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Have a 1989 4.0 has 265 thousand miles. I have replaced everything I can think in the cooling system and still have an overheat issue. When running the temp gauge will cycle down in tempture one to three times then stop cooling. Temperate on gauge reads around 220 when water starts boiling out of reservoir. Have done several coolant system flushes with a few bottles of coolant system cleaner I got from orielys. Any help would be greatly appreciated. 

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could be air trapped in the system, could be that your coolant pressure tank can't hold pressure anymore and is popping. 

 

For air pockets I've found the best bleed procedure is to point the nose of the truck downhill and bleed through the coolant sensor at the back/top of the block.  fire up the engine, turn the sensor slowly until air hisses out.  never do this when the engine is hot and wear eye protection because coolant can spray out.

 

for and old pressure tank and cap, replacements are available and not much money through your local parts store or online.  or you can convert to the later style with an open system.  should be a thread on doing that in the link in my sig (it requires a new radiator with a radiator cap). 

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I'd go open system, man.  One of the best things I ever did.  May solve your issue outright, but at least it'll make it easier to troubleshoot in the future.  

 

You're thermostat may be dead now.

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Are you filling the expansion tank ? It's supposed to be roughly half full. Making sure you have a good cap/expansion tank is another good idea as some are just a  junk design. Drilling a small hole at the top of the thermostat allows air to escape . 

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Closed systems are a PITA. If you haven't you can upgrade you cap on the reservoir to the volvo one. It's flatter so it doesn't rub against the hood.  Proper burping off the system is a huge issue. When I did mine I did as Pete said and pointed the nose of the truck downhill, filled the expansion tank about way left the cap off and started the truck. I would slowly add coolant as it went down (but you need to be careful since pressure is building up and the cap is off the coolant can volcano). I did that for a few minutes put the cap on took a little drive to get it up to temp and checked the fluid level.  

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The temp sensor Pete is talking about is on top of the back of the head on the driver's side, a single wire to it.

You can change to an open system but if you want to keep your current setup I recommend one of these, a little pricey but you'll never have a problem with the bottle again. http://macsradiatorshop.com/catalogsearch/result/?q=jeep

 

 

 

MacsRad.jpg

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Alright thanks for all the help. I have come to the conclusion that the block itself is clogged. Is there any way y,all have been successful on unclogging the block without pulling it apart?

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35 minutes ago, Lakiniteasy said:

Alright thanks for all the help. I have come to the conclusion that the block itself is clogged. Is there any way y,all have been successful on unclogging the block without pulling it apart?

 

Just curious, what brought you to this conclusion? Have you done anything else since your last posts? Have you flushed the heater core in addition to the rad?

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It takes a lot to "clog" the block. If your coolant looks like anything better than thick, gloopy, rusty Dexcool mud then I strongly doubt your block is clogged. Even then I think it's a stretch.

 

The Renix cooling system can be a pain to bleed. Your pressure cap might not be holding pressure. There's a lot more things to check before you jump to your block being clogged up. In fact, if you're really boiling at 220 (there's another thing - you need to make sure your gauge is accurate) it's almost certain your system isn't holding pressure.

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