Alpha 1 overheating above 1500 rpm

[oigman]

I have a 1997 Mercruiser 5.7 Alpha 1 that I just changed the manifolds and risers. I had the outdrive pressure and vacuum tested at the dealer over the winter, they also changed the water pump. Put the boat in today and when I run it up on plane it immediately ovberheated. I let it cool and got back to the dock. I pulled the thermostat. Ran it out same thing. the risers are getting hot also. Noticed that the temp went from 130 to 200 to 130 in seconds. I suspected that the water pump is sucking air. I replaced the water line to therm housing with a clear line and sure enough when I run up the rpms over 1500 or so i start getting bubbles in the line, at 2500 to 3000 I get large (golf ball sized) bubbles and the water velocity doesnt seem to have increased much. Do you think that the water pump may have been put together wrong? Is it normal to have any air in this line? How could air get in this line? water pump output tube? gaskets?

Thanks for any ideas

 

[Don S]

Re: Alpha 1 overheating above 1500 rpm

Did you do those tests for air (with the clear line) on muffs or in the water.
Cause it won't work at all on muffs.

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

On the water, it ran great in the driveway. It only gets hot under load

 

[Bondo]

Re: Alpha 1 overheating above 1500 rpm

they also changed the water pump.

Did they change Just the Impeller,..... Or the Whole Thing,..??.....
Is there Any Chance that it's been run or even turned over Without a water supply,..??.....

My 1st Guess would be to Pull the lower unit,+ Reinspect the Water Pump.....
Sometimes the Water Pump Housing gets little Scores Lines in it,+ at that point,...
The Complete Waterpump,+ it's Housings should be Replaced........
And,........
While you're at it,.......
Backflush it from the T-Stat Housing(the In-coming waterline), Back thru to the Waterpump, with the Lower Unit Off.....
You might have some pieces of Impeller lodged in there somewhere,..........

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

They replaced the entire water pump supposedly. Large bubbles in the line to the tstat housing is probably the cause of the overheating right? How can air get in this hose? Thats why I'm thinking that they did something wrong with the pump.

Thanks

 

[Bondo]

Re: Alpha 1 overheating above 1500 rpm

How can air get in this hose?

From a Leak............

 

[Haut Medoc]

Re: Alpha 1 overheating above 1500 rpm

If you had a truly harsh overheat you can distort the water pocket cover......
But, you do not want to go there if at all possible.....
Check the hose that goes from the bellhousing to the transom shield for any signs of wear.....
You will need to pull the lower & look at the water pump, & while you have it apart, look at the water pocket cover, located in the upper half of the drive by where the lower drive shaft mates to the upper gear housing.....​

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

Ok I'm going to pull the boat today if possible. The water pocket cover is where the tube from the water pump seats into? Do you think these bubbles I'm seeing are causing the overheating?

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

Ok I pulled the lower unit and found everything looked good (crap!) The impeller was brand new, the key was in place the gaskets and o rings looked good that go to the water pump coupler. I plan on running water backwards then reassembling and running on muffs to check the hose in the outdrive for leaks. Any other ideas?

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

Ok ran water backwards through the upper half of the outdrive, came through like gang busters, removed the elbows to check the gaskets. all fine restrictor gasket on top of spacer, 4 hole gasket below. checked and set ignition timing. Still runs hot over 2000 rpm. I'm stumped. HELP!

 

[Bondo]

Re: Alpha 1 overheating above 1500 rpm

When you had the Elbow Off,......

Did you inspect,+ Clean the Ports that allow the water Out of the system,..??....

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

I checked them, they are brand new, just took them out of the box and put them on

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

Rechecked everything, took the risers off again, ran water through the y pipe, removed the outdrive ran water through the input passage, put in a new thermostat. checked the exhaust flappers. At wits end ready to burn the thing

 

[Don S]

Re: Alpha 1 overheating above 1500 rpm

Is the exhaust bellows hooked up the the drive bellhousing? If it's off and aimed down, you could be pushing air (exhaust) down to the water pickups on the drive.

 

[Bt Doctur]

Re: Alpha 1 overheating above 1500 rpm

check the copper tube for any corrosion holes and for the proper length.
the pump base gasket may be leaking hot exaust into the system .can be checked by dropping the unit and putting muffs on to see if there is any leakage from the base.

 

[oigman]

Re: Alpha 1 overheating above 1500 rpm

Ok Im splitting the drive tomorrow AM and am replacing the brand new pump again. I will check the flow with muffs for leaks. The exhaust bellows was replaced last year and is attached correctly. The outpu tube is intact and when I fed water from the therm hose back to the out drive (which I had the lower half off at the time) it held pressure, I had my thumb stuffed in the water pump coupler. Could it be a head gasket? Would that cause the mainfolds to get hot? The risers were running 165 starboard and 150 port. Sometimes the port would drop to 110.

 

[jakebird]

Re: Alpha 1 overheating above 1500 rpm

oigman, I don't think that it is the engine at all. I am having the same problem, and I redid the entire top half of my engine, heads, risers, exhaust manifolds.... there is still air in the hose when I run at over 3000 rpm. did you ever figure out what was wrong? There must be a solution to this.