Since it was 10F this morning and going to be the same or colder tomorrow morning I figured I will get some statistics on how well a winter front works. I want to get mileage and time as to how long it takes to hit 190F. I will do a constant 55mph for these tests and be on the road within a minute of starting.

Today was without a winter front going straight into 20-30mph headwinds at 10F. Although synthetic oil seemed to not do anything at 20+F, you will notice that it cranks slower now that the conventional oil I am running starts to get really thick, you can even see how long it takes for it to get oil pressure. So synthetic does have it's place in the colder temps.

My EGT's were around 680F at 55mph when the engine was still under 130F, this proves how thick oil and cold surfaces make an engine have to really work. On the return trip with the engine at 190 and still at 55mph, the EGT was around 400F.

http-~~-//www.youtube.com/watch?v=vvVo4QMXX4Y

Geez... Now I'm glad I don't have a 12V... All the junk that was added on your doesn't exist on a 24V...

What junk?

What junk?

Look at your videos. To the folks with no exposed wires in the cab of their trucks, yours looks like the old time telephone switching boards!!!!!!! Just saying!!

ISX,I've seen those bypass thermostats in Marine applications... because they had wet exhausts... some cooling (sea) water had to flowing out at all times to keep from burning out the reinforced rubber exhausts. When the thermostast opened, more water flowed out. I tried a good 160 bypass in a old Jeep wagon I had... had a Buick engine... overheated instantly. A new thermostat of conventional design fixed it. What does the book call for? Russ

ISX, I've seen those bypass thermostats in Marine applications... because they had wet exhausts... some cooling (sea) water had to flowing out at all times to keep from burning out the reinforced rubber exhausts. When the thermostast opened, more water flowed out. I tried a good 160 bypass in a old Jeep wagon I had... had a Buick engine... overheated instantly. A new thermostat of conventional design fixed it. What does the book call for? Russ

The stock ones are the ones that don't work (do all the swaying). I can't figure out what that hole is that the stock thermostat seemed to seal off. After ripping everything off that thermostat and making it as bare as the thermostat that works, the only difference is the design. The one that works is a superstat, the junk one is something else, maybe stant. If the air is out of the system then that bypass should be closed by design. The thermostats act identical in boiling water and the superstat one starts to open at 195 and keeps the truck between 195 and 200 which it is just like that in boiling water. But you put the other thermostat in the truck, and it's a different animal, even after making it identical to the superstat. I don't get it. I am going to do more testing tomorrow and see what else I can figure out.

But the stock thermostat is a NAPA one right? I read about lots of issues with non-Cummins thermostats..

But the stock thermostat is a NAPA one right? I read about lots of issues with non-Cummins thermostats..

The one that I have that is junk is a OEM NAPA one, the good one is a NAPA one also but destined for the 5.9 V8. I made a poll to see just how bad it was and it seems the cummins ones are the worst. http://www.cumminsforum.com/forum/general-diesel-discussion/346822-thermostats-swaying-coolant-temp.html

That lower chamber has a hole in the side... it's some bypass obiviously. Maybe we need a coolant flow chart. I know with the Marine engines with bypass thermostats, if someone use an automotive thermostat in there, they'd cook. The Cummins may be diofferent... dunno.Russ

Looking at the poll I wouldn't say there are any clear results as to it swaying excessively. I would bet most people are missing your comment on dropping to 150. Its not uncommon to see "sway" on these motors due to the size of the cooling system. But the sway you are seeing is not normal. I know they aren't super cheap but I would try a Cummins tstat. My stock one swayed a bit, but I put a new Cummins stat in and its rock solid. The only time there the coolant drops below 190* when the stat opens is when its cold outside and I am cruising at 40ish mph, the rest of the time it opens about 195 and drops to 191 and holds between 191 and 193.

The bypass hole is what I am trying to figure out. I found out the napa oem junk one is a "bottom bypass". The thermostat I got works perfect, the one meant for a 5.9 V8, it sways between 195-200 like it is supposed to, so finding a working thermostat is not my concern.My concern is that the junk one sways in the truck big time, but not in a pot of boiling water, it doesn't suddenly open full blast like it does in the truck. I have all of it stripped down to make it look exactly like the one that works, and it still doesn't work. I can come up with a dozen excuses about thermostat design but then I can revert back to the fact that it is perfect in boiling water so I don't get what it does in the truck that is different when the other tstat that works sits in there the same and everything. There is no reason why it isn't working. I figured out my mechanical gauge probe has the same threads as my bypass bleed line thing so I am putting it there which will be an inch from the thermostat right in the line of site of it so I can see exactly what that thermostat is seeing for temps.

Here are the 2 thermostats. The one with the smaller diameter disc thing is the good one (or the one with the lighter color gold). You can see that I have them to where they are identical but the junk one still does the huge swaying so I just don't get what the deal is with it when the other one, same and everything, works perfect in the same spot and everything.

Are you certain the 5.9 V8 one is blocking the bypass? If not and your at a light load that could account for the more constant temp. I don't think that the boiling test really shows anything here. It seems to me that the "junk" stat is probably weak, and thus opening full sooner than it should and blocking the bypass and allowing full radiator coolant to enter the block. The OE thermostat isn't supposed to go full open unitl 203*, and it seems yours is doing it much sooner, and then is slow to close and your getting huge temp fluctuations. The 5.9 V8 stat may work now, but I am skeptical it will operate as it should when you put a load on the motor. Honestly, put a Cummins stat in and try it.

Are you certain the 5.9 V8 one is blocking the bypass? If not and your at a light load that could account for the more constant temp.

It isn't blocking it at all, it doesn't have that spring loaded plate like the other.

I don't think that the boiling test really shows anything here. It seems to me that the "junk" stat is probably weak, and thus opening full sooner than it should and blocking the bypass and allowing full radiator coolant to enter the block. The OE thermostat isn't supposed to go full open unitl 203*, and it seems yours is doing it much sooner, and then is slow to close and your getting huge temp fluctuations.

That bypass hole is tiny, I don't see what difference it would make being blocked or not, I can't figure out what it even does. On top of that, I took that bypass cover thing off of the junk thermostat and it didn't change thing. In the pot, it was fully open at 203 or so and didn't open til around 195 which it's a 180 stat so it's stronger if anything. It opens in the engine at 220 or even 225, it is worse with a load on a cold engine. So when I hook the trailer up on a freezing day and take off it will skyrocket before it opens, then plummit to 150. It's like it takes a while to see the temp of the rest of the engine.

The 5.9 V8 stat may work now, but I am skeptical it will operate as it should when you put a load on the motor.

I will hook the trailer up and see.

Honestly, put a Cummins stat in and try it.

Those things cost $30! The one that works is $8. I will wait until I see it fail before I get a cummins one.

I'm not talking about your external bypass, but the internal one. When the thermostat is closed the bypass takes the coolant from the head and feeds the water pump thru a passage. When the thermostat opens the thermostat will block that bypass and force the water pump to suck from the radiator, when thusly pulls coolant from the head. If the 5.9 V8 doesn't have the same internal casting it wont operate the radiator bypass properly. $30 vs 8$.. how much do you value your time? Even at minimum wage your time has exceeded $22 worth of labor. The symptoms you are describing are classic failed thermostat, high temp spikes and swings. I didn't realize it opened late, so weak may be the wrong term, so maybe slow is better.. My original tsat was very slow to react, big spikes, but it never dropped below 180*.

This explains a lot. http://www.are.com.au/feat/techt/thermostat.htm If that tiny hole is supposed to be the bypass then I don't know what to think. Figured it would be bigger. Plus that bypass thing that I cut off on the thermostat never moved in the pot so I don't see how it would open and close, I imagine it just stayed closed the whole time so the coolant never really circulated until the thermostat finally opened. Which explains why the rest of the engine would get so hot since the thermostat isn't that close to the engine so it wouldn't see the heat as quickly. I might have to stare at it in my truck some more. I think I just realized how it opens when the thermostat opens by getting pushed away. But it still doesn't answer why the thermostat is still junk with the bypass valve disc thing completely removed.

i noticed that when you started your truck that it took along time for the oil psi to rise...didnt know if ya knew that.

This explains a lot. http://www.are.com.au/feat/techt/thermostat.htm If that tiny hole is supposed to be the bypass then I don't know what to think. Figured it would be bigger. Plus that bypass thing that I cut off on the thermostat never moved in the pot so I don't see how it would open and close, I imagine it just stayed closed the whole time so the coolant never really circulated until the thermostat finally opened. Which explains why the rest of the engine would get so hot since the thermostat isn't that close to the engine so it wouldn't see the heat as quickly. I might have to stare at it in my truck some more. I think I just realized how it opens when the thermostat opens by getting pushed away. But it still doesn't answer why the thermostat is still junk with the bypass valve disc thing completely removed.

:smart:easy way to get the right thermo. is call cummins with your E.S.N....problem solved!

That hose is not the bypass that is commonly refereed to. I have no idea why it's there. And its not shown in the coolant diagram. But I believe the junk thermostat has a faulty spring, and the bypass doesn't get bypassed until the thermostat is fully open. [ATTACH=CONFIG]2247[/ATTACH]

i noticed that when you started your truck that it took along time for the oil psi to rise...didnt know if ya knew that.

my 99 take about the same amount of time to build pressure...is that not normal?