Why 20psi, not 15??

[JMBT]

Hello all, I am finally headed home to do my rods and mains!! (CM2250 764,000mi) I had planned on doing them at 750k but a few unplanned loads made it 764k.

Anyhow my question is about pressure testing the coolant system when I drop the oil pan. I have read many times on the forum to pressure test at 20psi overnight. (Forgive my ignorance, but I have to ask), why do we test at 20psi if the system is designed for 15? Could the extra pressure during the test “cause” an OLD component that is not leaking at 15psi to start leaking?

I got to thinking about this when I recently replaced the pressure cap on my Onan genset. I looked up the part number on QuickServe, got it at Cummins and put it on. Over the next few days I noticed the coolant level was going down. I took the cabinet apart found a few leaking hose clamps, and fixed them. A few days later more leaks. Apart again replaced a hose and tightened some more clamps. A few days later more leaks.....that’s when I saw the new cap was 16psi and the old one was 13psi. I checked the number again on QuickServe it was the right part. I called the dealer where I got the cap and he said, yep they changed the pressure from 13 to 16psi but didn’t know why.
Long story short, no matter what I did I could not keep coolant in the little bastard at 16psi. I found a dealer that had a “new” 13psi cap, (the old number) and it has been leak free since.

Just wanted to see what you guys thought. Also if there is anything crazy you guys have experienced when doing rods and main on one of these motors, please let me know. Tear down starts Monday, wish me luck!!