88 overheating question...

i've done some searching, so don't yell at me for posting this overposted topic i have an 88 xj w/ the 4.0l in it... i've never had any problems w/ it overheating. never gets higher than 210, and then randomly today, when i was heading up to do a trail in the mtns, when i stopped to disconnect and check my oil/etc (just for safety's sake), i heard something. i popped the hood, and the overflow bottle was spewing out antifreeze from around the cap. i didn't notice it overheating (although i didn't check right b4 i stopped so i guess it could have been running a little warm). once it stopped bubbling, etc, i took off the cap, put a little water in it and continued on the trail. it started heating up, so i popped the hood again and it was again spewing out from the cap, steam, etc... so where should i start? does the cooling system work on a pressure system (so if the cap doesn't seal completely it would overheat?).. any help at all would be great. i just replaced the t-stat for safety's sake.. didn't help at all.... also, my overflow bottle is relatively new.. A half a year at best... also, i eventually plan on switching to the newer cooling system, i'm just not done researching it yet, and i won't be able to for at least a month for monetary reasons.. thanks for all the help and input.

vladthedog said:

(so if the cap doesn't seal completely it would overheat?).. . [/B]

Click to expand...

ABSOLUTELY!!!! In fact, unless something else is obvious from the outset, that cap seal, on the closed system, is the very first thing to suspect. And, the replacement caps, IMHO, are not very good quality. I have gotten around this by making my own cap gaskets from innertube rubber and carrying a couple of spares . . . . . with a caliper, determine the two sockets that will match the inside and outside diameters needed for the gasket and its hole and carefully cut them out with an exacto knife.

I've been through this cap-pressure-leak business many times with my '88 . . . . the latest being last week . . . . 'gasket had blown-out just a little . . . . . 'replaced that gasket with a fresh one, wetted both sides a little before application, no more overheating.

Beyond this (if it isn't the cap), as you've noted from the archives, there are the thermostat, the fan clutch, the "fjthfdf frog" bottle, etc.

I have opted to keep my closed system because, over the 13 years and 288000 I have owned/driven mine ('had 50,000 miles when I bought it), I have become fairly familiar with it and its "quirks." I'm not telling you that's the best decision . . . 'just the one I have made . . . .

the burp

Also search this and other forums for details about how to burp the air out of your cooling system. It involves purging the air through the coolant temp sensor. You can replace every part in your system and it will still overheat if it's holding air. Very common issue with the XJ.