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Old 07-30-2020, 08:57 AM   #1
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Thermostat Cummins 6BT5.9M 210 HP (1989)

Our engines have always had inadequate cooling. It is a design flaw in the early Cummins 6BT engines. The boat runs 180-190F in warm (75-80F) water at 1500 rpm. It will not run at 2500 rpm for more that a few minutes without going over 205F and continuing to climb. 2600 rpm is WOT. Previously I have increase fitting sizes and hoses on some of the recommendations from Tony at SBMAR. I came across an article he wrote about the older Cummins cooling issues an it recommended a larger opening 160F thermostat that they had developed. I installed them last week. The new cooling performance is very good. I now cruise at around 170F at 1500 rpm. I ran at 2500 rpm for an extended period of time and the temps stayed below 200F. Previously I had to focus on the temp gauges hovering around 200F anytime I went above 2000 rpm to catch a bridge opening. This is one of those "I should have done this 10 years ago" moments. The old thermostats were 180F, stamped 6/1988.
Just sharing
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Old 07-30-2020, 09:39 AM   #2
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Great upgrade happy it worked out for you.
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Old 07-30-2020, 09:51 AM   #3
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I bet if you got a new factory thermostat rated at the same temperature it would also allow more flow compared to your 1988 thermostats.
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Old 07-30-2020, 10:12 AM   #4
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Quote:
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I bet if you got a new factory thermostat rated at the same temperature it would also allow more flow compared to your 1988 thermostats.
Only if the new one is designed like the one on the right. This picture is an OEM 180 on the left. SMX 160 on the right. OEM has much more restriction of flow.Click image for larger version

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Old 07-30-2020, 10:14 AM   #5
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Right.
But Cummins updates their parts as well.
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Old 07-30-2020, 10:18 AM   #6
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Right.
But Cummins updates their parts as well.
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Old 07-30-2020, 10:19 AM   #7
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This is the 1988 thermostat.Click image for larger version

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Old 07-30-2020, 10:20 AM   #8
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Quote:
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Only if the new one is designed like the one on the right. This picture is an OEM 180 on the left. SMX 160 on the right. OEM has much more restriction of flow.Attachment 105683

The OEM one in that picture looks like one of the Robertshaw style high flow thermostats. The whole brass sleeve moves on them, so they're pretty good flowing. Better than the style shown on the right in many cases, although if that style has a big enough opening, they'll flow plenty.
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Old 07-31-2020, 01:44 AM   #9
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2500 rpm just under 200 is still pre ty hot.
Are you certain you’re not overpropped?
Should be more like 180.
Do you have exhaust gas temp numbers?
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Old 07-31-2020, 05:30 AM   #10
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2500 rpm just under 200 is still pre ty hot.
Are you certain you’re not overpropped?
Should be more like 180.
Do you have exhaust gas temp numbers?
2650 is the max rpm for this 1989 engine. It runs 2600 at WOT. I believe the props are correct. What is your basis for " should be more like 180"? This is an intermittent engine. The 210 HP is not continuous rated engine. The data I provided was while running in 80 degree Chesapeake bay water. Cold water would help the numbers. My understanding is this is normal.
Can anyone with a pre 1990 210 hp Cummins give their thoughts?
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Old 07-31-2020, 07:06 AM   #11
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Nothing wrong with being at 200 when at WOT and cooling back down when you pull back on power. Every diesel I've ever owned did that. Even the brand new 6BTA I repowered my old Mainship with would run up to 195 ish at WOT then back down to 175 when I returned to normal cruising rpm. My Dodge/Cummins truck followed the same pattern.
Regarding the older Cummins 210s, I never ran one but I did consider re-powering with one. In doing some research I vaguely remember there being an "upgrade" to the cooling system by re-plumbing, actually re-routing some of the plumbing. I just don't remember the specifics.
(I think it was Paul Foulston who turned me on to this. He may still be on boatdiesel.com. Paul is a British engineer/marine tech that knows the old Cummins better than anyone IMO).
But in my mind you are good with the new thermostat.
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Old 07-31-2020, 07:46 PM   #12
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Thanks for input jleonard.
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Old 07-31-2020, 10:16 PM   #13
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My 99 6bta runs at 195 or so all the time once it is warmed up.
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