mechanic

I am going to be changing out my thermostat. I have the new one from<br>Cummins already, along with a black rubber seal. I remember reading<br>on another post, but can't seem to find it now, that there might be another<br>gasket needed, between the outlet and the block. Is this true, or is RTV<br>good enough?<br><br>Thanks in advance.

turbo thom

When I replaced mine the rubber gasket was all I used. Why the RTV? I don't see how you can use it.<br><br>..PT..

mechanic

Great, thanks. This is just different than other thermostats I've changed <br>in gassers. I don't want to get it apart and then have no way to go get<br>parts if they are needed.

vssman

Just for good measure, I used a bit of RTV on both faces of the engine hoist bracket. I made sure it was only enough to smear on the surface and not get inside the coolant system. No one had either o-ring when I did mine. I just reused the old ones.

mechanic

Well, I replaced it last night, very easy to do. I didn't use any RTV, so far<br>so good. I used the 3934373 thermostat, and a 3925466 seal. When I took<br>it out for a test drive, I noticed that it didn't get very warm before it opened<br>up, and it appeared to cycle about 4 times before I got back home. I thought<br>I had another cycling thermostat, but on the way to work this a.m. it came<br>up and stayed there, no cycling. I am concerned though about the maximum<br>temp reached with this one. It appears to only get up to about 165 or 170.<br>It's alot cooler than the old one that opened at 190. The heater doesn't seem<br>to get very warm now. <br><br>Does anyone know of a thermostat that doesn't cycle and opens at 190* ???<br><br>I may go ahead and put the old one back in just to get the heat back in the cab.<br><br>Thanks.

vssman

Gee - we must be living duplicate lives. I am experiencing the same thing. I've tried a 370-180 stant equivalent and found it to open too high the first time. I spoke to Stant tech support and it had soemthing to do with pressure &amp; temp requirements to open properly. I guess it is a requirement for the &quot;witches hat&quot; style t-stats. The &quot;top hat&quot; style t-stat is a more robust design and opens on temp only. That's that the Cummins t-stat is. Let me know if you find anything.

mechanic

Does the Stant 370-180 exhibit the temperature cycling? If not, what<br>temperature did it open at first, and what temp did it settle out at?<br><br>Thanks

mechanic

Well, I just talked Cummins tech support, and he said that they don't have<br>any other thermostats that open at different temperatures, and reduce the<br>cycling. So, I guess I will be putting the old one back in and block off some<br>of the radiator to help reduce the cycling. I suppose a real winter front would<br>help, too.

vssman

I tried the Stant T-stat and it really cycles. It would go to the white line just before 230 before it would open for the first time. It would then dump back down to 145-150. Then it would cycle between 210-220 and 145-150. That's why I called Stant tech support and found out about pressure/temperature requirements. I felt it was better to run at a constant cooler temp than cycling like that. I figure Cummins might have determined it was ok to run that cold.... I hope.

mechanic

Thanks for letting me know about the Stant. I went ahead and put the old<br>t-stat back in on Friday. I also went ahead and slid a couple pieces of<br>cardboard in front of the radiator that covered the outer 2/3rds, leaving the<br>inside 1/3 open. I did a 900 mile round trip over the weekend like that and<br>this is what I observed. At outside temps around 55, driving 65 MPH, which<br>was about 2100 RPM, the engine was operating at about 8 lbs of boost and<br>the pyro was reading about 750 pre-turbo. At these conditions, it took 2<br>hours of driving mostly on a slow incline from 100 feet to about 1200 feet<br>before it actually stopped cycling and stabilized at about 180*. After that,<br>the outside temp dropped to below 50 and the cycling returned. It was still<br>reading higher on the temp gauge than the t-stat that doesn't cycle. I like<br>the engine to run a little warmer, I have seen charts that show less internal<br>wear on the engine at higher temps.

yrusoslow2

I am having the same problem. I just replaced my thermostat with the updated one that is supposed to get rid of the cycling but it is not working. Still goes from 190* to 145 or 150 and then back up. I drive about 800 miles a weekend on open roads and back roads. If anyones finds out anything could you let me know. I would really like to get this cycling to stop. Thanks again.

mechanic

How long have you had the new thermostat in? Mine cycled about 5 times<br>on the first test drive, but then I parked it overnight. The next day on the<br>way to work it cycled once, then stopped. It didn't cycle after that. I think<br>it was cycling before all the air was out of the system, maybe.