Cleaning and Maintaining the 5.7 Cooling System?

[BrodyO]

I've got a 96 Larson 194 SEI that I'm currently doing headgaskets on. My headgaskets blew out in the same spot on both sides, between the middle two cylinders. I've always kept a close eye on my temperature gauge which, to my knowledge, has never gotten above 175 when running. I want to make sure the engine doesn't get too hot before running it again so I'd like to know the steps in cleaning and maintaining the cooling system on one of these. So far I drained the water jackets to make sure it wasn't full of sand or anything, I got a little bit of rust to come out but otherwise it was very clean. I ordered a new pump impeller to throw in, haven't inspected the current one yet. I plan to throw a new thermostat in since it's apart anyways. It's probably worth noting that this boat DOES NOT run in saltwater, freshwater only. So I'd like to know is there some type of screen for the incoming water that could be partially plugged? Is it worth running the engine in a garbage pale with fresh water and some time of radiator flush? I don't have any reason to believe that the pumping system isn't working properly but I want to ensure that it is. If anyone else has theories on why the headgaskets blew the way they did I'd love to hear them. I am of course having the heads crack checked, valve job and resurfaced as well as checking the block for straightness and cracks. Thanks!

 

[Lou C]

If you blew 2 head gaskets only running at 175* something else is up because that engine in an auto application with a pressurized cooling system can run at a steady 195--210* without problems for thousands of miles. Either there was a clog in that area of the cyl heads or block that restricted flow, or other problems caused those cyls to run hot such as:
1) excessively lean fuel mixture (what did plugs look like, bleached white?)
2) over-advanced ignition timing (should check base and advance timing)
3) Boat was over propped, with too high a prop pitch. This will be evident if the engine never reaches full max rpm while wide open (ie 4600-4800 rpm). This will cause the engine to really labor when pulling the boat up on plane and will cause localized overheating of the combustion chambers, and damage to exhaust valves due to them overheating even if the engine temp is only 175.

So not only do you need to really make sure the heads are clean, not cracked and resurfaced but a valve job may be needed because the valves could be tuliped in those cyls. The middle cyls sometimes run hot on a GM small block V8 due to the exhaust ports being right next to each other. The combination of lean fuel mix and over-propping is a recipe for disaster on a marine inboard. And if you are putting this back together yourself be prepared to spend a LOT of time, cleaning the block deck surface, cleaning out bolt holes, etc. Take the intake manifold to the machine shop also and have them check the sealing surfaces and re-surface as needed.

PS some V/P outdrives used a screen on the water intake but if you had cooling system restrictions it would have been plainly obvious due to overheating right away. That is not likely your problem but the other things I listed probably are. Have you owned it since new? Was it ever overheated before you owned it? Anything is possible. One thing for sure, you must make sure that:
fuel mix is not too lean
timing is right on
boat is propped right

these three things are most likely the root of your problem, unless it was severely overheated in the past. One bad overheat can weaken the HGs and then they may blow right away, or even a few seasons later. Happened to mine back in '16 from an overheat in '13

 

[BrodyO]

Well I think you might be onto something. I haven't gone through the carb but it seems to be good, I pulled the plugs out before major compression issues and they were all golden brown, no black or shiny specs so I believe it's good there. The boat does not get up to the proper WOT rpm's usually 4100-4200 at 42-45MPH. I actually got a new pitch prop in the day after the HG failure that should've fixed that but I suspect that was my main problem. I ordered a new water prop and 160* thermostat for it when it all goes back together. I verified timing with a timing light before I had any problems and it was spot on.

 

[BrodyO]

Only owned the boat since March, put about 20 hours on it before the failure. Previous owners told me they never went over 3000-3500RPM and that may be the reason they never had this failure or knew that the boat had the wrong prop. It's also entirely possible that it was overheated sometime before I owned it but I am unsure of that.