Siphon issue during impeller change?

[motorboatdiaries]

Hi, attempting impeller change on Penta AQ205 with outdrive. Loosening the water pump cover plate bleeds water but the flow just keeps on going and intake hose remains firm which would indicate it remains full of water. I suspect it’s siphoning water up through the intake on the leg but no obvious anti-siphon valve I can see. There is a U-bend and join in the hose as it enters the engine compartment I could loosen to introduce air and break the siphon, but that seems a workaround. Question is: am I missing something obvious here, or perhaps is this an unconventional installation? Nothing on forums and websites indicate there is ever any need to release hoses when changing the impeller. Any pointers much appreciated, thanks.

 

[Laysula]

Close the seacock.

 

[stelican]

I have used the method you described remove hose at u bend.

 

[Elessar]

Close the seacock.

Outdrive !

 

[spannerman]

This is quite normal for outdrive engines as the pump is often near or below the waterline hence inflow of water. The easiest solution is to just bend the hose to the pump upwards or bend it flat with a tie wrap for the few moments you need to pull off the old impellor and fit a new one, or stuff a bung into it.

 

[ChromeDome]

Locking pliers / Vice Grip is your friend.

 

[motorboatdiaries]

Thanks everyone. So it looks like the answer is to play with the inlet hosing to either crack the siphon or block the end. Makes sense, but a couple of take-aways: 1) I don't think I've ever seen this referenced in any instructions, videos or forums, everything I've seen starts with addressing the faceplate and focuses on the challenges of pulling out & replacing the impeller itself, no mention of isolating the inlet feed or taking other such precautions, and 2) without taking such precautions there would appear to be a risk of significant water flow into the boat if the faceplate was removed and the water allowed to bleed continuously. I'll return later next week to finish the job, thanks again for the feedback and tips, more boating lessons learned, and no doubt many more to come...

 

[ChromeDome]

The logic behind it is that if you open anything connected to the water outside the boat, located below the waterline, water should be expected to come in.

Sort of the nature of water

 

[Bilgediver]

I drive an outdrive boat occasionally and thought this problem could be dealt with by lifting the prop clear. Is this not the case?

 

[ChromeDome]

Not really. The water intake is so low on the drive that you'd have to tilt it way above the transom plate, to get the intake out of the water.



Early OMC drives were capable of 75 dgr. tilting but I'd be surprised if any are still around (OMC went under 20 years ago)

 

[stelican]

Thanks everyone. So it looks like the answer is to play with the inlet hosing to either crack the siphon or block the end. Makes sense, but a couple of take-aways: 1) I don't think I've ever seen this referenced in any instructions, videos or forums, everything I've seen starts with addressing the faceplate and focuses on the challenges of pulling out & replacing the impeller itself, no mention of isolating the inlet feed or taking other such precautions, and 2) without taking such precautions there would appear to be a risk of significant water flow into the boat if the faceplate was removed and the water allowed to bleed continuously. I'll return later next week to finish the job, thanks again for the feedback and tips, more boating lessons learned, and no doubt many more to come...

No problem if boat is out of water!