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Intermittent Overflow of Coolant

Whitlock

NAXJA Forum User
Location
Pennsylvania
2000 xj 4.0 109800 miles. About every 3 or 4 short trips (less then 10-15 miles) Coolant is expelled from the overflow bottle, after the vehicle is shut off for a few minutes. Had the system flushed and filled. New Stant 16lb. radiator cap. Water temp runs around 205. The coolant level is correct in the bottle and I checked the radiator is full. Its like the pressure builds as the heat soaks the coolant and it releases all at once, forcing coolant out of the botlle. ??? bottle is only 1/3 or so full, but it blows out the vent.
 
In 30 years I've never had a problem with a Stant Cap. Mopar does not make a venting cap, which is real nice to burp out the air in the system.
 
Liquids don't expand much, so if your getting pressure it's probably steam, exhaust gases or air. What is your coolant mix? Do you have any very small leaks, that could be sucking air during the cool down cycle, is there excessive moisture coming out of the exhaust or a whole bunch of bubbles and foam when you run the motor with the cap off?
If your thermostat is sticking and cooking the coolant in the back of the head to steam, it usually causes some rather wild swings in the temp. gauge, when the thermostat finally opens.
 
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It seems to have stopped. Now the coolant bottle fills up when the engine is hot and pulls back down to the add line once it cools down. Must have had a air bubble in the system. I bought this vehicle used from a dealer 5 minutes after it was traded in.(Nothing done to the vehicle.) When I took off the rad cap for the first time, it was a plug of rust the same diameter as the filler neck. The people who owned the Jeep never service the radiator. Oil was change at a quick lube every 3k miles though. (I checked with the Quick Lube) So much for their "Full Service" oil changes. Thanks to all for the replies. I still plan on putting in the 180*Jet thermostat from Jeggs that came yesterday.
 
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