I have a '97 Ford F53 Superduty chassis motorhome and the radiator has developed a leak. It only leaks on cold start, then once warm if fine. My mechanic says it is the seal between the core and the plastic tank. Since it only leaks cold I wonder if a seal sweller would fix it. Do they make such a product for radiators, or should I put some Auto Rx or other seal sweller product in the radiator? I can't imagine that a little motor oil product would harm the cooling system. BTW, this is a normal green antifreeze.

 

TP, if this was mine, I think I would simply pull the radiator out, and have a good radiator shop fix it. Try and determine right where the coolant is exiting when cold. (Dye treatments with a black light, do the trick in seconds).

I for one, do not like those "sealent" treatments, you pour in a radiator. Sometimes they cause more problems than they're worth, and many times they plug narrow coolant passages or bends, unneedlessly. In fact, those sealents have no clue where the leak is, or what they need to seal. Unless you are out on the road, traveling, I would avoid those additives for the radiator. ESPECIALLY, if it's only leaking when cold, then stop when everything heats up and expands. In other words, if the motorhome is not your only source of transportation, then fix it correctly.
Also, those sealents for cooling systems sometimes work for awhile, then all of a sudden stop working. They cannot be trusted. Who knows, a year from now (or more) you might be traveling, when the high coolant temps, and extended engine operating times, cause your radiator to start leaking again. Then, you might be forced to use those additives by neccisity, while you are far away from home.
A small leak along a seam on the outside of the radiator's plastic tank, you might be able to fix with some JB Weld, or similar product. This would not comprimise anything internally, in the cooling system, if this was the case.
Just my $.02. ED

 

Leaks on a plastic tank seam can't be repaired with JB weld or anything else externally.
Get it repaired properly by a radiator shop.

 

Sometimes, the thermal cycling of the radiator causes the aluminum tabs of the header, or side supports to loosen up. I have repaired a few leaking radiators over the years, by recrimping the tabs. Make sure you don't squeeze so hard as to crack the tank, but this technique sometimes works.

 

Talked to a Valvoline tech about their sealer. The tech was familiar with the crimping fix and said I should be able to find a shop that will crimp it. But the issue will be getting the rad out. Will have to call some shops this week and see.