5.7 Alpha High-RPM Overheat - Can't Solve

[Brentathon]

I think this high-rpm overheating has been plaguing me for seasons, now. I believe I’m getting just barely enough water in from the drive, to not overheat up to my normal cruising rpm’s (3600rpm). While trying out a new prop (WOT runs), I now confirmed I overheat. Plus, bay water is now in low 80’s, which seems to make noticeable difference in my cruising temp. Temp is pegged at 140 at low rpm’s, but creeps and stabilizes around 170-ish while cruising….probably too much rise, indicating borderline cooling.
So much history, so I just list things...
Engine: 1990 5.7 Alpha One, raw water cooled. Replaced w/ re-built in 2007.
Drive: SE106 from 2005.
Replaced: T-stat & T-stat housing, Circ Pump, Impeller/upper housing, elbows, plastic water pickup inserts, Y-pipe & flappers.
The water hose from bell housing-to-gimbal housing replaced a few years back and looks fine. Manifolds are only couple years old, and looked good when replacing elbows. PS cooler seemed wide open (1 inch open water passage, no blockage felt).
Just replaced on 7/20/12: water pocket/seal in upper drive, impeller, impeller housing & impeller pump base.

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

So, I installed a clear hose from the transom to the PS cooler, and clear hoses from the check-ball Tee on the T-stat housing to the elbows. The gauge stays pegged at 140-ish at low speeds, but once I get on plane, it creeps up and I see lots of aeration in the hose coming from the transom. I tried revving it in nuetral to around 3200 rpm for 30 seconds, and the gauges stays down at 140-ish with very little aeration in the hose.
It sure seems to be an issue when I'm at cruising speeds. The plastic water pickup inserts are not clogged, nor is the internal cavity of the drive. I do have a depth finder transducer mounted 8 inches off keel, about 3 feet forward of the transom....I've always had one in this spot, but 3 years ago I replaced it with a Norcross Hawkeye. It sticks down very little from the hull (i.e. 3/16" maybe?).
I just don't know why I get aeration at speed, and not while revving it in neutral.
Any ideas?

 

[Bt Doctur]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

might want to replace the water pump base, could be sucking air in . the water pump is not under water when running a boat at cruise.
once the cavitation plate is above water ,the pump is above water.The low pressure side (the base) is sucking in hot exaust gas if you see bubbles. the hi pressure side(from the top of the pump to the engine) can only lose flow but not produce bubbles.
Can be checked by splitting the unit and putting muffs on the lower unit . check for leakage at the pump base.If leakage is found ,replace pump base using contact cement on the gasket.

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

Ah yes.......thinking about it, that makes good sense Bt Dotur. I replaced the impeller and upper housing over the winter, but I'm not sure if anyone's ever touched the pump base since I got this drive in 2005.
I just confirmed the transducer protrudes only 3/16, so I'm thinking that's not likely an issue.
Thanks!

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

Update:
Had boat hauled, and mechanic replaced impeller & housing, and the base assembly, as well as the whole "water pocket" assembly in the upper half of drive.
He found that the Sierra impeller had a spun hub (I just put it in, in winter ); the rubber was loose around the brass hub; you could see the square profile on the hub.
**HOWEVER** I'm STILL getting aerated water coming in from drive at rpm's above 1200-ish, and replacing all those drive components had NO change in the temperature rise. I still overheat at fast cruise.
Port manifold was 260F degrees, while starboard was 160F. And even though it's aerated water, there's always a decent amount of water coming out of the Tee on the T-stat housing, even at high speed when temp is rising. I'm stumped. :facepalm:

 

[Grub54891]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

A bit of a reach but when you replaced the flapper's did the old one's fall down in the bullhorn causing a restriction? Mabe the water can't go away fast enough under power? Just a thought.........
Grub

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

The flappers were replaced, and then the entire Y-pipe was replaced with the new engine(flappers were still fine, and re-used), so that's not likely the issue....there were no pieces in the drive.

I just spoke to mechanic, and he's wondering if there's some kind of crack or casting flaw in my SE106 drive..lhe said he's only seen that twice. He plans to swap out the drive, and install a different one for a good troubleshooting move. I'm glad he has a spare drive, so that we could do that. I was thinking about a possible casting crack/flaw, because it only seems to aerate when you're in gear (1200rpm and higher), and when exhaust is exiting thru prop hub.

A bit of a reach but when you replaced the flapper's did the old one's fall down in the bullhorn causing a restriction? Mabe the water can't go away fast enough under power? Just a thought.........
Grub

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

Update!!!
I was finally able to get the boat hauled, and my mechanic swapped out the SE106 drive with a used Merc drive.
I immediately noticed that the idle temp was a little higher than the SE106's, but as soon as I powered-up to plane, I also noticed the temp gauge barely rose at all......even at a 4600rpm WOT!
That's it! Countless hours and many $$$ over multiple seasons.....to find out that for some reason (???) my SE106 provided aerated water; despite replacing impeller, impeller housing, pump base, and entire water pocket assembly....basically all the serviceable parts in the drive. :facepalm:
I'm wondering if Sterndrive Engineering will offer to do anything with my case.

 

[Brentathon]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

I'd also like to add more to this LONG story:
It appears, that the impeller issue I thought I had in 2010, was probably not the impeller at all. The old thread is http://forums.iboats.com/showthread.php?t=433560
I though I received a defective Merc impeller, which was inside a Sierra kit I bought from my marina. But the Merc impeller was probably ok. It's likely that each impeller I had in the boat operated slightly different, and because I was getting boarderline enough cooling from the aeration issue, some impellers would allow for overheating more than others.
However, the Sierra impeller I just replaced earlier in this post, definitely was defective from the hub separation. I plan to send it back to Sierra.

 

[ktbarrentine]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

Do you plan to (or have you already) done a water-pressure test on the SEI drive to try and narrow down where the flaw is, and if it might be repairable?

 

[NHGuy]

Re: 5.7 Alpha High-RPM Overheat - Can't Solve

Check the path from the pickup to the leg pump. Maybe it's sucking some air when you get the pump over the surface of the water. There's a chance it's a quick or easy repair.
SEI's warranty is only 3 years. You just didn't know it was that problem on your 2005 til now.