Heat Exchangers


Jul 22, 2023
Monterey Bay, California
Boat Info
SeaRay 340DA 1999
Twin mercs

Thanks everyone who helped me to understand proper salt water cooling methods.

We have installed heat exchangers on my SeaRay 340 DA, 1999 with twin MerCruiser 7.5 MPI engines.

We flushed both engines with salt away and then rinsed it thoroughly. Before adding antifreeze, I did test run using fresh water as coolant. Before test run I fill both coolant expansion tanks to lower level mark. I took the boat out and run it on 1000, 2000, 3000, 3500 and 4000 RPM for total of 30 minutes.

Now have few questions:

1 On low RPM temperature was stable and hold at 170 Degree Fahrenheit. At the moment when I raise RPM to 2k-3k and above, temperature moved up to 185 for ~~15-20 seconds but then get back to 170. Is that normal?
NOTE: I used dashboard temperature gauges to read temperature, it not very precise and it hard to say exact temperature value when it climbing above 170, but I noticed that gauge arrow moved right for about 2-3mm and then return back to 170.

2 Right after I return back home I noticed that Extension Tanks are about 1.5 inch below the level I filled them. I thought on hot engines it should raise to expansion tank to upper mark? What does it means and what should I do?

3 I noticed that newly installed hose which connect fresh water to riser is touching the riser - is it ok to hose to be touching the riser? Will it have an abrasion over time at this point and should it be reinstalled ?
Sorry for bothering ppl. I have this sorted, I hope.
1 - is ok
2 - leaks, have it fixed and to be tested more.
3 - will put and zip tie bigger hose around working hose where it is touching to protect.

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