Water backflow in VP gasoline V8 sterndrives

[MapisM]

I've been told during a sea trial that in pre-2000 versions of these engines/sterndrives (5.0 GL and 5.7 GL) there is a risk of water flowing back from the prop, up through the risers, and finally inside the cylinders, when throttling back rapidly (as in an emergency stop).

Apparently, this is due to the fact that:
a) the pressure from exhausts gas is suddenly missing when throttling back (and that is normal of course);
b) the flap meant to avoid the water backflow has been modified afterwards (around 2000), to be actively controlled and closed immediately upon loss of exhaust pressure, whilst in previous engines it was passive and only worked upon backwater flow pressure (thus reacting with some delay, giving a chance for some water to flow back into the cylinders while the boat stern "sits" deep in the water upon deceleration).
The only workaround to avoid such risk is to avoid sudden throttle releases, but obviously there migh be occasions when that's actually necessary.

I never heard of such problem before.
Can anyone confirm, or let me know if by chance I was told porkies (though the person reporting that seemed to know what he was talking about)?
Ta!

 

[enterprise]

Yes, I have heard of this in mercruiser before but it covers all of their range of sterndrives.
There is a one way flap system installed in the risers, it's there to stop the water flooding the cylinders in an emergency stop situation or while reversing.

If rapid deceleration is required I was always told to put the boat hard over to starboard so as to stop the sternwave slaming up against the transom.
Another problem is when droping passengers off at a beach etc with the engine off, if you are stern on to waves then this might also cause water to be pushed up the exhaust.

I suppose the same could be said for Volvo penta.

 

[andy59]

I had a 5.7 mercruiser in 2050 Bayliner and had misfiring problems , when we stripped it the flaps in the exhaust were completely gone , (corrosion) and some water managed to get as far as rear port side cylinderwhen slowing (waterskiing) . Previous owner said all was okay but...... So IMHO it can happen and those flaps seem very fragile for the life they lead (mercruiser) but not sure about volvo but i think they are similar design .

 

[tinkicker0]

Yep, I replaced my flap last year (Mercruiser), the old one was hinged and stuck in the open position.

The new one was moulded rubber and was self closing.

The main culprit for these failing is if the engine overheats due to a bad impellor not supplying enough cooling water or a rust clogged water injection elbow. This will rapidly destroy the flapper as well as the riser connecting hoses.

 

[MapisM]

[ QUOTE ]
If rapid deceleration is required I was always told to put the boat hard over to starboard

[/ QUOTE ]Thanks everybody, there's always something to learn...

Just curious about the above statement, Enterprise: why stbd?
I guess you're talking about a single screw boat with right hand prop, but if you need to avoid a MOB on your stbd, hopefully you won't steer toward him anyway... /forums/images/graemlins/smile.gif

 

[enterprise]

I cant remember the reason for Stbd, perhaps it was our instructors preffered side but your right, it wouldn't make any difference.
Just a word of caution, always look behind before you do make a drastic turn in either direction. /forums/images/graemlins/ooo.gif

 

[MapisM]

Well, actually it does make some difference on single screw boats - that's why I mentioned it. Hard over to port would be stronger and tighter than to stbd. In this respect, stbd is less dangerous.

Good point re.looking behind, though probably if you're forced to make such maneuver you'll hardly have the time for anything else...

 

[phockit]

I wonder how much of the perceived problem is due to folks not following the maintence schedule - on Mercruisers you are meant to have the manifolds removed and checked/replaced every five years, which is how you get to check/maintain the backwash valve as well. I'd imagine there is something similar in the Volvo schedule.

Paul

 

[AndieMac]

The back-flooding flaps are mounted in the top of each side of the Y pipe, which is below the elbow and rubber pipes leading aft of the risor.
They are definitely part of your maintainance programe. With the early models, the hinge pins sat inside a rubber bush mounted in the walls of the Y pipe (V8's). Of course over years of heat and salt water, the rubber bush would fail, and needed replacement. The inside walls of the Y pipe can accumulate crusty salt deposits, over time, which make normal operation of the flappers, restricted.

The later models are of course better, with an improved system, but will still need checking.

As far as I know all sterndrive configurations would have a similar system, whether its diesel or petrol. No doubt one of the mechanics can confirm or deny this.

If you like to do your own maintainance, its a very simple process to remove the rubber pipes and steel elbow to inspect the condition of the flappers. Just run a screwdriver around inside the rubber pipe where it joins the elbow, to release it.

 

[AndieMac]

The references above are made about Mercruiser, I would imagine VP would have a similar arrangement.