I've got an 89 renix 4.0 that has a new water pump, thermo, and a less than 2 yr old radiator. I keep blowing through those dang reservoirs like there's no tomorrow and it's making me nuts. It usually runs no hotter than 210 until the reservoir cap starts to fail, then loses pressure.

The fan(s) come on and run just fine, no obvious leaks either.

1. I want to convert to a closed system--can anyone provide me a with a link to step by step instructions?

2. I read in another post to check the heater valve--how would I know if this is what's causing my jeepsadness?

 

I am not sure of any sites,but I just converted mine to a 91 style radiator and got rid of the bottle. You need the radiator out of a 91 and up,just make sure they are the same deminsions,the hoses going to and from your heater core need to also be changed along with the heater control valve. remove the old plastic bottle and the holder,find a overflow bottle that will fit in this area and run hose from the radiator overflow. I got everything I needed from the local wrecking yard for less than $100 and have had no problems at all.
I think there may also be some write ups on this site.

 

If you are from washington state, there is a radiator shop in vancouver washington that custom make the over flow tanks and it will stop the over heating problem, i have done it in mine and have sent other people there and it works just fine. Its about 150 dollars. if you are interested let me know i will get you the contact infomation.

 

The problem with your system is that is really a PIA to get rid of all of the air in the radiator.

I solved my problem, using a left side tank from an old copper rad with the filling neck adapted, but if yours uses plastic tanks, you can replace it with a 1992 or later style tank. I used the overflow tank with the lower hose, the way is supposed to and the upper hose of the tank let it hang as a breather. The other thing you might need to do is change the heater hoses for straight ones or blog the lines that used to go to the overflow tank.

When I was replacing mine, the rad had less than 1/2 a gallon of coolant mixture on it.

 

did you do replace your radiator and hoses, etc with the 91 or later radiator? I have the same problem with my 89 4.0 and I am tired of replacing the bottles like you mentioned in your post. if you did, did you have to make other modifications?

 

is the shop in Vancouver still in business? if so, please send me their contact info...even at $150 that is far less than buying an overflow bottle on ebay every 3-6 months! I have to tell shops to NOT touch my cooling system so they don't overtighten the cap