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Oil temp high

Old 04-09-2020, 02:38 PM
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Default Oil temp high

Hello all.
new to owning big blocks.. had all 350s.
I recently bought a Sunsation with 502 bored out to 509 and dyno for 550 hp. I ran boat yesterday on lake st Clair and was seeing idle oil temp 230, 3000-3500 250-255 3500-4000 260-270. Water temp stayed steady at 160-170 degrees.

Called previous owner and he stated all oil temperatures is what he would see. When WOT I should see 300 degree.

Looking for guidance on if this normal range? Im using 20w50..

Last edited by scampbell90; 04-09-2020 at 02:42 PM.
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Old 04-09-2020, 03:16 PM
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That's very hot for my big blocks. In my experience anything over about 240 degrees starts to have an adverse effect. Oil should be around 200 at full temp and hard usage can see 220-230 but 250+ or even 300 is really hot. Oil needs to get hot enough to evaporate moisture and do its job, which occurs around 200-215 degrees. At 240+ it's going to break down and not be able to do its job.
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Old 04-09-2020, 03:26 PM
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thor Thanks for your reply. If temps are that high, could it indicate I need a replacement oil cooler? I personally do not feel comfortable running even 240..
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Old 04-09-2020, 03:31 PM
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We don't have a "performance boat" per but do have a 7.4 mpi- What you reported is a little warm for oil temps. Thor39 I believe is correct that at 240+ the oil most likely will start to break down. Years ago we had a 165 hp Straight 6 and was running it very hard on a VERY hot Labor Day - the engine temp stayed cool. All of a sudden the engine oil light came on and the oil pressure dropped. I immediately shut the engine down- the oil was full nothing out of the ordinary. Took the boat to a local marina and was expecting the worse - We ALWAYS ran 30W Strait Rotella in this engine from day 1- Turned out the oil broke down and couldn't maintain pressure - we switched to 20W-50 and the engine just purred. Your engines probably have oil coolers - Is it possible that these have become plugged or their internal thermostat isn't opening/closing as it should? I'm not a mechanic but I would start with the simple stuff first and then go from there. All the best

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Old 04-09-2020, 04:30 PM
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If that was after the cooler I'd be really concerned. But before the cooler it would be something to monitor. I know the boat is new to you and you don't know what is normal. (do you trust the previous owner) I'd pull the oil filter, cut it open and inspect the pleats. If it's clean that's a good start. This is a highly debated subject here. There is the thought of monitoring before or after the cooler. I personally want to know temp after the cooler and what is being delivered to my engine/bearings. Others talk about it needs to be measured in the pan. (who cares what the pan temp is...) Oil sees extreme temperatures in the cylinders and 350+F on the valve springs. Just look at the color valve springs turn (you know they are over 400F) and oil is the only cooling they receive. Just too many variables. If concerned install a larger cooler. The best thing is to know base lines and go from there. Differentiation from normal is what you are looking for.
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Old 04-09-2020, 04:34 PM
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3pointstar what a fortunate break you had there! Oil cooler is real simple to get to. Thank for the advise. Hopefully its a simple clog but only time will tell.


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Old 04-09-2020, 04:40 PM
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02 bored out to 509 and dyno for 550 hp
Had the same issue until I changed the size of the oil cooler after repower. If you have the standard 502 oil cooler then your going to need to get a bigger one. Mine was about 8 inches long and mounted on the port side of the motor. Wound up getting a 2/75 by 18 inch cooler which took care of the issue. Probably should also install a oil thermostat b\ut that's another day

Highest OIl Temps that are safe
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Old 04-09-2020, 07:31 PM
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Is the gauge right? Right senders in there?

Seems pretty hot
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Old 04-09-2020, 07:41 PM
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Locked one up running about 260*. I was running down the river hard, then just cruising about 3500 rpm.....no knocks a couple tics from the lifters and seized up. Temps always came back down to around 200-215 once I was out of the throttles. Maybe I lost an oil pump, Idk... But it def took me by surprise.

Maybe a larger oil cooler, or larger pan is in your future. Perhaps both...
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Old 04-09-2020, 08:27 PM
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the size of the cooler lines also comes to play,i like -12 but -10 will work.the stock merc lines are maxed out on a hp500 that is stock.
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