Coolant boiling after shut off

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Gur88

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2002
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4.7 magnum
Good morning. I just bought a used 02 1500 4.7 that needs/needed a lot of little things. After getting the misfires sorted out without the obd2 port (I think it had a dps?) The radiator started boiling over.

First thing I did was let it cool down. None of the plugs I replaced had signs of coolant and the oil is black, not milky. After it cooled down and I put distilled water in it I pulled a vacuum and it held overnight. I topped off through the vacuum and let it run with the cap off to see if I could see combustion fumes boil into the res, but there was none. I locked myself out of the truck not realizing it does that, so it ran a good 30 minutes while I fashioned some Romex into a sturdy enough hook to get the handle.

After I got in and shut her off, it boiled the water off rapidly but there was only a faint coolant odor.

My diagnosis would be a faulty res cap not holding pressure. I'd like a second opinion though since this is my first dodge and first vehicle with a pressurized holding tank.

I would also like to know what everyone recommends for a coolant flush, since the previous owner has the green crap in there and I'm going back to oem (hoat?) once I fix the issue.

Also, it appears my fan clutch is stuck on, so there's a lot of air passing through the rad for sure.
 

grpht03ram

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Bad cap or wrong PSI rating
Should be 16 or 17 PSI

Wrong coolant ratio. Should be at least 40% coolant
The coolant raises the boiling point too
Use Zerex G05 or OEM coolant

when you say "boiling over" do you means its puking out of the overflow hose on the tank?
If that's the case it could just be overfilled
Where does the coolant level sit in the tank if you let the engine completely cool overnight?
if it's between the 2 cold lines leave it alone
 
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scott lass 18

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I had a thermostat 195 was 7 years old was sticking close on my truck would see temp way over 220 then open up ! replaced it with a 180 works well now was 107 yesterday temp 195 on engine
 

Sandevino

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Pulling a vacuum isn’t going to show any leaks. You need to pressure test the system to 15-16 psi and look for leaks. If it holds pressure, all good.
 
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Gur88

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Found the leak, previous owner jb welded a crack in the radiator plastic.
 
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