Yet Another Overheating MJ

[Comavalanche]

So I recently purchased a 1988 MJ 4x4 Automatic AW4 with the "closed", "pressurized", "radiator cap-less" whatever you want to call it cooling system. Drove it on the highway for quite a ways after I bought it. Everything was peachy and I was grinning from ear to ear since my first car was a 1999 Cherokee (I still have it as it's my DD). Ever since I knew these trucks existed I wanted to own one. I finally saved up some cash and I bought it. I drove it to school a few days later and the thing puked coolant all over the ground. I limped it back to my house with the heater on full blast. This time no puking and no boiling of coolant in overflow tank. It now overheats at idle within 15 minutes. BUT here is the plot twist: PO had put an inline radiator cap on the upper radiator hose with a smaller hose running from the cap which hangs down and is open (a little bit of coolant came out and almost got into my eyes). So now I am sitting here researching everything from "burping the renix's cooling system to "open cooling system" conversions and blown head gaskets/cracked heads. Question is: Could this setup that the PO had put in be letting air into the cooling system? Should I get a new upper radiator hose? I recently bought a mopar thermostat from the dealer and was going to install that this weekend.

[Pete M]

nah, that's a common addition to the older trucks.  mostly it helps filling the radiator. :yes:  fwiw, the little hose should be fed to a catch tank.

 

try the bleeding technique that uses the coolant sensor at the back of the head.  :thumbsup:

[mik666]

also make sure the cap is tight on the overflow/reservoir tank - i replaced mine with one from a volvo which is supposedly a better cap and fit nicely

 

[Comavalanche]

I'll look for a little bottle for that hose. And I bought both caps- the blue Volvo one and the one meant for the truck. Both didn't seem to help keep pressure in so I assume it's something else.

[mik666]

do they screw down tight (the caps)? if not the tank itself may be past its expiration point from age etc

[Comavalanche]

Yeah they both screw on almost perfectly.

[Comavalanche]

I bought a 13mm deep socket today and burped it. Temp gauge pegged a little over 230 after driving around my neighborhood so I drove it home and shut it off. Sounds like i'm SOL  :doh:

[Megadan]

IMHO, I would convert to an open system.  Most likely the radiator on that truck is clogged up ( it is well past 20 years old) and it wouldn't hurt to put a new one in.  This way you can do an open conversion and kill two birds with one stone.

I am still running a closed system, with a new WP, Rad, and T-stat and I can leave it idle in 90 degree weather with no fan shroud and it stays fairly cool (spikes to 220 then fans kick on and comes back down).

[Crash]

Do you know what PSI the cap is rated at? Closed systems, especially renix, typically work at a higher pressure than open systems.  IF the cap is any less than 16lb, replace it. If it has the stupid lever piece, replace it. Since you have "overheated" your heep, chances are the pressure bottle has cracks and/or leaks.