It’s already been said, High AFR’s will make high EGT’s and cause high oil temps. That Hardin cooler keeps my 540 under 230* and I have closed cooling. Good luck and safe boating

Yeah just took a measurement on mine , the cooler is 3x16 and that’s from threads to threads so actually cooler tube is only 12 inches. Now that I’m typing this and reading it out loud that seems way to small, for what the motor is putting out. Even with all thermostats removed.

That would be the issue. definitely needs a larger oil cooler.

3 things come to mind before you condemn the oil cooler/lines as being too small
1. Is gauge actually correct, you can shoot a infrared gun on dark areas on pan, cooler, fittings and see IF its close to correct, surface temps will read 20 to 40 less than actual
2. IF a high volume pump is used and a std bypass valve was put in block by accident, its possible to have it open and bypass hot, unflitered oil to your motor
3. as mentioned, if your running too much timing or lean afrs, alot of heat will be created and transferred to the oil

Are these coolers just more efficient than the stock Mercruiser stuff, or are these being used as a secondary cooler? I replaced one of the factory coolers a couple years ago, do to a crack over the Winter. Was nota Merc brand, but looked just like it. Does the same thing as others here, run it hard for a few miles and the oil temp heats up. New sea pump impellers do not make any difference.
BTW, my oil temp sensors are on the remote filter mount.

No to both
Your motor is producing more heat due to the increase in hp. The old cooler was designed to meet the minimum needs to meet the original motor build. Your new motor probably has a high volume oil pump (do verify and see if bypass valve has been plugged) and therefore it needs larger oil lines and cooler.

If you look thru my thread in previous post it will show what I went thru to get my heat down. You may not need to do all of what I did, and may need different things, but its one look at the issue and the process

Please excuse this question, but are you referring to the bypass that would be in the adapter or in the remote filter housing? I run a Wix 51649 filter without a bypass in the filter, if that changes things?

The valve is located on the block where the filter would attach or where the remote oil filter attaches. Mine was removed and plugged

https://www.offshoreonly.com/forums/...safe-oil-2.jpg

my motors are 547"s...
my oil coolers wont open till 210 temp,
posted num,ber for them,,,

Before you buy anything shoot it with a temp gun.