The other day the wife picked my truck up from inspection and said it smelled like coolant. I never had any issues before so I looked for leaks. Found none. Took it out and it seemed to take forever to get to 190 but it smelled like coolant before it got there telling me it was warm. The temp. (scan gauge and factory gauge) would go as high as 204 and then all of a sudden drop to 188 in a matter of 20 sec or so and then climb back up and repeat. What gets me is the radiator cap is cold even after a 1/2 hour drive and the bottom hose is cold but the top is hot so the stat must be opening. I thought the stat might have failed 1/2 open which would explain the symptoms so I put a new NAPA one in. Wanted a 190 but saw I got a 195 but put it in and now it won't go above 180. It's in the 30's out here but I thought the radiator should be getting warm. It's like the coolant isn't getting circulated but it never puked or seemed to get over hot. Is it possible the water pump isn't working right? Any other ideas? I'm looking at it all through a microscope now so to speak and I'm not sure whats normal. And just for my knowledge, the coolant flow is from motor through thermostat into top hose through radiator and back to the motor through the bottom hose right? It took me so long to get used to the reverse flow system in my LT1 that now I forget the which way is normal:banghead:. Also, I did a flush this summer and all looked good so I'm pretty sure I don't have any blocked passages.

Hey Dave,Just letting you know you are not alone. Mine does the yo-yo temps too. I only climb to 204ish before it opens the first time. After that it goes up to about 199ish, and drops to about 188ish. I also felt that only my top hose and that very top corner of the radiator was warm/hot. The cap, and the other side were cold to the touch as well. My coolant is still full, and clean. It is not leaking, or blowing out anywhere. I think it is just that we are not getting enough temp into them in the colder weather without pulling/hauling something to keep the stat open.I can only guess it is normal because I have never had any problems with temps leading up to this, and only noticed it swinging this much after it had gotten cold. My other Cummins was the same with temp swings as the thermostat opens, and closes. I forget if it was exactly the same or not though. I think it was about the same because I remember a bit of concern initially like I was with this one.I do not know what is going on now that you can not hit 180 though unless the bypass hole in the NAPA stat is larger allowing more coolant to pass through without the thermostat having to open, and your not getting it warm enough to get up there. I am by no means an expert though. Lets see what everyone else says.Good Luck!

Depending on how far you are driving you might not get to operating temperature. Cooling system is very efficient and over-sized on these rides. Winter fronts should be considered a requirement. The smell of coolant is most likely your heater core which is a real treat to replace.I have some concerns with the latest round of thermostats Napa sells. I too just replaced my thermostat with the exact one you mentioned. The stat is stamped with 190 but the box says 195 yet I went from being able to maintain 190 degrees to it running around 188 sometimes a bit less. The thermostat I took out was also a Napa but it was about 4 years old. I buy very few parts from Napa any more. They keep changing their suppliers out and making it harder to tell exactly what part manufacture you are getting.

My napa 190 rides about 188, maybe 186 in the colder weather. If im cruising slower, then it runs closer to 180.

My napa 190 rides about 188, maybe 186 in the colder weather. If im cruising slower, then it runs closer to 180.

I would say that's right around where mine runs. My 4yr old Napa thermostat did not do that. I'm still waiting for the really cold temps to hit. Been very mild so far. Look at that. Only my second post and I lost my limited status!

Thanks for the replies guys!! Glad to see I'm not alone. What bothers me is that I never had a problem. 90 degrees pulling a trailer or 10 degrees empty once it hit 190 it stayed there. What could have happened? My old 'good ' stat failed and now I got a junker? I'm just concerned that the engine is getting really hot and it's just not showing it because it's not getting circulated to the temp sensor at the thermostat housing. Is that possible? Wouldn't it puke if it got that hot or lose power? Is it possible for the water pump to be working at only 1/2 it's ability? Sorry for all the ?? I'm just worried about this. It's already my second engine and I don't want to go for 3.

Even with full temp 190*F my cap side will measure much lower. As for the up/down is a sign of sticky thermostat which may or may not get better with time. Some get worse.

Thanks for the replies guys!! Glad to see I'm not alone. What bothers me is that I never had a problem. 90 degrees pulling a trailer or 10 degrees empty once it hit 190 it stayed there.

Normal if it warms up to 190*F and sits that's a good.

What could have happened? My old 'good ' stat failed and now I got a junker?

Happens. Some people get lucky and some end up with crappy batch of thermostats.

I'm just concerned that the engine is getting really hot and it's just not showing it because it's not getting circulated to the temp sensor at the thermostat housing.

No it doing just fine because water pump failures are very very rare on these engines. As long as its below the other high side mark your fine (215-220*F)

Is that possible?Wouldn't it puke if it got that hot or lose power?

If it passed 225*F it will trip the Check Gage light and Check Engine light set a error code for excessive temp and then go into limp mode.

Is it possible for the water pump to be working at only 1/2 it's ability? Sorry for all the ?? I'm just worried about this. It's already my second engine and I don't want to go for 3.

No. There are some 12V engine owner that cut the fins off the pump to reduce coolant pressure and flow. Cummins water pumps rarely fail. But if it bothering you... you could pull it out and check.

Thanks Mike! I feel better now but still wondering what suddenly happened. Let me ask a dumb ?? I assume the gauge and check engine lite for over temp take their info. from the temp sending unit at the stat housing. Is it possible for the temp back in the engine to be hotter than in the front where the sender is? I'm thinking air bubbles but there is no bleeder on these motors i don't think.

Typically when doing a cooling filling I will pull the thermostat out fill the block till it just about to run over and then assemble the thermostat and finish filling the cooling system. As for the rear of the block its slightly warmer but typically nothing that going to cause issues in normal operation.

OK, Thanks. I guess I'll just run it and keep my eye on the temp. I have another sending unit I think I'll put in and probably try another $28 thermostat. I just want it to be the way it was before whatever happened hapened. I probably should'nt let the wife drive my truck anymore:lmao:

OK, Thanks. I guess I'll just run it and keep my eye on the temp. I have another sending unit I think I'll put in and probably try another $28 thermostat. I just want it to be the way it was before whatever happened hapened. I probably should'nt let the wife drive my truck anymore:lmao:

Over all there is really nothing wrong with how it is operating. However you're skipping over the coolant smell. If you are not seeing any visible signs of coolant loss, yet keep smelling coolant your heater core is highly suspect. They typically start as a really slow seep. Just enough to smell, with some slight hazing of the windshield in front of the defrost vents over a long period of time. The system is self bleeding and may take a while to finish. If there are air pockets in the system you will probably hear the bubbles running through the heater core. It will sound like a little water fountain in your dash.

The allen head screw/bolt that is by the "E", if there is one in the head of your Cummins, opens into the coolant passages and will allow you to bleed the system without removing the thermostat.

Over all there is really nothing wrong with how it is operating. However you're skipping over the coolant smell. If you are not seeing any visible signs of coolant loss, yet keep smelling coolant your heater core is highly suspect. They typically start as a really slow seep. Just enough to smell, with some slight hazing of the windshield in front of the defrost vents over a long period of time. The system is self bleeding and may take a while to finish. If there are air pockets in the system you will probably hear the bubbles running through the heater core. It will sound like a little water fountain in your dash.

Thanks. The coolant smell came when it hit 204 which it never ever did before all this started happening. That's why I'm concerned that it may be getting hot but not showing hot on the gauge. I flushed last summer and it's been OK til now so I doubt air is a problem but that's how it's acting. I'm scared the engine might be boiling and the gauge only shows 180. Maybe I worry too much now that I see others have fluctuation but I'm confused as to what suddenly changed. Thanks for all the replies.

Temp Gauge information... http://forum.mopar1973man.com/threads/4597-Anyone-know-what-the-factory-engine-temp-gauge-lines-actually-are?highlight=temperature+gauge http://forum.mopar1973man.com/attachment.php?attachmentid=3181&d=1316638408 http://forum.mopar1973man.com/attachment.php?attachmentid=3182&d=1316638423 http://forum.mopar1973man.com/attachment.php?attachmentid=3183&d=1316638439

Oh... The smell of coolant might be a heater core issue staring. Mine didn't fail from corrosive coolant but the thin solder joint failed and was weeping on top of the HVAC box.

Temp Gauge information...

/threads/4597-Anyone-know-what-the-factory-engine-temp-gauge-lines-actually-are?highlight=temperature+gauge">http://forum.mopar1973man.com/threads/4597-Anyone-know-what-the-factory-engine-temp-gauge-lines-actually-are?highlight=temperature+gauge

/attachment.php?attachmentid=3181&d=1316638408">http://forum.mopar1973man.com/attachment.php?attachmentid=3181&d=1316638408

/attachment.php?attachmentid=3182&d=1316638423">http://forum.mopar1973man.com/attachment.php?attachmentid=3182&d=1316638423

/attachment.php?attachmentid=3183&d=1316638439">http://forum.mopar1973man.com/attachment.php?attachmentid=3183&d=1316638439

Mine never got that hot so I guess I'm OK. Looks like your only getting about 10 volts too? Thanks for taking the time to take those pics for me! I feel better now. I guess the rock solid 190 I was seeing is something all wish for but only a few are rewarded with. :cry:Don't say that! I guess I'd better start searching for heater core replacement threads!

The engine not even running for most of it. What I did was plug in different resistors into the ECT plug to show the different digital / analog temps. So people can understand what the analog gauge is tell you. As long as the need is in between both lower and upper tick marks your good. Heater core article... http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/27-hvac-heating-ventalation-and-air-conditioning/57-air-conditioning-evaporator-core-heater-core-replacement

Well I put the thermostat that I saved from my 53 block partout in. Also put on a new radiator cap. Now it goes to 204 the first time it warms up and then bounces between 188 and 195. I guess I can live with that for now but I think I'll put in a new sending unit and maybe my spare pump. It just bugs the heck out of me why it changed.

Well I put the thermostat that I saved from my 53 block partout in. Also put on a new radiator cap. Now it goes to 204 the first time it warms up and then bounces between 188 and 195. I guess I can live with that for now but I think I'll put in a new sending unit and maybe my spare pump. It just bugs the heck out of me why it changed.

Hey Dave, I am glad to hear it is now back to the way it was, and what I have experienced as "normal" on both this truck, and the 6.7 Cummins I had before this one. When we build up enough heat in the engine, and the thermostat finally opens. All of that ice cold fluid from the radiator hits the warm thermostat/engine. That causes the stat to slam shut and have to build heat again to re-open. Rinse, and repeat. I think since we can not get enough heat into the coolant/radiator it just constantly opens, and closes causing the swings up, and down. I know we all know how a thermostat works, but this guy shows it with a small torch, and then spraying it with water to show it opening, and closing. I think this is exactly what is happening to you/us.

http-~~-//youtu.be/T8cGKdICJmc

I just put on some grill inserts today. It is not really a full winter front, but should help restrict some of the air flowing through the radiator. I will report back when I get some drive time if it helps at all. Take care!

Oh... The smell of coolant might be a heater core issue staring. Mine didn't fail from corrosive coolant but the thin solder joint failed and was weeping on top of the HVAC box.

I don't get the smell but the windshield fogs up all the time,it's a real pain. I wondered if the heater core was the cause but I usually can smell even minute amounts of coolant.