I recently changed almost all parts (water pump, Mopar t-stat, hoses, radiator, fan clutch) and flushed out my cooling system. I replaced the radiator with a Mopar heavy-duty cooling one (part number 52080104AC) and reused the cap, which was a 2-year-old Mopar cap (part number 52079880AA). The cap looked fine and worked fine with the old radiator, so I saw no need to replace it.

The cooling system works normally now, except the system doesn't seem to be making any pressure. I blocked off the grill after a drive and got the coolant up to 218 degrees where the e-fan kicks on, but the upper radiator hose is still as squeezable as when it's cold and the cap is off. I don't think there's any air in the system, and the coolant rises and falls is the expansion tank when hot and cold as expected.

It seems to me the cap is not holding pressure in the system. Before I try to test the cap or replace it, can someone confirm that the HD radiator uses the same cap as the regular-duty radiator? I ask because of some confusing language on Rock Auto under the listing for the Mopar cap I have that's making me wonder if it doesn't fit the HD radiator.

Here's what Rock Auto's site says about my current cap. I bolded the language in question. Does this mean it fits the auto trans cars except for the HD engine cooling? The "Exc." wording is confusing me, and RA's site FAQs don't address the particular abbreviation.

MOPAR 3781830 Info
Fits w/ Automatic trans.
or Exc. Heavy Duty Engine Cooling; Manual Trans
or with Manual Trans

Pics of part numbers for what I'm using now:

 

So not sure what to make of this.
In my '98 parts catalog, using the year in your profile, I come with the part number 3781830.
And there is only the one listed.
And at the RockAuto site finding it under that part number and it also indicates yours as an alternate part number.
Here is the strange thing. Googling your part number get and old number of 4677493AA.
And I see what you mean on the RockAuto sight.
My thinking is you have the right cap.
My question is are the HD radiators all metal?

 

The HD radiator has plastic end tanks like the regular ones. The only difference from what I can tell is the core is a bit thicker.

 

Also, I measured the dimensions of the filler neck and sealing surfaces between the two radiators, and they appear to be the same. Maybe I should just buy a new cap and see if that helps.

In the meantime, here are some pictures of the new (HD) radiator neck.




And the old (standard duty) radiator neck



And the Mopar cap

 

So the reason I asked about what the radiator is made of.
I run the cheap Napa caps we sell at my store.
And over the years I have had to put it on just so.
Oh and this is with a Spectra radiator that I had to put in at the very beginning.
I found that if I bend the tabs ever so slightly, I mean just give them on nudge on the cap, that they work better.
I never really blamed the caps but more the plastic radiator for the cap not fitting quite right.
Wondering if that is the problem here.

 

How do you bend the tabs on the cap? Not what tool, but in what orientation do you bend them?

 

With a pair of needle nose and a smidge up and in.
So it would lock around the neck of the radiator better.
And I mean just a smidge.

 

Ah, so the goal is to get it to seat more firmly against the filler neck to seal better?

 

Pretty much.
Just never seemed to go on quite right without playing with it.
Also though I try not to remove it unless necessary.
Again though. Is this what might be happening here?
Not a clue.

 

I emailed RockAuto to ask about the description of the Mopar 52079880AA cap on their website (the same one I'm using now). Here's their response. It's interesting that the other caps don't have the HD cooling exclusion in their description, so maybe I'll just order another brand and see if it'll work. Fortunately, they're pretty cheap.

 

RA's response is almost meaningless, shame they didn't think to say which was the right one but as the only real difference between any two traditional rad caps is their relief pressure, if the HD one is different, that might explain your issue.

 

I just ordered a Gates cap with the red locking tab from RA. We'll see if that one will hold pressure. It feels like it's not sealing at all on the inner surface of the radiator neck rather than being the wrong pressure spec. I bet I could pop the cap off while it's hot and not have coolant spill out everywhere since there's no pressure in the system. Fingers crossed the new cap works.

 

But it's not the cap that creates the pressure but rather it releases pressure when it exceeds its spec. allowing coolant to escape via the overflow.

I know why cooling systems are pressurised - but, must admit, never thought about how they are, s'pose I just assumed it was result of the coolant expanding as it gets hot.

I have one of those caps (although I think it's Stant) & it's worked fine on at least 2 of my XJ's (& 3, possibly 4, rads) - but it's not XJ, or even Jeep, specific, just a generic, aftermarket 16(?) psi one.

 

I put one of those 16 psi caps with the red lever on my radiator and took it for a drive. It's holding pressure now! For some reason the Mopar cap wasn't sealing properly, though I have no idea why. Oh well, I found the solution.

Cooling systems operate under pressure when hot for two reasons. First, the coolant expands when it heats up. Second, the inner (smaller diameter) gasket on the cap (the one that's spring-loaded) prevents the coolant from flowing past and into the expansion tank. When the pressure gets to 16 psi, the pressure of the coolant on the inner seal exceeds the pressure of the spring, and some coolant starts to leak by into the expansion tank. It's the same concept at work in a pressure reducer valve you might find on your boiler, for example. For some reason my Mopar cap wasn't sealing internally, so the expanding coolant just pushed its way by the cap's inner seal and into the tank without any pressure behind it.