Cummins ISC overheats at idle **most of the time**

This is an old problem that I've kind of just.. let go.

2009 Cummins ISC "off-road spec" since 2011. Engine had a full in frame rebuild back in October of 2021 and ever since then, the truck would over heat at idle. We tried multiple thermostats (175* thermostats) , coolant flushes and new cooling lines but regardless of outside temperatures, the truck will eventually overheat at idle.

If it's 80* out side, it will over heat in about 15 minutes of idleing and will stay at about 200* with the engine fan always running.

If it's 10 degrees out side, it will eventually overheat after about an hour or more of idleing. The engine fan will keep it at about 185* in below freezing temps

But here's the kicker. Every once in a while it won't overheat at all. Water temps even on the hottest days will sit at 175 to 180 and never rise above that when idleing. I can usually tell when it will or will not be "hot" just based off of hight the water temp is when driving.

I have been putting this issue off because we could never find the issue, and using insite shows all temp sensors reading normal and there are no unusual hot spots on a temp gun indicating a blocked cooling passage that we can see.

However the other day I noticed something. Was going to idle the truck through the night a few weeks back for heat since it was going to be a chilly night. I noticed that the truck was behaving and wasn't going to over heat and so I left the engine fan off all night and sure enough it was fine. Well the next day I shut the truck off to get loaded up and then restarted and went on my way. That night came and I was going to idle again but noticed it was running hotter and again, wanting to creep up while idleing. What I noticed though while laying in bed, was that the engine harmonics were different. You wouldn't notice it sitting in the driver seat or standing out side. But while laying there I started paying attention to it. It would have a very inconsistent vibration to it and it would come and go. Basically the harmonics were all over the place. But then when the engine is running cool and not wanting to overheat, it would idle smooth as butter with no inconsistencies in the vibration or anything

I don't know if that helps out case in figuring out why it's like this but it's the only thing that I've noticed different when it's acting up, which is more normal than when it behaves.


Any ideas?

And yes the engine fan will always kick in and keep it from actually overheating, but that only happens at 220*. One day it didn't come on due to a failed water temp sensor and it actually tripped the high water temp alarm during idle. I had to jump in and hit the engine fan switch to cool it down.

Thanks

 

Water pump is the only thing we haven't taken apart. I've since had a new radiator installed with new upper and lower hoses after the engine fan exploded on me last year. And the shop rag or something has always been my fear. But since we've gone though most major cooling lines, I'm kinda worried if it is a foreign object, it may be wedged in the water jacket of the head.

But would any of that cause the engine to be fine one day, But as soon as you shut it off and restart, it could be running hot and "rough" for the next week before it decides to clear up? Not trying to dismiss what you are suggesting but just wondering.

 

46924665

and yes all genuine cummins parts were used and it was a new head as well.

 

I think my best bet is to do the cleaning route. I'm going to guess that since the radiator and all the large hoses have been replaced since all this happened, there's something clogging a smaller line or even worse, a passage in the head. I'm gonna look into this here in January when we do some other work

 

Thanks. This is a ISC, not an ISX but the idea is about the same. Believe it or not but this engine actually doesn't have a coolant filter. But I'll still look into blowing through the lines with air when I drain the system next.