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Old 11-19-2015, 02:04 PM   #1
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Cummins Thermostat location.

Alright. I have been around engines my entire life. I could always locate a thermostat within seconds. I do know where the coolant goes back into the block from the heat exchanger, but it just appears to be a pip that goes straight into the block....with no housing!!!!!! Now I know that is where it is...that is where it has to be. But in all of my experiences, there has always been an obvious housing associated with a thermostat. I see none here. I am sure it is in there somewhere because it has to seat. But dammit!!! And does it require you to remove the alternator to get to it???

Anyway, in the past, the two easiest things to perform on a boat were the impeller and the thermostat. With a Cummins, that is not the case!!!

I am still chasing a heat issue on the left engine. I have done a bunch of stuff that needed to be down(impeller condition, aftercooler service, radiator flush and replace coolant, and I have even got after the engine with an IR gun to confirm gauge operation). Now before I hire out someone to nip this thing in the bud, I figured that changing the thermostat would be easy(and cheap) enough to check that off the list of offenders. I am not sure how easy it is though. The only thing I have left is the thermostat, oil cooler,heat exchanger, and finally the exhaust elbow...in order of expense.

While I started this thread about thermostat location, I would gladly listen to y'all as it relates to my heat problem. I use the word "heat" instead of overheat. It is not overheating. It has been a steady creep. And now under normal cruise load(2300rpm and 17 knots), It is getting close to 200....every bit of 195). With that said, it has been somewhat random. Sometimes it will not do it. Sometimes it will. I just made a 450 mile journey in early October and it performed perfectly the whole time. But it was still 185ish...188ish. Just hotter than it used to be and definitely hotter than the right side. ANd now even hotter. SO the "randomness" of the occurence could be a symptom of a sticky thermostat. I do know that thermostats generally do NOT cause heating issues...but it is just so easy and cheap to cross off the list.

Next step would be hiring the local Cummins guy to take a boat ride and shoot differential readings along the cooling system.

Also, my boat is severely underpropped(Almost 3100rpm on a 2800rpm engine). So it is not overloaded. The boat has been dove on...and it has been quickhauled...nothing to note. Strainers are clean. No collapsed hoses under load. Water flow APPEARS to be normal....but who knows.

Thanks...I thought this was gonna be a short post...hahaha!!!
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Old 11-19-2015, 02:37 PM   #2
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What Cummins engine?

You might have 2 thermostats, my 8.3 does.

Cummins are very picky about coolant. Have you checked the additive package? Have you changed your coolant this century? Have you changed the coolant filter? As soon as I changed the coolant in my engine the over-heating issue I had went away. The filters (I don't know if they all do, but Fleetguard does) contain the additives needed for the coolant but eventually you have to change it anyway. Try this before you throw parts at it.
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Old 11-19-2015, 02:40 PM   #3
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Have you run Rydlyme or Barnacle Buster through the HE with a pump and a bucket yet? I followed Flyright's method on my engine and it helped. Got a lot of fizzing and gunk out of the HE and my temps are down now.
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Old 11-19-2015, 02:42 PM   #4
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Originally Posted by Xsbank View Post
What Cummins engine?

You might have 2 thermostats, my 8.3 does.

Cummins are very picky about coolant. Have you checked the additive package? Have you changed your coolant this century? Have you changed the coolant filter? As soon as I changed the coolant in my engine the over-heating issue I had went away. The filters (I don't know if they all do, but Fleetguard does) contain the additives needed for the coolant but eventually you have to change it anyway. Try this before you throw parts at it.
Ok...coolant filter? That is a new one. And it is a 6BTA. 330hp. 2001 vintage.
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Old 11-19-2015, 02:45 PM   #5
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Your BTA 330 is not fitted with a coolant filter as standard, The thermostat is located behind the alternator mount bracket...
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Old 11-19-2015, 02:47 PM   #6
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Your BTA 330 is not fitted with a coolant filter as standard, The thermostat is located behind the alternator mount bracket...
I will take another look at it today....Does it require taking the alternator off to replace the thermostat???
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Old 11-19-2015, 02:54 PM   #7
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I will take another look at it today....Does it require taking the alternator off to replace the thermostat???

That depends on your installation. You can usually swing the alt out on the lower bracket, It you have a hard pipe from the T-stat to the heat exchanger
you may need to remove the header tank bolts to get the pipe off. The Cummins B series T-stat seals with a square o-ring and No Gasket, so you need a little room to ensure the O-ring seats correctly.
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Old 11-19-2015, 05:58 PM   #8
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Coolant?
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Old 11-19-2015, 08:06 PM   #9
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Coolant?
If you are asking what type....It is Fleetguard Compleat. Made by Cummins. Blue stuff!!!
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Old 11-20-2015, 08:12 AM   #10
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We just came due for our periodic antifreeze replacement. Drain. Flush with Fleetguard Restore. Drain. Flush with fresh water. Drain. Install new coolant filters. Refill with 50/50 Fleetguard Compleat. Kind of a cumbersone project, but not particularly difficult.


Absolutely no change in coolant temps. Gotta do it, though, per service manual.


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Old 11-20-2015, 09:33 AM   #11
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Erratic temps are often caused by some junk in the tranny cooler or other location blocking flow sometimes and sometimes not. Especially impeller pieces.
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Old 11-20-2015, 10:24 AM   #12
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Erratic temps are often caused by some junk in the tranny cooler or other location blocking flow sometimes and sometimes not. Especially impeller pieces.
Makes sense. I do not think the left engine has ever shed an impeller though. And if it did, it would never make it any further than theafter cooler...that is the first stop after the water pump.
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Old 11-20-2015, 10:30 AM   #13
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The after cooler will indeed stop stuff. Mechanics are know for not bothering to chase down missing impeller pieces so unless you did all the work since new I would start there.
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Old 11-20-2015, 01:02 PM   #14
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I have the same motor.
Just FYI the stock cummins thermostat is rated to 200 degrees, so I wouldnt be too alarmed at that temperature (yet). I completely disassembled and cleaned or replaced all the components on my cooling system. By far the biggest offender was the transmission cooler. It was 90% clogged. Much worse than the intercooler. In fact, I replaced that and the heat exchanger as well. I had the old heat exchanger cleaned, rebuilt and tested and I have it as a spare in my garage if necessary. If you have not fully serviced your cooling system on your motors in the last 5 years, do it. Check the HX and especially the tranny cooler.
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Old 11-20-2015, 01:03 PM   #15
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Oh, and the thermostat is under the alternator.
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Old 11-20-2015, 01:07 PM   #16
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I should also note that after I serviced everything, I still had temperature fluctuations until I flushed the coolant several times and replaced the thermostat as there was crud in the thermostat and antifreeze side of the cooling system preventing it from opening and closing properly. Everything has been great now for the last year plus.
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Old 11-20-2015, 01:09 PM   #17
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I have the same motor.
Just FYI the stock cummins thermostat is rated to 200 degrees, so I wouldnt be too alarmed at that temperature (yet). I completely disassembled and cleaned or replaced all the components on my cooling system. By far the biggest offender was the transmission cooler. It was 90% clogged. Much worse than the intercooler. In fact, I replaced that and the heat exchanger as well. I had the old heat exchanger cleaned, rebuilt and tested and I have it as a spare in my garage if necessary. If you have not fully serviced your cooling system on your motors in the last 5 years, do it. Check the HX and especially the tranny cooler.
Thank you sir....the tranny cooler has not been serviced/replaced. So I will take a look at it. A 200 degree thermostat might be a holdover for over the road applications??? I know applications like Dodge Ram trucks struggle to keep temps up due to being underloaded the vast majority of their lives. But Boats, planing boats in particular, don't have that issue. Anyway, I will take a look. There does not appear to be a flow problem, but that is hard to quantify...especially if there are bypasses anywhere.
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Old 11-20-2015, 03:53 PM   #18
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If this is new to you I suggest getting the proper Cummins manual and parts list books for your exact CPL. They are well done and very helpful.
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Old 11-20-2015, 05:33 PM   #19
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Thank you sir....the tranny cooler has not been serviced/replaced. So I will take a look at it. A 200 degree thermostat might be a holdover for over the road applications??? I know applications like Dodge Ram trucks struggle to keep temps up due to being underloaded the vast majority of their lives. But Boats, planing boats in particular, don't have that issue. Anyway, I will take a look. There does not appear to be a flow problem, but that is hard to quantify...especially if there are bypasses anywhere.
i ordered the thermostat from seaboard marine who used my serial number to get the thermostat. and I asked them about the temperature range and that was what they told me which was also in agreement with my Cummins manual that came with the engine/boat. My engine is a 1999 or 98. Max engine temperature operating range is 203 F according to the manual.
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Old 01-07-2016, 09:37 AM   #20
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I am still battling this. But the battle is less serious right now as water temps are quite cool. I do not know if that would have anything to do with it. In theory, it should not. I did get a Cummins guy to ride along with me to shoot temps. His temp samples were primarily of where the coolant comes out of the block at the thermostat....the actual metal hose that comes off of the thermostat. And at that location, the temp was...wait for it....180 degrees!! Ironically, in the same location, the temp on the right engine was about 185.

Anyway, would this be a proper place to measure temperature for this issue??? I am still not feeling all warm and fuzzy about this. Basically, we accomplished nothing except we now know the temps coming out of the engine are 180. Should I just shrug it off as a gauge problem.....
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