How long does it take to fry an impeller? Thirty seconds!

[natemoore]

I started my engine with no water flowing to the muffs. It had been a long day of troubleshooting and I just screwed up.

I realized it and shut down the engine in less than 30 seconds. After fixing a coolant leak and getting everything back together, I could tell right away that the engine was beginning to overheat. The temp gauge was racing past 160.

I pulled off the outdrive and replaced the impeller. The old impeller looked fine. No burnt rubber. No scorching. But I replaced it anyway.

Sure enough, that fixed the problem. Engine is back to normal.

Moral of the story: it only takes a few second of running with no water to ruin your impeller.

On the bright side, I didn't fry the exhaust shutter.

 

[joewithaboat]

Re: How long does it take to fry an impeller? Thirty seconds!

Ive always wondered if it only took that long or if people just say that. I have always hooked up to water or run in a tub of water. Good to know.

 

[ziggy]

Re: How long does it take to fry an impeller? Thirty seconds!

yep, them alpha1 impellers ain't up for any abuse imho.
i change mine every year. i pull the drive and store it in the basement then mid winter i R&R the impeller and gear lub. + drive inspection. something to do for the boat mid miserable winter.
here's one that got the low water flow treatment.

melted leading edges, cracked back sides at the bend. which is always visible even after only one years use.

the three are one year use only mc-1 (alpha1) impellers. that first pic one was warmed up due to low water flow on the hose. the 'ol lady turned on the washing machine.
got me some smoke from the drive, + the temp gauge got up to pretty much right at the beginning of the red. my temp gauge is old style, yellow is cold, green is ok, red is to hot. i was at the start of the red. only damage was the impeller. imho, these alpha1 impellers are fragile. don't abuse them at all, and for peace of mind, R&R them yearly.

 

[achris]

Re: How long does it take to fry an impeller? Thirty seconds!

At Merc school they show us impellers that have been run without water, for how long and at what engine speed. At 3000rpm, fully fried in 30 seconds!

What you may not be aware of when you run without water in the exhaust gases. The exhaust gases are at full temperature and they pass over, as well as the exhaust shutter, the water pocket cover in the bottom of the upper drive shaft housing. That little cover is plastic, and takes very little to distort. Once distorted it will then allow exhaust gas into the cooling water, leading to engine getting hot at higher RPM. So next time you're out on the water, just be aware you MAY have a melted water pocket cover. If you have, good luck. The bolts holding the cover in are 1/4" S/S and the ali housing corrodes them solid, and usually break off when you try to remove them.

Chris.......

 

[natemoore]

Re: How long does it take to fry an impeller? Thirty seconds!

Ive always wondered if it only took that long or if people just say that. I have always hooked up to water or run in a tub of water. Good to know.

I was hooked up to water, just forgot to turn the spigot back on. :facepalm: I need to add a placard right above the ignition switch.

 

[natemoore]

Re: How long does it take to fry an impeller? Thirty seconds!

At Merc school they show us impellers that have been run without water, for how long and at what engine speed. At 3000rpm, fully fried in 30 seconds!

What you may not be aware of when you run without water in the exhaust gases. The exhaust gases are at full temperature and they pass over, as well as the exhaust shutter, the water pocket cover in the bottom of the upper drive shaft housing. That little cover is plastic, and takes very little to distort. Once distorted it will then allow exhaust gas into the cooling water, leading to engine getting hot at higher RPM. So next time you're out on the water, just be aware you MAY have a melted water pocket cover. If you have, good luck. The bolts holding the cover in are 1/4" S/S and the ali housing corrodes them solid, and usually break off when you try to remove them.

Chris.......

You're good. When I was torquing down the bolts to 60 in-lbs (that's only 5 ft-lbs), the plastic cover collapsed under one of the bolts. The nut just kept turning and turning. Guess it got heat damaged. I ran up to my local Merc repair shop and they gave me a serviceable used one.

 

[achris]

Re: How long does it take to fry an impeller? Thirty seconds!

You're good. When I was torquing down the bolts to 60 in-lbs (that's only 5 ft-lbs), the plastic cover collapsed under one of the bolts. The nut just kept turning and turning. Guess it got heat damaged. I ran up to my local Merc repair shop and they gave me a serviceable used one.

I'm not referring to the water pump impeller housing, I'm referring to the plastic water pocket cover on the lower side of the upper housing.... most people don't even know it exists...

Chris......

 

[natemoore]

Re: How long does it take to fry an impeller? Thirty seconds!

I'm not referring to the water pump impeller housing, I'm referring to the plastic water pocket cover on the lower side of the upper housing.... most people don't even know it exists...

Chris......

Oh, that. I did look at it. Nothing jumped out at me. Luckily I replaced that when I did the rebuild two years ago, so those bolts should come out okay, again. Thanks for the heads up, though.

 

[joewithaboat]

Re: How long does it take to fry an impeller? Thirty seconds!

good catch achris, great info.... fileing away in the the data banks

 

[Don S]

Re: How long does it take to fry an impeller? Thirty seconds!

Just thought I would post this from a 1987 service bulletin.

 

[joewithaboat]

Re: How long does it take to fry an impeller? Thirty seconds!

Wow! They should put that in the front page of the owners manual.
I see impatient folks at the marina waiting to launch, warming their boats up still on the trailer (w/alphas). I bet they have a bad day after that.

Sometimes i see the owners of the marina lined up at the window, on stools, drink in hand, just watching and Laughing at folks. Good stuff!

 

[achris]

Re: How long does it take to fry an impeller? Thirty seconds!

Wow! They should put that in the front page of the owners manual.
I see impatient folks at the marina waiting to launch, warming their boats up still on the trailer (w/alphas). I bet they have a bad day after that.

Sometimes i see the owners of the marina lined up at the window, on stools, drink in hand, just watching and Laughing at folks. Good stuff!

And now you can be one of them...