housing

That's because the AF flows out of the elbows, down the y-pipe and into the drive through the exhaust passage. Some of it will run down the exhaust reliefs at the bottom of the gimbal and it does appear as if the AF is coming out of the water intakes. I can assure you, if the impeller and everything associated with it are in good condition, water does not, can not, flow passed an impeller that not moving!

Also true that after such a set up has been running and then the boat is taken out the water on trialer with drive up. Afterwards, if you lower the drive, you’ll get a load of water out the intakes running out and draining.

Yep, the suction side of the base plate is at the rear. When you tilt the drive, you trap water between the impeller blades and as the suction port is now higher than the impeller, it has nowhere to go. Drop the drive down and the water then flows from the cavity of the impeller that is open at the suction port, down through the intakes....

Chris...........
 

woodguy81

Petty Officer 3rd Class
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Mar 30, 2019
Messages
98
Discovered the flappers were missing while troubleshooting an overheating issue last year. Turned out the riser gaskets were in backwards. I’ve only had this boat a year and a half and don’t know why they are gone. I’ll check out the impeller tomorrow. Thanks.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
At no stage have you given us an engine serial number, or even a year (and we do ask that you include those details in the first post of all new threads).

Shutters can be missing because of a lack of water, and the hot exhaust gases have burnt them away, or if the shutters are the old style (a serial number will tell us that), through wearing out the shutter shaft, and the shutter drops down and most of the time ends up on the seabed. Some unfortunate souls end up with them stuck in the bottom of the y-pipe and then rubbing a hole in the pipe and filling the boat with water. :eek:

Chris.........
 

QBhoy

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Mar 10, 2016
Messages
8,286
The facts don't really care whether you agree with them or not. They just are.



That's because the AF flows out of the elbows, down the y-pipe and into the drive through the exhaust passage. Some of it will run down the exhaust reliefs at the bottom of the gimbal and it does appear as if the AF is coming out of the water intakes. I can assure you, if the impeller and everything associated with it are in good condition, water does not, can not, flow passed an impeller that not moving!



Yep, the suction side of the base plate is at the rear. When you tilt the drive, you trap water between the impeller blades and as the suction port is now higher than the impeller, it has nowhere to go. Drop the drive down and the water then flows from the cavity of the impeller that is open at the suction port, down through the intakes....

Chris...........

Chris

Im 100% aware of the passage of flow and it’s route. You can be a cheeky b**t...d sometimes.
I can only presume you aren’t a people person at all, perhaps you have been self isolating for way too long...and before all this ever started.

anti freeze runs out my intakes every winter when I pour it down from above. There is nothing wrong with my impeller and it cools tremendously.
These are facts.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Waaay more fun to tease 'im back. He is an ozzie after all. Deserves ever bit of it.

So now we have a yank, goaded by a jock, taking the **** out of an Aussie. They're right, the world's seriously screwed up at the moment.... :D
 
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