Why do I say your cooler is too small? Well, for starters, some of us prefer to have an oil system that cruises with oil temps in the 190-200 range, and maybe get to 210-220 when holding it at 6000RPM for several minutes at a time. With no oil thermstat, and cruise oil temps of 220-240 degrees, your clearly taxing your oil cooler. Hold it WOT for a couple minutes, and you'll probably start cooking the bearings out of it. But hey, if you are into cruising along at 3500 rpm , and can care less about wot oil temps on hard runs, by all means, its your boat.

What Eddie Marine said about keeping pressure up in the exhaust, is a valid statement. This ensures that the risers/tails are FULL of water, even at idle. We get that reducing flow, made your oil temp hotter. I agree that increasing flow, will remove more heat. What would solve both your issues, would be a larger cooler. This way your tails would stay cool at idle, and your oil/heat exchanger, would remove more heat per gallon of water flow.

Just because water boils at 212, doesnt mean if it doesnt get to 212, you will forever have moisture in your oil. Theres a million stock black merc motors that rarely get over 170 deg oil temps, and dont have issues for thousands of hours. Get a soup pan, put a cup of water in it. Heat it to 170 degrees on the stove. You will see, that while it may not "boil", the water will still be steaming, and eventually evaporate. Condesation in oil can be an issue, especially if you run in 65 degree water, have 95 deg water temps in the block, and 180 degree oil. Its just like when you put an ice cold 35 degree beer out on a table in a 100 degree day. But, an 80 degree beer can, on 100 degree day, won't condensate.

I asked what your ignition setup is, because low timing at idle, can make the exhaust really hot at idle.

You came here with a question, pretty much told everyone they were wrong, and now are teaching Eddie Young about cooling systems. The man has probably rigged more big power offshore boats than anyone on this site.The ideal setup, is a cooling system that keeps all part temperatures in line, and an oil system that keeps oil temperatures in line. Theres a reason Mercury racing used an oil thermostat setup on the HP500, along with a larger oil cooler than the stock 502 mag oil cooler, with no oil thermostat. The oil thermostat elements begin to open at 180-190 deg, and are fully open by 210. If you have a merc Hp500, 525EFI, or pretty much any merc racing engine, and see 240 degree temps at only 3500RPM, something is WRONG. Matter of fact, the smartcraft alarm on the 525 will be buzzing in your ear.

If you have hot spots in your tailpipes at idle, which can really get things hot, and then give it throttle, throwing cold water on hot stainless, eventually it could split a weld, and send water into your engine. This was a common problem with stainless headers over the years, and a reason why many of them have ruined engines over the years.

Either way, sounds like you got it figured out. Good luck.

Ya, I was just putting some tech behind your findings.

I understand that completely. I never said anything about increasing or decreasing water flow. I was only talking about the direction of flow of oil and water and that we haven't seen a measureable difference between having the oil and water flow the same direction or opposite of each other.

By reading temps before the cooler, we are only guessing. What is the temp of the oil after the cooler.....the oil going into the engine? By reading it before the cooler, you are taking the cooler out of the equation. For example, lets say the oil before the cooler is 250*. If you have a 2 x 8 cooler, then the oil going into the engine may be 220* and if you have a 3 x 18 cooler, the oil may be 180*. That's a huge difference. One is a problem and the other is not. IIRC, you have about a 3 x 12 bundle for the oil cooler (longer overall but has P/S cooler also). That cooler should remove at least 50* (with enough water) when the oil is over 200*. If your oil before the cooler is only getting to 240-250 at WOT, then you don't have a problem. It's not uncommon for engines to see 230 going into the engine. This is for higher HP engines at or near WOT, of course. How hot will it get? That's my question.
Eddie

Thanks for the explanation. So my filter should be after the cooler as previously posted so I can read the oil temp after being cooled. Interesting, for now I am going to check the accuracy on my gauges and senders with a infrared thermometer. Cruising @ 32-3400 rpms it shows on port side 225-230 Starboard side is cooler.
215-220. I never saw my gauge go past 240-250 on a WOT run. Maybe i'll swap the senders. Oil lines are going with the flow on both engines. I don't know what the water temp is now on the river.I'm sure its a least 75-80 cooler would be better. I'm going to call Eddie Marine and ask if I can dump some water without hurting the risers.

It's not only the cooler, the stock merc lines, fittings, block/filter adapters are junk, very restrictive, upgrading all to run #10 lines is the way to go, I use a eddie marine cooler that works great and is priced well, also added a canton thermostat as well, as with my procharger crossover oil temps were always very low, there is a very in depth thread on here discussing the stock oiling system...

I asked what your ignition setup is, because low timing at idle, can make the exhaust really hot at idle. And I answered you several posts back 34deg. Locked

You came here with a question, pretty much told everyone they were wrong, and now are teaching Eddie Young about cooling systems. Sorry you feel that way BUT No i did not say everyone was wrong and I was not teaching Eddie about cooling systems. We were talking about direction of flow.

Good point Thanks

Here is my cooler http://bpi.ebasicpower.com/p/HEXMER817577T

You seem concerned about your water psi being too high at 15-16 lbs. water pressure is a good thing in my opinion. My boat has over 30lbs at wot and never had a problem with hoses or gaskets. With a oil thermostat my oil stays between 190 and 220. Stock merc setup.

The benefit to higher water pressure is that it increases the boiling temperature of water.