Impeller failure.

[nitsuj]

Recently bought a pre owned 1996 Mariah with a 7.4/Bravo 3. Previous owner said he recently replaced the impeller. I knew I should have replaced it myself, but I thought surely it was good for one test drive before I tore into it. I was wrong. I knew better, but I did it anyway. Kids, replace the impeller when you buy a preowned boat. I get the boat off the trailer, it seems to be idling fine, running great. As I sat at the dock waiting for my passengers, I kept an eye on the gauges. Oil pressure good, temp good. Everyone gets on and away we go. at about 100 yards of no wake speed, alarm buzzer sounds. Heat is rising. I shut it down for a while to let it cool some then limp back to the dock. Get the boat home and pull the impeller housing. It's toast. ALL the vanes are gone. And the cooling system is still full of antifreeze, presumably from when the previous owner winterized it. So apparently, when he ran it for me on the muffs, the impeller was already toast, other wise the anti freeze would have flushed out of there. Seriously, replace the impeller when you buy a boat. Even if the owner says he just did it. The seller of my boat is someone I know. He's honest as hell and wasn't hiding anything from me. And he took excellent care of the boat. My guess is one of two things happened. Most likely, he's getting to an age where things you did 8 years ago seem like things you "just did". Hell, I still refer to the house down the street as "The Edwards house" and it's occupants as the "new people". Old man Edwards died 20 years ago. Or almost as likely he ran it on muffs when he was winterizing it and didn't have good water flow. I make these assumptions because I assume if the impeller were fine when I put it in the water, the coolant the system was full of would have lubricated it enough that it wouldn't self destruct when I fired it up.

So, a few questions. One, what the best method for back flushing the system to find all the rubber impeller that must be in there some where. A few big chunks were in the housing, but there has to be more. I've seen references to back flushng it, but haven't seen and instructions, and haven't found anything on youtube. And 2, what should I check for signs that the engine was damaged from over heating? Nothing looks amiss, and and no point did it run poorly or stumble. And it was never above idle. I was moving it around by moving the shifter from forward to reverse, never applied throttle. I don't think I damaged anything but want to be sure. Oil is clean and honey colored, no sign of cloudiness or milkiness.

Thanks guys!

 

[Rick Stephens]

I’d pull the line from the transom off the thermostat and hook up there to flush back. Then pull thermostat. It’s really the choke point.

 

[nitsuj]

Thanks for the advice! I also need to figure out of I did any damage from the over heat. Hopefully someone will chime in.

 

[achris]

.... I also need to figure out of I did any damage from the over heat.....

You shut it off as soon as you heard the alarm. Likelihood of subsequent damage is low. The alarm switch settings on marine engines are lower than cars, as the general operating temperature is also lower. Hence the alarm will sound even if the engine hasn't reached a high enough temperature to blow/warp heads and head gaskets.

If you want piece of mind, do a compression test...

Chris........

 

[nitsuj]

You shut it off as soon as you heard the alarm.

I wish I could say I did. I shut it down when I heard the alarm, then fired it up a few minutes later to idle to the dock, with a few shut down on the way. It ran, at idle, for maybe 10 minutes total with the alarm buzzing.

If I killed this motor, forgive the pun, I'm sunk. Wife wasn't thrilled with the purchase to begin with. If I have to repower it, I'm going to need a more comfy couch!

 

[Rick Stephens]

One has to keep the admirals happy, no doubt. Fix the cooling system and do your compression checks. Then take her for a spin without anyone else on board. Give you time to run her around and think about what's happening. Unlikely you torched a boat motor at idle.... possible, but not likely.

 

[nitsuj]

One has to keep the admirals happy, no doubt. Fix the cooling system and do your compression checks. Then take her for a spin without anyone else on board. Give you time to run her around and think about what's happening. Unlikely you torched a boat motor at idle.... possible, but not likely.

I’m so glad you said that. It’s what I was thinking, and I’m glad I wasn’t being over optimistic. I’m going to go through the cooling system and look for impeller corpse. Going to pop the elbows off and have a quick look at the flappers, then replace the impeller and housing. Fingers crossed I can have it ready for water trial next weekend. I’m testing it in a tank this time, so I can rule the muffs out as the reason the impeller grenaded.

 

[thumpar]

Happened to my right after I got my current boat too. I pulled the hose off the impeller housing and ran the garden hose in it at the thermostat housing. All the pieces came out. It has been fine. I change mine every other year now and carry a spare.

 

[havoc_squad]

I’m testing it in a tank this time, so I can rule the muffs out as the reason the impeller grenaded.

Please, do NOT use a test tank on sterndrives for running outside of the water. Use appropriate flushing/muffs attachment with the faucet wide open and keep the motor within 1500 rpm or less.

If you fear/suspect your muffs water pressure being not enough at home, take it to a safe lake/river to test it.

If you want to grenade an impeller, running a sterndrive in attempting to use a test tank on it will do it.

I'm suprised no one caught this being said.

There are countless people who have reported cooling issues after running the boat with the outdrive in a test tub/bucket, only to find out they burnt up their impeller.

We just had an example of that happen around four weeks ago on the forums.

The tank water level has to come up to the boat's waterline with the drive in the fully down position (like it is in the water) for the water impeller to work, which is nearly impossible to do with sterndrives. So, it's not an option.

 

[nitsuj]

That’s a new one on me. Not disagreeing, but I think I’ve heard of other people reccomending a tank vs. muffs. Something about the bravo drawing enough water to collapse a hose. So, like if I filled a cut in half 55 gal plastic barrel with water, and lowered my drive into it, so the water level is above the cav plate, that wouldn’t be sufficient? I had this grand plan in my head of mounting a spigot to the barrel and leaving a running hose in the top so I could drain the warm water out as it’s replaced with new cool water. I don’t doubt you, but I’m shocked to learn this. Ok, plan b, I’ll use
muffs. Thanks for the heads up.

As an aside, I don’t understand why running an engine on land has to be so sketchy. The muffs have to be hooked up just so, then it has to draw water up the leg into the pump, with your fingers cross the whole time that water makes it to the impeller before it destructs. And no tell tale like the old drives and outboards had that would signify 100% that you had flow. I had an old OMC stinger drive that had a spot to screw a hose on and a tell tale on each side of the drive. You knew 100% that the impeller was wet before you hit the key. That was a great system. With this bravo and my Volvo before it, the amount of time it takes to verify water flow is about the same amount of time it takes to melt an impeller. While you wait to see water coming out, you don’t know if waters coming or a parts bill is coming. And when it’s in the water with no tell tales you have no idea what’s going on until it over heats. Why can’t they still drill that stupid little 1/8” hole somewhere?

 

[thumpar]

The problem with running in a tank is that the water level needs to be as high as the impeller. On bravos that is hard because the drive is so massive and the impeller is at the level of the bottom of the engine. With muffs the pressure of the hose helps push the water into the impeller. The bravo won’t collapse the hose but can use all the water it can give. I’ve run mine on muffs with no problems but I don’t rev it up.

 

[nitsuj]

That makes total sense. But I am curious about something (not arguing, trying to understand) when the boat is in the water, the water line doesn't seem like it would be above the impeller, as the impeller is on the engine. Next time I'm in the boat working on it, I'll have to look. Sure seems like the impeller housing is above where the water line would be, but maybe its mounted lower than I'm thinking. But maybe the forward motion of the boat forces water up the leg?

 

[thumpar]

It’s below the water line. That is why if you plan on doing an impeller while on the water you have to be fast to either plug the hose coming in or shove a longer tube in the hose to put up high.

 

[nitsuj]

Bravo 7.4 Impeller replacment

I have another thread going about my recent disappearing impeller woes. I wanted to start a new thread with a more narrow focus. Which impeller kit do people recommend? Any reason to pay double for OEM? Is it one of those "you get what you pay for things" or one of those "it doesn't matter how much you pay, they're all made in the same factory" things?

Also, my impeller had a paper type gasket between the wear plate and the pump. None of the sea water pump kits I can find show this gasket. Is this something that's been phased out, or am I looking at the wrong kit?

 

[nitsuj]

Probably no one cares or is following this, but I figured I’d document my progress on the off chance anyone ever searches out impeller failure, maybe some info will be helpful.

I backflushed the system and got out some of the impeller pieces, but not all of them. I’m going to have to take off some hoses and visually inspect. That’s tomorrow’s job.

I ordered an impeller and housing, riser gaskets and exhaust shutters. I popped the risers off tonight and had a look at the shutters. I ordered them thinking they were probably toast. I’m glad I did. They’re not disintegrated or missing but they’re all blistered and curled. They definitely wouldn’t function as they should. Once the new ones and gaskets come in, and I find the rest of the impeller shrapnel, I should be ready to button it up.

Added a pic of the shutter and impeller I’ve found.

 

[SDSeville]

Wow. I have never seen an impeller that toasted. I guess I have been lucky.

 

[thumpar]

That’s what mine looked like too.

 

[nitsuj]

Wow. I have never seen an impeller that toasted. I guess I have been lucky.

Unfortunately, it was running dry for a little while before it registered it wasn't pulling water. Luckily that seems to be the only damage. Other than the flappers, also pictured. All the other exhaust rubbers appear to be in good shape. The boat over heated a bit, but I got real lucky. I learned a few very valuable lessons. 1. Sea trial every boat you buy, even if you know the seller and he's honest. 2. ALWAYS replace the impeller in a new to you boat. Even if the seller just did it. 3. Always check for water flow in a new to you boat. The last 3 boats I've bought all had cooling issues. One clogged with scale, one clogged with sand, and this one, apparently clogged with impeller.