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Discussion Starter #1
I've been remote tuning this week a customer's 3900 with HPTuners and once again I've run into a 3500/3900 that is running very hot that the owner previously didn't know. The temperature gauge on the dash really is a dummy gauge, you won't see a difference between running at 200*F coolant temp or 215-220*F, which is where his vehicle is running.

This is not uncommon, I've consistently seen this issue on 3500/3900s, including my father in law's brand-new-at-the-time 2010 3500 Impala. When the engine runs this hot, performance suffers due to spark knock retard, and it'll bring up some engine longevity issues, especially if you're not running synthetic oil and/or not changing your oil often enough.

My attention once again turns to the thermostat. The factory 3500/3900 is a 187* opening temperature thermostat. I'm not convinced its doing its job, simply because its the thermostat's job to regulate the temps. My main concern is the factory thermostat is a bottom bypass unit. What this means is there's a flapper valve on the bottom of the unit. It's meant to block off the coolant flow from recirculating in the block once the thermostat opens, forcing flow through the radiator and not allowing any natural recirculation. A traditional thermostat will allow cooled coolant to join the normal circulation of coolant when the thermostat opens, rather than trying to force a bypass flow. I don't know for sure if there's a flow issue that's created, but I do wonder.

I purchased a boatload of thermostats to find an alternative, and here's what I found...

The 3800 thermostat flange is a direct fit, its the same diameter, its basically the 3900 thermostat without the bypass flapper at the bottom. The 3800 seal is close in size too but the 3500/3900 seal fits the 3800 stat. The 3800 thermostat is a non-bypass, so it will open and allow coolant flow through the radiator, but won't obstruct coolant flow. It's available in a high flow unit if you get the right part number. Its also available in 160*, 180* and 195*, of which I think we should look at trying the 180* high flow unit, which is available from Motorad using part number 2003-180 (note that similar part number 203-180 is the non-high-flow unit, don't get them mixed up).

So here's the deal, I don't have a 3500/3900 vehicle to test this on, so I'm calling on the community to take over from here. Someone should try installing this thermostat, and in particular someone who has access to a scan tool to monitor coolant temperatures and do a real before and after comparison.

Again, Motorad part number 2003-180 gets you the HIGH FLOW 180* thermostat, and Felpro 35614 gets you a new 3500/3900 seal. This thermostat will be a direct swap for the factory thermostat.

 

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Will my car run hotter potently by swapping this? I've been suspecting my thermostat as being stuck open/close for a few months now. My car would run a little warm. Around 205-210 when cruising. The hose going into the radiator is very hot while the one coming from the radiator is always very cold. The car is a little slow to heat up sometimes too. There's a very good chance I would test this to a. change mine and b. help others out. Would a tune be required? I just got some more coolant so that shouldn't be an issue. I got all the tools to do it. It's pretty easy. I just want to make sure it's a direct swap and that I won't run into any issues. I have dash command and can log temps. Parts are around $11 on Amazon.


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Discussion Starter #3
Would be awesome if you can try it out. It should run cooler, the idea is we want the engine to be running 190-205* rather than the 210-220 I often see, which is too hot. Even if it ran 180-190, very happy with that temp range.

No tuning required.
 

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Please post a DIY and the out come. I want to flush and swap out my thermostat soon along with the better coolant.
 

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What is the better coolant? The green stuff? It will also help with getting my coolant to water ratio from 20:80 to 50:50. Should I go a little higher with how cold it gets here?


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Discussion Starter #6
Dexcool gets a bad rap but it's ok to use. Mix 50/50 with distilled water, don't use tap water (or get it premixed).

I use the HOAT coolant (Zerex G-05) but it would need a full flush before adding it in.
 

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Would it be better or worse to use say 60 coolant and 40 water to help with the cold temperatures we get here. I will be ordering the parts tomorrow as well. And I will debate on doing a full flush.


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Discussion Starter #8
More water is better than more coolant in terms of heat transfer, but gotta watch the freezing point of course. Best just to do 50/50 IMHO.
 

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How many quarts of coolant for a full flush?
 

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Any idea why GM would want to have this flapper and not allow coolant to circulate like with the other thermostat?


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So iam new to the Imapla world and Fwd world and owning a new car world, so that being said. I'm also new to the forum so if I ask dumb things im sorry just trying to learn. I recently purchased a 14 limited Impala with the 3.6 lfx, (only engine offered on the limited if im correct) back in july and I love it but I have a few concerns.One being the temp gauge it always seems to be towards the higher side once it warms up from cold it goes to just a smidge past the half way point and never moves. So my question is that it normal? Can you switch out thermostats on this car and not ruin the car? And also could it be that maybe since its Direct injection does it run a little hot for that reason? Any help would be appreciated.
 

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So we need a 3800 t-stat and a 3500 seal for it to work. Correct?
 

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You got the 180° tstat?

I wonder if i can get them both at autozone. I have a 20 dollar credit i can use and get the parts free.
 

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I'm kinda curious as to why GM would such a different thermostat design on the 3500... Would bother me not knowing - I mean are there any downsides to changing to a thermostat with a different design? I'd just be a little nervous changing the design of a part that is so important to the engine cooling system without knowing the "whys"....

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Yes I got the 180*. You very well could do that. I would try checking thru their website to see. Also I'm not sure either. I'm waiting to here back from Overkill as to why because I'm curious too.


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Discussion Starter #20
I can't tell you the whys, only the whats.

As I mentioned, I've done the initial legwork on this, happy to see KG2014 pick this up from here and give this a try. I hope we find this is a simple solution to cooling these engines.
 
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