1991 Mercruiser Bravo 1 Raw Water Blockage

[dorchard]

Hi All,

I have a 1991 Mariah 2000Z (open bow) with a 7.4L Mercruiser Bravo 1. We have had one successful boating trip this year, but when running on the trailer for another checkout (with muffs on, of course), no water was making it out of the engine. The first place I have checked is the hose coming from the lower unit to the water pump (merc bravos have a belt driven pump). Water will flow from this hose (by the water pump) out of the intake in the lower unit, but when I hook up the muffs and try to see water coming in, nothing...it seems that there is some blockage between the lower unit and the hose, but that it only blocks flow in one direction...

Any troubleshooting ideas? I would prefer not to have to drop the lower unit if at all possible, but it is looking like that is where the problem lies...

I haven't gotten my service manual in the mail yet, but I think i have seen that the bravos don't have a rubber impeller in the lower unit...is this the case? What else could have blocked the water flow?


Thanks for your help!

Dan

[Reel Poor]

There is no impeller in the drive. There is nothing else I can think of that would stop pressurized water flow in only one direction.

One thing does comes to mind. Does it have one of these installed in the hose between the transom assembly and the raw water pump. If so it may be installed backwards.

[dorchard]

Thanks for your reply...i take it that image is of a check valve...that was the first thing that my wife thought of...unfortunatly...unless it is in the outdrive somewhere, there isn't any equipment like that. I disconnected the hose from the inside of the outdrive, so only components in the outdrive are in play.

The really odd thing is that this whole thing was working fine on our initial test fire of the year, and for our first day of boating...just stopped working when I tried to check it out again..


Thanks again,

Dan

[HT32BSX115]

There's really nothing in the drive. other than debris plugging up the pickup ares of the drive...You could have a small hole somewhere between the pick up and the pump that is causing a vacuum leak and subsequently sucking air....... The raw water pump would cavitate due to this.

The other problem could be debris from a previous impeller failure that usually gets stuck in the oil cooler heat exchanger downstream of the raw water pump....Depending on how the pieces plug....they could allow some water to pass and then move around a little and then completely plug the system.





Regards,


Rick

[Reel Poor]

You can have a look here since you don't have a manual yet.

http://www.boatfix.com/merc/Servmanl/11/11covr5x.pdf

[dorchard]

Does anyone know any specific place in the outdrive to look for such a blockage? Also, if i pull the lower unit, do i need any new set of gaskets or anything, or can i just put everything back together?

Thanks,

Dan

[Reel Poor]

ALWAYS replace seals and gaskets before reassembly.

[dorchard]

I called a marine service shop on this and they told me "its a raw water pump issue", and regardless of the troubleshooting info that I gave them, they insisted that this must be the problem. Am I missing something here, or is that guy just not getting it? If I have disconnected the input hose from the water pump, and simply tried to see free water flow inbound from a pressurized source...how can it be a component that isn't even hooked up at that time causing the issue? Is there anything in the Bravo I outdrive that would respond differently to pressure from a hose than from the suction of the water pump?

Thanks again,

Dan

[HT32BSX115]

Did you say that you disconnected the hose from the transom to the pump and you cannot get water to flow from the muffs thru the drive to the pump?

You should be able to connect a water hose to the above hose and force water out thru the drive backwards. It would come out thru small 'water Pickup' holes in the lower unit.

The only thing I can think of is that a colony of killer bees built a nest in your drive plugging it completely

[dorchard]

You are correct interpretting what I have tried for troubleshooting. The water will flow from the transom, back out of the intake, but not from the intake in to the bilge area.

I will have to check with my local biologist as to the likelyhood of a killer bee--outdrive infestation ;-)

--Dan

[HT32BSX115]

Ok....You got a hose from the transom to the raw water pump. Have you tried disconnecting it and running water from the pump end out thru the drive?

[dorchard]

Yes...water will run from the pump-hose back out through the drive...but not in the reverse direction.

Additionally, as another troubleshooting method, i placed a garden hose into the raw water pump's intake, and ran the engine...everything worked properly, so I am now confident that this issue must be in the outdrive somewhere. This is a bit worrying, because with no moving parts (that I am aware of) between the intake on the lower unit, and the hose connector on the bilge side of the transom, I don't know what could be blocking water flow this completely....I wouldn't imagine that any debris from the lake would clog this so bad..

Thanks,

Dan

[HT32BSX115]

I have a bravo hanging on a hoist in the shop.

I cannot imagine how something would do that unless it got sucked up into it...like a platic bag etc....I cannot think of anything in there that would do that......It would have to go in thru the holes......pretty unlikely but if you can run water backwards thru the drive then there must be something in there plugging it up.....possibly a piece of gasket material that was dropped in there from long ago and it's just now moved in such a way like a flap.

It looks like you're gonna have to pull the drive and/or drop the lower.


When you do ......take some digital pictures!

[dorchard]

Hi All,

A followup--i broke down and took this to a marine shop, because I don't have a good place to be pulling an outdrive apart right now. The marine mechanic said that he pulled the whole drive, pulled it in half, checked in both halves, and there was nothing in there. He did say that the raw water pump was cracked--so that is getting replaced. He said it looked like it froze and cracked...so i will be bringing that to the attention of the (different) marine shop who winterized my boat last year! I won't be bringing it there for winterization any more!

One thing that makes no sense to me though--water wouldn't come into the bilge through the disconnected intake opening on the inside of the outdrive--the mechanic said this was probably just because there wasn't enough pressure to do this...but the muffs had full pressure, and were squirting everywhere on the outdrive, so I can't imagine how that could be the case...and when I put the garden hose into the raw water pump's intake, the whole boat ran fine...and the mechanic has said that the hose had enough pressure to run water through the whole system...this seems almost like a contradiction to me...enough pressure to run through an entire engine, but not enough to rise 1 foot through an open passageway in the outdrive?

I am just trying to figure out what I did wrong or misunderstood here...I would like to learn from my mistakes. If anyone has any insight, please clue me in!

Thanks,

Dan

[paulrfrancisco]

Convert the "through the drive" path to a "through the hull" path... Pretty simple installation if the boat is out of the water...Just need the through hull fitting and a shutoff valve and a different hose... as long as you're changing the pump maybe now is the time.... Then you'll not have to worry about this again.

This is the way my boat is setup.....

[NateZ]

Ever find out exactly what the problem was? I'm having the exact same problem on my boat right now.

Nate