Radiator cap pressure

95 jeep cherokee sport. How important is cap pressure. Mine says 16lbs on a search online it says it takes a 18lbs cap…also it looks a little tired. Is there supposed to be a hole in the copper? im guessing where the water flows when the cap opens due to over heating. The hole (if its supposed to be there looks kind of crude.


A new cap is cheap. Your Jeep is 15 years old. If you’re in doubt, verify the pressure rating required at the parts store and replace it.

Have you been having overheating problems?

It’s impossible to see the hole from here, but it might be a problem if it has no flap over it. The cap is designed to allow flow into the reservoir when the pressure exceeds 16psi and free flow from the reservoir back into the radiator. Typically that’s accomplished with a spring loaded plate with a hole in the center covered by a rubber “flapper valve” (for lack of a better term). The “flapper valve” is actually a check valve that allows free flow from the reservoir to the radiator and zero flow from the radator to the reservoir.

Im not overheating However. I was told my temperature fluctuating 30-40 degrees is not normal and wondering if the cap has anything to do with it

The whole in the copper part…is it supposed to be there

Heres the hole. I have to guess its supposed to be there…is it?

Unlikely that it’s causing your problem. But a bad cap could have caused you to lose fluid and you could have air in the system now. Have you checked the coolant level?

My recommendation would be to pressure test the system, and easy and inexpensive process, and assuming no leak is found to drain the system, change the thermostat and the radiator cap, and refill the system with fresh coolant if it hasn’t been done for a number of years. Park the vehicle somewhere nose-up (ramps work great) to be sure that the radiator fill hole is the high point in the system and be sure to allow all the air to escape. Fill it to the brim, run the engine with the cap off while continuing to add coolant, until the T-stat is open and the coolant no longer drops. Then fill the reservoir, run the engine for a while, allow it to cool fully, and check the radiator to be sure it’s still full.

This all sounds like a lot of work, but it really isn’t. And all of this together will cost very little, well under $100 including the pressure test kit, and should take care of the fluctuations. Check the cooling fans for proper operation while you;re doing all this.

I was typing while you posted your photos. Yup, that hole is normal. That’s the return hole through which fluid travels from the reservoir to the radiator.

But your fluctuating temperature is not. Odds are that it’s a sticky thermostat. See my other post for my recommendations. I sometimes engage in overkill, but if your temp is fluctuating it’s a good ideat to get it all back in good shape.

Thanks Mountain. Will do…

Ive already done most of what you say. With the exception of a pressure test. Its 30 below and until it warms up im thinking if it aint broke dont fix it… Thanks for the help…

Just replace the radiator cap and see if it gets any better. If it doesn’t, you didn’t waste that much money replacing a 15 year old radiator cap.

it is very important chance the cap

it looks bad to me

My question is…The cap cold be opening randomly to allow for coolant to and from the reservoir where there cold coolant causing temperature swing of 30-40 degrees

How important is pressure. Mine is 16lbs i did a search online and is says my vehicle 95 cherokee sport takes 18lb cap

Radiator caps never open randomly. Pressure has to force them open.

Pressure is important enough to make a new radiator cap worth the price.

I wouldn’t worry about the information you found online. Just go to the parts counter at a dealership or at an auto parts store you trust, tell the clerk the year, make, and model of your vehicle, and he or she will give you the right radiator cap.

Any changing of the temperature that makes the pressure in the radiator go down below ambient atmospheric pressure will make the coolant flow back to the radiator.

“I was told my temperature fluctuating 30-40 degrees is not normal”

How old is the thermostat?
Replace if over 5 y.o.

I would replace the thermostat, cap and “long life” coolant every 5 years.

I have no clue as to the age of the cap or thermostat. u did change the coolant. Thanks guy n yes i will change the cap and thermostat

I would have to argue that a fluctuation of 30 to 40 degrees is normal. I have tested this theory on plenty of vehicles through the years. for the last two years i have been driving a 96 astro. The cooling system is good. No leaks, heater works good. Just went on a 600 mile trip last Saturday. I have a habit of keeping an eye on things, since I’ve been a mechanic as long as I have. The temp gauge fluctuated between 160 and 200 the whole time. I have also watched this happen on scanners. the temp gauge is going to change as the temp comes up, t-stat opens, then you have a fluid exchange. The cool coolant in the radiator moves to engine, and warm coolant in engine moves to radiator. t-stat closes and process happens all over again, and again, and again.