have a 95 auto ctd longbed club w/3.54 rear and 210k+ miles. we use it primarily to pull a 5k 5ver. but even not pulling the temp guage rises and falls like the thermostat is sticking then opens. the temp goes a little over 200 when towin but only about 180 unloaded before dropping down to 140. i automatically changed the thermo thinking it was bad, but still does the same thing. any thoughts?


john

 

Totally normal

Cummins says,

THE COOLING SYSTEM USED ON THE CUMMINS DIESEL ENGINE EQUIPPED VEHICLES PROVIDES THE CAPACITY AND PROTECTION OF THE HIGHER GVWR AND GCWR RATINGS AVAILABLE ON THESE VEHICLES. OWNERS SHOULD BE ADVISED OF THE NORMAL SLOWER WARM UP AND LOWER ENGINE OPERATING TEMPERATURES ASSOCIATED WITH DIESEL ENGINES.

THE COOLANT TEMPERATURE GAUGE WILL FLUCTUATE ON DIESEL ENGINE EQUIPPED VEHICLES. IT IS INFLUENCED BY VEHICLE LOAD, OUTSIDE AMBIENT TEMPERATURE AND EXTENDED IDLE TIME. THIS FUNCTION IS CONSIDERED NORMAL

 

Yep, exactly what infidel said.
You will notice the most fluctuating while on long cruizes, I can watch mine move up and down while cruizing on the interstate.

 

i appreciate the feed back. and thats good to know. but why would it start happening after a year of travelin and 20,000 miles? it had been holding steady at 180 after warm up.

john

 

Unless i am mistaken , there is a "slow opener" thermostat available from cummins , i just don't know the p/n , if anyone could chime in with that info i would appreciate it. My understanding is the "slow opener" will hold a more steady temperature. On really cold days (even with the winterfront on) i have seen my temp gauge go from 180 (middle of the gauge) and drop right down to the stop pin, i know that can't be good for the engine.

 

you can also get the optional t-stat from Dodge- it'a an old big block stat that keeps the temp alot more stable. Could it be you didn't notice the temp swinging before? Depending on the ambient temp it may have been more stable- summer mine swings alot more than winter for obvious reasons. Might be time for a coolant change as well- mine seems to be more widly swinging lately.

It's totally normal and won't hurt anything, but if it bothers you you can change it.

 

glad to know it's normal. i wondered if i just noticed, but i keep an I on things and sure it hadn't been happenin. winter and summer it hadn't gotten over 180. it seemed to start after doin a 16 mile lowgear dirt road uphill on a mild day. on the last pull it red lined and i shut the engine off to cool down, didn't boil over though. bout an hour later it was ok, no leaks or runnin bad. scared me though. maybe not immediately, but it seems it started after that. i'll not fret. but it kinda made my heart miss a beat the 1st time on the open road i saw it go over 200. we were a long way from home. thanks to all for the feedback

 

An oil temp gauge doesn't swing like the coolant, in fact it stays pretty much rock steady. This leads me to believe that block temp doesn't swing as much as the coolant.

 

For future reference never ever shut down when the temp is at it's highest in a situation like that- unless you know there is a catastrophic failure imminent. The temp on the gauge is relative- it fluxuates and does not reflect the exact actual temp in the engine based on where exactly the sender is located in the circulating path- if it's high in the engine, for example, and you have a coolant leak air leaking in will form a pocket and insulate the sender creating a very false reading (typical on FOMOCO V8's).
Now, the reason that practice is bad is simple- the engine is generating heat all the time it's running, right? The block is cast iron, which holds heat, right? Alot of heat in fact. When the engine is running the h20 pump is always circulating the coolant through the block helping to cool hot spots and even temps- when the overall coolant temp reaches the t-stat's opening it does so and the coolant exchanges through the radiator until the overall temp drops below the t-stat's temp confining the circulation to the block again.

Now, the Dodges fluxuate because of the tstat's narrow temp range and massive radiator. The stat opens and the radiator cools it down quickly dropping below tstat close temp but the engine creates heat quickly heating the coolant and opening it again- a wider range tstat will open earlier and allow more flow through the radiator keeping temps more even, hence lower overall.

Ok, with that knowledge in mind- the truck starts heating up beyond 180, the tstat opens at 180 but it takes a few seconds for the coolant exchange to drop the temps, even longer when the airflow through the radiator is slow (like 16mph driving) and it still climbes at the sender (showing you the 200ish) before the tstat sees a close temp. Hauling uphil slowly is alot of work and generates alot of heat with little cooling airflow. So, if the temp starts to spike and the engine stops running (key shutoff in pannic) the h20 pump stops, so coolant circulation stops which means the block is releasing all that built up heat into the already heated coolant in it instead of trading it through the radiator causing the engine to actually heat up more for a period of time.

In extreme situations (like an aluminum block near overheating temp) this alone can cause overheating and damage where as putting it in neutral and letting it idle (or running it to 1Krpm for a minute or so just as you should do to cool an overheating auto tranny) will provide more airflow (from the fan) to cool the engine down. the CTD is a durable engine (heard about them being overheated and blowing all the coolant out with no damage) but anything can happen, and cumulative damage can happen too.

 

at the time it happened i was scared shitless. i had read a post here before that stated the quickest way to cool a diesel engine is to shut it off. no one disputed it at the time. i did try lettn the engine run and reving it a little to get the fan goin. but when the temp didn't drop i shut it off.


john