1987 Ford L9000 Big Cam overheating under load

We recently purchased a 1987 L9000 single axle daycab as a backup tractor for our semitrailer repair business. It has a Big Cam Cummins 855. The truck sat a lot before we purchased it. Only 7,000 miles have been put on it in the past 5 years. This afternoon, I hooked up to a J.B. Hunt container to deliver it. After about 4 miles, the temperature started creeping up. Temperature light came on before I was able to get it pulled off on the shoulder of an off-ramp. Called our road service guy (who fortunately was at the shop at the time). He brought some water and coolant out to me. By this time, the engine had cooled off pretty much. We found that it had been puking coolant out of the overflow tube. We filled the radiator back up, and got on the flip, and brought it back to the shop. Under a load, getting back onto I-85, the temperature slowly started to creep up. By the time I got to the exit (just a mile), the light came back on. As I let off of the throttle, and got the RPMs back down, the temp went back down, and stayed around 175 until I got back to the shop (just a couple miles).

Two questions: first, what is the difference between the Big Cam III and the Big Cam IV, and how can I tell which one I have? Engine tag says NTC-315. I've mostly driven trucks with Detroits in the past; don't have a lot of experience with older Cummins engines. And second, what could be the culprit for the overheating? I've read a few things on the internet about some of the Big Cam engines having an overheating problem. Since the truck has sat for so many years with little use, we wondered about a sticking thermostat. It would be nice if it were that simple. But I don't know. Didn't mess with it after I got the trailer back to the shop this afternoon. It was quitting time by then.

We only gave $3500 for the truck, with 794K miles on it. It's in pretty decent shape other than this little issue. Any input will be greatly appreciated.

 

That's a lot of information, and I sincerely appreciate it. It gave me something to go on. One of our trailer mechanics used to be a diesel mechanic before he came to work for us. He said something this afternoon about doing a pressure test as well. He also mentioned 'stop-leak' and said that he had one of those engines in the shop one time where the customer had used it, and it totally screwed up his cooling system. And like you mentioned, it does have some very small hoses. I noticed there was a little slack in the belts, not too much, but more than it should be. I guess it could be something as simple as that.
Thank you again. I really do appreciate your help.

 

We'll check that for sure. Thank you.

 

I was running the interstate about 2000 or so, 65 mph (it's geared pretty low).

 

That's really interesting. The thought was in the back of my mind, that yes, it's a 31 year old truck, and we didn't have those high speed limits back then. We're going to get it totally gone over, full service, belts and hoses and the whole nine yards. You're probably right. Nice to have experienced folks like you for advice!

That's interesting about the 315 vs. 350. I'm 41, so I wasn't driving back in the day when these engines were more widely used. I got my start with Burlington Industries when I was 24, back in 2001. I always liked the 60 series Detroits myself. I was born too late to drive in the good old days. But I love the older trucks.

Only other question I had is what was the difference between the Big Cam III and IV?

Thanks again for all your information and help. I really do appreciate it. I've taken notes of all you said. We'll get this old girl right. I like keeping old equipment alive. Past my bedtime.