Engine/Transmission (1998.5 - 2002) 180° thermostat increased mileage

[Lazy-Boy]

I posted not to long ago about changing out my stock 190° thermostat. Truck was two years old so I bought one from Cummins. It shocked me to see my temps down about 10° because I had thought nothing would change. So I started digging around to make sure I had bought the correct part. I checked and posted on the TDR and most said don’t worry about it, so I didn’t.



Anyways, on that TDR posting one of you said that some had reported better mileage. Well, you can add me to that list. I was consistently getting 20. 5 but now I’m consistently getting 21. Not a huge difference but I’ll take it.

Wonder what else gets better or worse with a 10° difference.



I’m still not concerned, just curious.

Why does Cummins put a 190° stat in the engine at the factory and then suggest 180° when you switch them out? Why not just put a 180° in at the factory?



Are all you 2002 owners running just under the 190° mark with your stock stat? That seems to be where the needle sits when you’re using the 190° stat. Just curious if Cummins or DC has made any changes.

 

[Cobra]

Lazy Boy,

I don't know why Cummins would suggest a lower temp stat I cannot address that but I can tell you why the higher temps are put in at the factory. The higher the temp of the engine the lowere are your emissions. Most modern car engines are running 195 or higher in the thermostat for that reason.

 

[pwr2tow]

I think Cobra nailed that one down.

I have a question though, why does running a colder thermostat get better mileage, it is usually the other way around the warmer the bettter the mileage, or am I thinking too much in gasser terms.



Ron

 

[PRout]

Funny you bring this up, I just changed mine out for a 180 this weekend. Was surprised in how much difference it made on the gauge. Its alot lower. Also you can hold your hand on the valve cover.



Havent noticed any difference yet in performance, but will check it out & report back. Would be cool (no pun intended) to get an increase in mileage, but wasnt the plan.



I did notice that it might feel like its not fully heated up. Theres a certain smoothness it gets when its at full temp, that it may not have w/ this therm. Could just be imagination. We'll see.

 

[Lazy-Boy]

Yea, it was the gauge reading that shocked me. It registered a 10 - 15 degree difference. At first I thought it may have been stuck open a little. I read on the TDR that the stock temp gauge isn't perfect. So I just trusted the stat was dropping things down only 10 degrees.



Also, you mentioned the valve cover is cooler. That reminded me - another thing I noticed was that the cab heater doesn't crank out as much heat as it used to. That thing use to burn me up. This may actually be a solution for folks living way up north who say the cab heater doesn't get warm enough. I won't ever have that problem down here in Texas though.

 

[jjohn]

L.B. 2002 right at 190

Hum i wouldn't mind a little extra mileage.

 

[Mark_Kendrick]

me too. . a thermostat is cheap.

 

[RClark]

Does the lower engine temp change the signal to the computer(s) enough to affect the way it works? Seems everything you change does something to another part of the system you never figured on.

 

[Steve Campbell]

I would like to see some "long term" fuel mileage numbers from going to the 180 T-stat. Is this just symtomatic of the warmer weather and fuel that has been cut less, or is there actually (not calling anyone a fibber here) a fuel mileage enhancement from going this route?

 

[JPS]

thermostats

I think most thermostats we buy now days are junk. Not a very precise piece of equipment for such an important job. Talking to a few mechanics they all told thermos vary a lot with the same temp stamped on them. I changed one out on wife's car, stamped same temp. car warms up slower and does not seem to get as warm! As to better milage, . 5 miles to the gallon is hard to detect. Not saying it isn't happening, just a lot of variables there! Have a good day! JPS

 

[pwr2tow]

I still would like to know how lowering the temp can increase fuel mileage, It is useually the other way around.



If the ECM is reading a lower temperature then wouldn't it adjust with more fuel for a colder engine, Then more fuel would be consumed.



I was thinking of changeing my stock 180 to the 190 to see if I would gain any in fuel mileage. The reason is it seems like the newer than 99 models get slightly better mileage after DC went with a 190 degree thermostat. Maybe I'am wrong, could someone who has done this chime in and give us their experience.



Ron

 

[Marco]

Engine temps vs. Timing

Gang,

The ECM adjusts the timing upon several inputs. One of them is the engine coolant temp. Then higher the coolant temp then lower the timing. ( And vice versa )



Having that said, it WOULD make sense that Lazy-Boy get's better mileage with the lower temp t-stat.



I say it would, because there is NO real world testing that will show increased MPG due to the changed timining.



Advancing the timing by 2°(over the stock timing) gives:

HP remains the same

NOx emissions become 60% higher

Fuel consumption is TWO (2)% lower



Do you think it is possible to see that 2% gain driving on the road???

I don't think so!



Marco

 

[Snow man]

My stock stat ran right at 190,right on the line,all the time,well my truck was running hot,so i changed the T-stat,the new one fro mCUmmins ran a hair below the line,no difference in Fuel economy,its so bad anymore i dont even check it,i used to get 16. 5, since i got the cold wether reflash(hi idle),my mileage went to 10-12,from bad to worse.

 

[pwr2tow]

Marco,



Thanks, I understand know.

Nice to hear from you again, how's the testing comeing. If you need someone from the frozen north just let me know.



Ron

 

[juicedcummins]

My exp would say that to get better milage with a 180 deg t-stat would be against the grain,unless the mickey-mouse fact clutch-fan is running more often or all the time with a 190.

I went through this a few years ago on a big rig and found you need a certain amount of temp spread between the t-stat, clutch-fan and shudders if equiped. The exact temps I don't remember and it varies on appilcation but ballpark of at least 10deg on these trucks and maybe even 15deg.

Did chrysler put in the same clutch fan with the 190's?

 

[JJW_ND]

Higher temp should be better

I can tell you for a fact that if I could keep my 12V an where near 190 or even 180 it would get better MPG. I get my best MPG in the spring and fall when air temps of cool (between 30 - 60) and I have my winter front on which will bring the coolant up to near 180. In the dead of summer with AC on, 90s in temp, the coolant running interstate speeds will be 175 or so. A couple 100K miles and half a dozen Tstats. BTW, only the first factory Tstat was different the the ones that followed. The Tstats since have always been very consitent in temp control. Even when changeing to a new one (coolant change) you would never know the stat was changed. I just wish they would keep the coolant temp warmer.



You 24V can only be so lucky to be able to control your coolant temp so easy.



Beings most MPG records are set with a engine running with no load, being able to keep the little heat being generated will help get the most out of the fuel.



jjw

ND

 

[LSMITH]

I just put a new 180 in mine about a month ago, and only put it in because I can't get a 190 easily and I needed one right then. The new thermostat keeps the coolant temp needle swinging, 160 up to about 185 and right back down. The needle was solid as a rock before, it would go to 190 and drop one time to 180 and sit there forever. If I could find the gasket seperatley I would put the original back in it. I would like for it to run up at 190-195 personally. MPG improvement with a 180? Not on mine.

 

[illflem]

I used to run four identical air cooled diesel irrigation pumps 24/7, load never varied. Saw a 20% increase in in gallons per hour from 50 to 90° outside temps. Diesels get better economy with warmer running temps to a extent. I have no idea how it relates to a computerized rig.

 

[pwr2tow]

Thanks guys,

I always thought that a little more heat would help, I guess I will have to change out the t-stat and find out for myself.



lsmith,

You say that you couldn't find a gasket?? When I changed my coolant last fall I don't recall a seperate gasket on mine and I just reused my original t-stat. could you tell me what you are referring to?



Ron

 

[monkfarm]

what Jps said about the today's thermostats being junk. . . My oem thermostat went at 40000, the dodge stealer pulled one out for a 12 valve, and said it was $75, very well made, it was beautiful (by cummins), said it wouldn't fit mine (is that right?) and pulled out this junky looking one for ?$27?, made by chrysler, said he'd been selling quite a few of the (cheap) ones. you could tell the difference just by looking at it which was going to last. . .