radiator cap PSI rating

Hi,

Im in the middle of upgrading to a open coolant system on my 89 XJ 4.0. And I am using all 92 XJ stuff with a modine 2 core. I got a radiator cap.. and its rated 13psi. This sounds low to me.. Is this the right cap ? Or is it correct but I could go with a higher PSI cap ??

Thanks,
Dan

It should be a 16psi from all my experience.

IIRC, the RENIX cap is 16psig, but 13psi will give you plenty of safe margin for the 4.0 operating temperature.

Water boils at ATM at about 212*F. Add 3*F for each additional pound retained in the system by the pressure cap, so add 39*F for the 13psi cap. 212 + 39 = 251*F. With an operating temperature of 210-215*F, you've got plenty of room (redline on the gage is about 250*F anyhow.)

I still don't understand why everyone is switching to the open system tho - I've got four RENIX XJ's (one was already converted when I bought it) and I don't have any trouble with them running hot. Is it just me?

5-90

5-90 said:

IIRC, the RENIX cap is 16psig, but 13psi will give you plenty of safe margin for the 4.0 operating temperature.

Water boils at ATM at about 212*F. Add 3*F for each additional pound retained in the system by the pressure cap, so add 39*F for the 13psi cap. 212 + 39 = 251*F. With an operating temperature of 210-215*F, you've got plenty of room (redline on the gage is about 250*F anyhow.)

I still don't understand why everyone is switching to the open system tho - I've got four RENIX XJ's (one was already converted when I bought it) and I don't have any trouble with them running hot. Is it just me?

5-90

Click to expand...

The darned pressure bottles swell up, when the coolant/air expands in the cooling system and contracts when cool. The expansion and contraction, eventually weakens and cracks the surge tank (probably the heat helps make the plastic a bit more brittle also). I think 16 PSI is a little high, mine is labeled 1.2 bar. Get the feeling with a lower venting pressure, things would last longer. I´m on my third surge tank in two years (Crown Automotive) and have the start of some cracks in it, that will probably soon start to seep. Think I´m gonna fab a metal surge tank for the next time and put a regular radiator cap on there, so I can trial and error the best pressure. I have no overheating problems, my system just seems to build a bit too much pressure, I have a new cap and it´s not corroded shut. Possible I´m leaking a little compression into the cooling system, but actually rarley use any coolant, if I had a headgasket compression leak into the coolant system, it seems logical I´d eventually miss a little coolant.

There is nothing wrong with the closed system, alot of brand new vehicles have actually gone to it. If the plastic bottle bothers you eagle did an aluminum conversion using an aluminum bottle from summit racing and some fittings. Alot of the new closed systems also use brass or aluminum tanks with normal bungs on them as well as plastic so there is also a source.

There are 2 caps that can be used,13 or 16 psi.
91 -93 use a 13#,94-01 use a 16#.
A 4.0 in a ZJ uses a 18# any or all could be correct for a 4.0
Wayne

5-90 said:

I still don't understand why everyone is switching to the open system tho - I've got four RENIX XJ's (one was already converted when I bought it) and I don't have any trouble with them running hot. Is it just me?

Click to expand...

No, it's not just you. I currently have an '88 XJ, an '88 MJ and an '87 MJ all still running the closed system, and I have no overheating problems with any of them. The XJ has a replacement closed-style radiator, the two MJs are still using the OEM radiators.

The rush to convert comes because long ago someone who had an overheating problem solved it by sticking in a new-style radiator, and rather than simply realizing that the old radiator was probably incapable of cooling lawn tractor he mistakenly assigned the improved results to the fact the replacement was the open style. In fact, a direct replacement would have accomplished the same result.

Both styles of radiator provide the same cooling area/volume (given equivalent core construction so you compare apples to apples), so to think that whether the radiator cap is on the radiator or on a pressure container back at the firewall is ridiculous. But the myth is constantly perpetuated by people who read an article and become convinced that the road to Renix salvation lies in an open cooling system.

Then they wonder why their auxiliary fan doesn't work ony more ...