gripit

I was woundering what the garden hose water pressure should be when flushing the engine so there would be no trouble with reversion. Also, am I correct that the mercruiser service bulletin sayes to fun engines at 1300 rpm during flush?
The engines in this case are BBC. Thanks

articfriends

iTrader: (1)

until yesterday i never heard of having too much pressure from your hose. I have a hard time getting enough water from the hose to even run boat. Most city water is around 40-45psi but once it expands into outdrive it's no where near that because of the volume its trying to fill/provide,Smitty

gripit

I never heard about to much water pressure either until yesterday, but now I'm concerned about reversion.

US1 Fountain

iTrader: (1)

How do you get reversion from a garden hose?
A motor running on the hose won't pull as much water as when it is in the lake.

Whenever I have ran on the hose, water just pours out past the muffs until the motor starts, then the motor will take all the water the hose will supply. (City water)
Use all types of muffs, the U-shaped, the ones wiht the pin thru the side pickups, and now the low water pickup. All same results.

hillbilly24

Yes it can happen. I brought iy up in another thread and someothers don't seem to believe in the theory, but it can happen. I have experienced it 3 times and one of them was with a brand new reman from Merc. Had just finished install and was running for a while while tuneing and leak checking. After 20 or 30 minuits of ideling, the oil was milkshake. I called Merc and said motor was no good and needed another one under warranty of course. They asked me for a play by play of what went on. Thi was the first time I had heard of the situation and i diddn't believe it myself but they insisted that i thourouly flush the crankcase and clyinders and retest with motor over 1200 RPM. No more problems. They say that boats with level through transom exhuast as oppossed to those with a dwonsloap to them are most affected. The seawater pump PSI should be 2-5 PSI at idle and no more then 30 at WOT. At idle the exhuast pulses are to far apart to prevent reversion if water pressure is to high. Thry additional pressure that a water hose provides as oppossed to sucking it out of a lake can be ebough to cuase reversion problems. You don't need to worry too much about it if your exhuast has a downward slope past the elbow and you keep the bow high when running. Hope this was informative to y'all.

US1 Fountain

iTrader: (1)

Hillbilly,
I don't see how a garden hose will push more water past the seawater pump impellor than what the impellor will pull. In order for more water to pass the impellor, it would have to blow past the vanes, but do to the natural twist in them, any water pressure on the inlet side of the impellor would actually force and seal the vanes tighter to the housing. Whenever I run on the hose, my WP is usually zero-a couple psi at idle. This is on city water with the faucet on full tilt.
Am I on a different wave lenght here?

hillbilly24

Depends alot on condition of impellor and HOUSING as well. On many boats if you turn the water on but don't start the engine, water will come out the ehxhuast tips. Raw water pumps are postitive displacement in a perfect world and when they are brand new. There are alot of other variables involded. My water pressure at house is 110PSI it aint legal but don't tell anyone We have regulators at the street that you aren't suppost to screw with but I couldn't resist. At the shop that I was working at when this insident occured the water pressure was 150PSI as it is in many commercial buildings. Also the angle of you pipes between elbow and tip has an effect. Bottom line is that it isn't very common but it can and does occur, and often enough for Merc to put out a service bulletin on it. I only suggested it in Dockrockers situation becuase the symptoms seem to fit, Milkshake after ideling on hose with valve all the way open, and from his pictures his exhuast does not appear to have much if any downslope.

gripit

So, whats the ideal PSI on the garden hose for flushing with engine at 1200 -1300 RPM on a system with hose connected to engine not transom pickup? Or it dosn't matter as long as engine is 1200-1300 rpm?

BadDog

I bet it has a lot to do with the slope of the boat exhaust system while on the trailer.

Badtoon

I read in my Merc Manual that the hose water valve to be open only 1/2 way and the 1200 to 1300 RPM for flushing. That's from there Service manual not a manual written by others.