Is the problem the pre-coolor or something else?

Your original cooler is bad, and so is the one from the junk yard. You should not see coolant with a good cooler. The coolant stains means it has failed.

 

I'd replace the coolant diverter valve diaphragm with that new cooler. They had a service bulletin a while back, but I can't find it now. It was restricting coolant flow to the pre-cooler and the higher temps in the cooler could cause premature failure. It was simple, 4 screws a replace the diaphragm. They should be checked when ever you replace the pre-cooler. I think the new diaphragm has a white center, but I'm not sure, I wish I could find the bulletin.

 

never buy parts from a junk yard

 

get the diaphragm installed or it's going to do it again

 

You don't have to pull the cooler back off, you just have to drain the coolant and find the valve. Not sure where the diverter valve is for your arrangement, looking at the pics, yours is different than ours. It'd probably be at the other end of one of the coolant hoses going to the cooler, I'd guess. On our Pete's, it's bolted right to the cooler.

 

I found the service bulletine, it is SEBD6867. I cannot find it online, so here's a quick summery:

C11 (S/N: KCA1-UP)
C13 (S/N: KCB1-UP)
C15 (S/N: BXS1-UP; MXS1-UP; NXS1-UP)

When replacing a precooler on the serial numbers listed above, inspect the coolant diverter valve diahragm. If the diaphragm does not have a white teflon disk, replace the diaphragm using the 319-6059 Diaphragm Kit.

It also says that if a coolant sample indicates engine oil, the diverter valve should be checked.

 

I have a C13 cat motor and I am getting oil in the reservoir. I have already changed the oil cooler. Could this be a pre cooler issue.