closing engine water intake
 

I know that if your A4 does not start before you crank your engine repeatedly, you should close your water intake. My understanding is that if you do not do this water can be forced into your engine.

I'd like to understand how this happens. I was trying to explain this to a fellow on the dock the other day and could not fully explain (or at all) the mechanics behind how this happens.

Thanks!

Exhaust gases blow the water out of the waterlift and overboard. Some installations will flood the engine if the engine is cranking but not running. The much lower volume of gases exiting the engine when it is not firing may not be enough to do the trick. I did a generator installation once where the entire genset was below the water line. We put a drain valve in the waterlift you could open when troubleshooting starting issues. No reason you could not do this for an A4.

It's a weakness of a waterlift exhaust system, the risk does not exist with straight pipe, water jacketed or standpipe systems. Waterlifts being IMO the most common exhaust system, the risk is real.

The way a waterlift system works is water injected into the exhaust line is pooled in a container and exhaust pressure pushes it up and out in batches, sort of like a high volume percolator. Where the system fails is when there's no exhaust pressure such as during no-start cranking but the raw water pump continues to deliver water. In time, depending on the volume of the waterlift and rise of the hot section it may be a very short time, water will fill the waterlift and hot section backing up into the engine through open exhaust valves.

Further, as the water builds it exerts greater exhaust backpressure making starting even more problematic.

Thanks Neil, that explains it very clearly. I'll share that wisdom with my friend on the dock that thought I was crazy :)

& full illustrations! Thanks!!!

You are not crazy. I have hauled a diesel off a boat that had bent and broken rods for just this reason.

I picked up a free seized M25 diesel from a Catalina 30 that had ingested water. The timing was perfect, I had just started the rebuild of my spare A-4 (also free) and the parts sales from the M25 fully funded the A-4 project, like about $1K so net expense was $0.00 and I ended up with a rebuilt A-4.

Even though the Universal M25 is a Kubota based engine there are parts no longer available, the flywheel housing for example. The alternator bracket attached to an unsupported and vulnerable spot on the housing and all but guaranteed a breakage. There is a redesigned bracket to remedy this but once the housing is broken the engine is a dead duck, running or not. That housing and the Hurth transmission commanded the highest prices. I wound up selling everything off that engine down to the short block which I hauled to the dump.

Your sketches are great, but you need an additional one. That's the one with the waterlift outlet full of water. That full outlet pipe acts like a potato in the tail pipe and the engine won't start.