So the previous owner had bad injector cups and they were all replaced as well as injectors before I bought the truck. He did not however flush out the radiator before he sold it to me and he DID tell me that he didn't get a chance to flush it yet. The trucks running great, good heater, normal operating temps but the coolant is very dark colored and smells like Diesel fuel. I WILL get to flushing it out in the next few weeks but my question is this: IS it doing anykind of damage to my cooling system or the truck by driving it like this?.

 

It will not hurt it to drive with a little diesel in the radiator. I know a person that drove thousands of miles with leaking cups and sucked the fuel off the coolant in the puke tank every night.
Run straight water for a month, flush it and put coolant in. You will still have some residual, less that 1/4 inch usually so you may have to suck it off several times before it stops. Put some Water Wetter in.

 

Think of it as water pump lubricant.

 

I ran water through mine for a while and put in a little purple power every now and then and drained the radiator almost every evening after work. It cleaned it out well. then I did a flush and fill on last time like you are supposed to.

 

Diesel will degrade the coolant hoses and degas bottle.

 

The esters in diesel will degrade natural rubber. Modern hoses and seals are all Buna N, Nitrile, Silicone or a combination of these and sometimes contain teflon also. The only oils that will attack these compounds are jet oils per MIL-PRF-26399 or PRI-QPL-AS5780. These oils will eat the soles off your shoes but are not used in diesel applications.
The puke tank composition will not be effected by engine oil but it will discolor it which makes it look like it is going bad.