Radiator cap: pressure/venting

I was chasing what I thought was a problem with my cooling system in my 2000 XJ. The system took a long time to pressurize. It never ran hot, got up to temp quickly, has no leaks, never loses fluid, and heat out put is good. It did pressurize but took quite a while to do so. I never noticed this before but may have not really checked. Being winter I believe this was part of my confusion. So I did some research and it appears to be normal operation due to the vented style radiator cap. It takes the 10231 cap mentioned below.

From Stant:

Vented Radiator Caps

There is one coolant cap that is quite different others: Stant #10231. Some people think this cap is broken when they first see it because the part that goes into the fill pipe to seal the system “wobbles.” This is a vented coolant cap. A “Vented Coolant Cap,” sometimes referred to as a “Partial Pressure” cap, may not immediately let the system realize pressure upon thermal expansion (warmup).

The cap has a small disc at the bottom of the cap that is a vacuum valve. That vacuum valve hangs down in the open position until the cap is subjected to rapid expansion of coolant or steam, which closes the valve in a sealed position. At that point, the system is pressurized and helps control overheating and enhances system efficiency. This also means the vehicle can be idling without having a pressurized cooling system and could be “venting” vapor into the overflow reservoir.

Click to expand...

Why would we not want the system to be pressurized sooner to make it more efficient?

In the process I compared the pressure in my 97 ZJ 5.2L. I noticed that it creates pressure much sooner. This led me to do a bit of research.

XJ 4.0:
91-95 13 psi non vented cap
96-01 16 psi vented cap

TJ 4.0:
91-92 13 psi non vented cap
93-95 16 psi vented cap
96-06 18 psi non vented cap

ZJ 4.0:
93-96 16 psi vented cap
97-98 18 psi non vented cap

ZJ V8: 18 psi non vented cap

WJ 4.0:
99-04 18 psi non vented

WJ V8:
99-02 20 psi non vented
03-04 18 psi non vented

This would explain the difference between the pressure in the ZJ vs. XJ that I noticed. However, why the bouncing back and forth for psi and vented vs. non vented caps? And why do the ZJ TJ and WJ use higher pressure caps for some years?

To me it seems like building pressure sooner and having more of it to be beneficial. 1) pressure increases the boiling point of the liquid 2) it reduces localized boiling of the coolant and as a result reduced hot spots.

When speaking about vented radiator caps the red lever style cap is not being referred to. It is the way the vacuum valve is operated.
Normally closed/non vented:


Normally open/vented:


Here is somewhat of a description on how the vented/non-vented works. However their conclusion is incorrect: http://www.coolcatcorp.com/Radiator Caps/Radiatorcaps.html The system will still bleed off air when it is higher than the cap pressure. When it cools down it will bring back in coolant only from the reservoir. The vented style maybe better at discharging trapped air.

Stant 10231 is the only one they offer in the vented style. 10230 is the non vented in 16 PSI.

Cliffs notes: the vent cap pressurizes the system when there is a quick jump in pressure, such as under a extended heavy load. Non-vented will pressurize the system as soon as the coolant begins to heat up.

Anyhow I find it odd that on some vehicles they went with a vented then non vented then back to a vented.. or from vented to non vented.

The pressure step up seems logical enough.. better radiators, better sealing, more pressure allowed. TJ went from 13 --> 16 --> 18 psi, ZJ 16 --> 18, and the XJ 13 --> 16... except in the case of the WJ.... maybe the 20 psi of the earlier cap was too much for the system to hold.

I have though of a few reasons to go with the vented:
1) Easier to relieve trapped air from the radiator. More like quicker because a non-vented cap will do the same thing. The system just needs to pressurize to purge the air first.
2) Less work on the water pump due to lower pressures.
3) Less time for a leak to happen.

I have also thought of a few reasons not to go with the vented:
1) The trapped air will still purge, just not as fast as the system needs to have pressure to open the pressure relief valve
2) Lack of pressure in the system can lead to localized hot spots int he heat and engine block.
3) Lack of pressure in the system can allow the pump to cavitation easier at higher RPMs.
4) Leaks will be come more apparent.

#2 and #3 bother me.

For example in this weather, 30-40*, my system will not pressurize even after a longer run until it sits and heat soaks. I drove to storage which takes about 20 minutes. Once I arrived I popped the hood and the radiator hose was soft. No pressure. I let it sit while I did some things at storage, around 10 minutes. Before I left I checked the hose and it was a bit firm.