open or closed cooling ?

Ok new MJ owner here - 88 with a 4.0L auto, 4x4 and AC. Whats the difference between open and closed cooling? I have the overflow bottle up by the passenger firewall, but where is the radiator cap? Under the plastice on top of the radiator? Is 240 too hot? I have an aftermarket gauge that reads 240 on a 65-70mph cruise in about 90 degree weather.

thanks for the help
Wade

I have a 88 XJ so I have some insights. The 87-90 XJ/MJs have what is called on this board as a closed cooling system. After 91 they went to what is called an open cooling system.

The closed cooling system has radiator without a radiator cap. What you called the overflow bottle is really a pressure bottle and that plastic cap on top is the "radiator cap".

The weak point in the cooling system is that plastic pressure bottle. A lot of people lean on it when doing work, this damages the bottle and at the worst time it fails and you are without any cooling system.

Moroso makes a an all aluminum bottle you can attach to the firewall and use a conventional radiator cap on it. They guys who did that say it works great. You can also get a new bottle from quadratec.com for $20.

Examine your heater hoses, if there are aluminum bands holding the hoses to the wye fittings then you need to change them, they are 15 years old.

You are running too hot, my XJ runs between 200 and 210.

My guess is your XJ still has the factory radiator, time to install a new one. Also time to change hoses. I posted on here yesterday all the stuff I think you need to change. Buy your radiator hoses at any parts store you trust but buy the heater hoses at the dealer. The aftermarket heater hoses just don't lay right. There are 6 sections of heater hose.

Before you change the radiator I would change the fan clutch for the mechanical fan, and install a new Mopar 195F thermostat.

Here a web site with articles which will help you figure out the cooling system. http://www.olypen.com/craigh/tech.htm

You can convert to the newer style system BUT what to do with the sensor for the electric fan is the problem.

Any more questions, just ask

Hey, that's a pretty cool site! Wish I had know about it when I converted mine. Well, "converted" is a bit strong. I simply had a newer style radiator installed just to get the cap for easier filling of the system. Oddly enough, my truck already had the fan's sensor installed in the lower radiator hose. I never realized it might not be the norm. Good to know!
Jeep on!
--Pete

Martin pretty well summed it up. Except for the location of the "radiator" cap, there is no inherent difference in the cooling capacity of the newer style radiator compared to the older style. They are the same size, same capacity.

The OEM radiator in my '88 started leaking in latw '99, at which time I replaced it with a GDI and a Moroso tank in place of the plastic bottle. Haven't had a hint of a problem since. If you still have the original radiator, as Martin said you are overdue. Even if it isn't leaking, it is probably severly clogged with many years of corrosion.

I converted to the open 3-row system last fall, and have been happy with it so far. My aux fan is controlled by an adjustable thermostatic switch that came from AutoZone (about $18).

My old closed system got to where it would belch out coolant every time I parked it......uke:

i had the closed system and i had no problem with it until it overheated really bad and cracked all my hoses and my pressure bottle cap an after that she never ran the same temp... so i think that if you take care of them then the closed sytem is fine...but if you are having alot of trouble with it, then they are easy to swap out.....but to your question i think that 240 on the highway is to warm and i find that weird that you have a pressure tank and a radiator cap???

I haven't had any pressure issues with the added radiator cap. Not yet at least. If I ever have a problem, I'll seek out a cap with a higher pressure rating. It didn't cost any more than the cap-less radiator and filling the system was SOOOO much easier. Maybe someday I'll switch to an open system. I've long since removed the air-box, so there's lots of room under the hood. One project at a time though...
Jeep on!
--Pete