My experience with I/O engines in salt water led me to understand that they last about 15-20 years (maybe) and then rust through issues can catch up to you.  So, with the ‘new’ boat and its 2 Mercruiser 7.4L engines, I wanted to take some extra steps in protecting the engines (which are mostly iron) from salt water exposure.  This would be done by installing closed cooling systems on each of the engines.  I chose to get ‘FULL’ systems that cool both the engine block and the exhaust manifolds.  This would be a great setup, as the engines already had Stainless Steel exhaust elbows, so they would be highly resistant to the salt.  Also, isolating the iron exhaust manifolds from the stainless steel elbows would cut down on the galvanic corrosion that trashed the original manifolds.

I did some research on aftermarket closed cooling systems . . . the 2 prominent manufacturers are San Juan Engineering and SeaKamp.  I chose San Juan Engineering as they seemed to have a kit for my specific version of the Merc 7.4 (7.4 LX MPI).  After verifying with the factory that I was getting the correct kit, I ordered 2 kits through a reseller.  The Kits arrived about 10 days later.

The kits weighed about 60 lbs. each . . . Of course, I had to open them up and take a peek at the parts and instructions . . . most of the weight is in the heat exchanger.

I previewed the instructions and planned for a weekend of ‘fun’ installing the kits on both engines.  The weather was getting cold, so the pressure was on to get the closed cooling system (CCS) installed before I needed to winterize the engines.

There is a fair amount of work that needs to be done in the water pump area. I actually had to remove the water pump to get some of the fittings installed.  Here are some pictures of the installation process.  The instructions were a little bit vague in places, so I had to figure some things out on one of the engines and then apply that knowledge when proceeding with the other engine.

Once the fittings, etc. were installed in the water pump area, and the mounting bracket put in place, the heat exchanger was installed.  I also had to remove the exhaust elbows and place an isolating gasket between the elbow and the manifold.

Upon testing the port engine, I noticed that it ran a bit hotter than prior to having the CCS installed.  The thermostat provided with the kit was the same temperature, but it seemed to be more restrictive in its flow.  I was concerned that under load, the temperature would get too hot.  Part of the problem seemed to be that the exhaust manifolds would not get any flow of water until the thermostat opened, which would take a few minutes of running from a cold start.  That few minutes seemed to allow the manifolds to get fairly hot before the flow would start.  I wanted to make a modification to the system that would provide some flow to the manifolds during the warm-up period.

So, I took the original thermostats and drilled a couple of holes in the flange to provide some by-pass and tested them on the stove for accuracy, etc.  I installed these modified thermostats in the engines and ran another set of tests with much better results.

Each engine took about 6 gallons of coolant at a 50/50 mix. I am still trying to find a place to attach the reservoir bottles . . .

I installed the closed cooling systems just before the winter layup in 2012, so there were a few odds and ends to complete on the systems before I put the boat back into service in 2013.  Sea water strainers are a good idea with closed cooling systems and also help add longevity to the sea water pump impeller.  So, I installed sea water strainers on both engines. (pics below)

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The engines previously had a fresh water flush systems, and I found an ‘interesting’ alteration to the system.  (above left).  Nice use of a spark plug !?!?

The starboard engine fittings needed to be adapted a little bit, as I was not able to open up one of the spare fitting plugs on the recirculating pump.  So, I made a custom “T” fitting for the large recirculating hose  (pic left) to accommodate the needed plumbing alterations.

I was able to find a location to mount the overflow reservoirs, as it turns out that the side panels in the engine compartment are made for mounting components, etc. (pics right).

So, that pretty much finished off the closed cooling system installation.  It was a bit tricky at times, but hopefully they will work out fine.