Back Flow Problems

[AQUA-HOLICS]

My boat has a problem with back flowing water to the cylinders from the exhaust. The boat is a 1980 Howard Custom Cruiser. The motor is a Chevy 350 with twin turbos. The exhaust system consist of esco logs with a turbo mounted on the end. From the end of the turbo it makes a 90 degree turn strait down 12" to a second 90 degree turn and out the transom (4" below the water line ). The exhaust tips have both inside and out side flaps.The water level is about 6" below the turbos when the motor is off. Even on flat water with no waves the problem will still occur. Thanks in advance for any help, Ray.

[jkh04200]

what is the lobe seperation of the cam?

[AQUA-HOLICS]

Unknown of that info on the cam, the boat was purchased used 5 years ago but I was told that it had an after market cam. One theory I have been told is that the cam lobe is big enough that it allows the valves to be open and the pistons are creating a vacuum on the down stroke and thus sucking water backwards through the exhaust ( when the motor is being turned off ). How does that sound to you? If that's the case would I only need to break the suction with in the exhaust tubes? Thanks for the reply, I am realy lost at this point of what to do! Ray.

[BDMar]

How long has this engine combo been in the boat? Has it always had a reversion problem? I doubt it is the camshaft unless it is totally wrong. They are usually a fairly wide lobe seperation and not that big for a pleasure boat application. I would look into a leak at the end of the exhaust manifold where the riser would normally go. Also, the 90 deg exhaust elbow mixes the water very close to the turbo, look into a problem there.
Don't look at the cam until you have checked the water system.

[AQUA-HOLICS]

The motor combo has been there for quite some time ( I have owned the boat for 5 years ) . The reversal problem has been there the entire time, it seems to happen only once a trip ( in 3 to 4 days on the water ) or not all. Yes the water is introduced close to the turbos but it strait downwards in direction when introduced. I have had the exhaust logs pressure tested and they checked out O.K. Another suggestion was that the water cooled air tubes that supply the intake manifolds on the pressure side of the turbos could possible be leaking water. Would this not create the problem more often than what I experience? Thanks for your comments, every little bit helps, Ray.

[texas-19]

If i understand you right,their is not much chance to get water back through the hot side of the turbo,you would have turbo problems.Hot turbine wheel and cold water,not good.
If the air tubes your talking about is an intercooler,like someone else said,check it out and get it pressure tested.Maybe too much water pressure on it?cracked tube?
Why do you think your getting water in the cylinders?
The engine can't pull water backwards when it is shut off.
With that turbo set up you probobly don't have alot of overlap anyway.

[jkh04200]

How much water are you getting? How do you know its there? If its been that way for 5 yrs I wouldn't worry to much about it. Reversion is not lake water. It is coolant water

[BDMar]

The water can go back through the exhaust side of the turbo after the engine is shut off. If it is as intermittent as you say it is not the camshaft. A hot engine will heat sink the water up the exhaust after it is shut off if the turbos are not high enough above the water line. If the engine rotates backward just a slight amount as you shut it off and the exhaust valve closest to the turbo is open it will be a very large water pump pulling water up through the exhaust.

[AQUA-HOLICS]

I will be checking the innercooled tubes for a water leak, The motor cylinders will get so much water in them it wont turn over (hydraulic). I remove the plugs and a strait stream of water will come out of some of the cylinders. BDMar: If the motor runs backwards (even for a secound ) do you think I can break the suction on the exhaust above the water line so when it trys to pull water, it would pull air insted? Ray.