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Strange Cooling Issue


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1992 4.0 4x4 etc

New thermostat, radiator, rebuilt motor, overflow bottle, water pump, hoses, head gasket (after rebuild), no exhaust leaks (all new to the tail pipe), 1998 XJ electric fan on a switch (always on), new clutch on the mechanical fan, rebuilt stock injectors, new o2 sensor/MAP/CPS/TPS/IAC, coolant temp sensor (front and back) etc I'm sure you get the point.

 

So the temps will hit 215 or so then drop back down to 160ish. After that it'll bounce back and forth for 30 min or so until it settles at 200 or so. It's been pretty cold around here so not a big deal but was having this problem last summer and watching it almost over heat. I've never had a vehicle (including this one prior to the rebuild) that bounced the temp around so much so figured I'd ask the experts. I've tested just about everything including the lower radiator hose to make sure it wasn't collapsing. Any ideas would be great. Thanks

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Don't turn your efan on all the time for a start.
 
Beyond that your thermostat is moving too fast.  The restrictor Don mentions will help.  Or try a different (better) thermostat.
I'm still working out some kinks with my wiring harness or it wouldn't be going all the time. Got a new harness though since it's causing multiple issues. The thermostat was working fine when I tested it. If the restrict doesn't work I'll try another.

Sent from my PH-1 using Tapatalk

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1 minute ago, SBpunk said:

If the restrict doesn't work I'll try another.

 

The restrictor works for this sort of stuff 99% of the time; some race classes it's common to not run a thermostat and just tune the system with a restrictor.  If either the water pump flows too much, or the thermostat's 'motor' moves the valve too fast when it hits rated temperature, you will get a massive rush of hot coolant from the engine into the radiator, which displaces all the cold coolant in the radiator...  Drilling a bypass hole in the thermostat can help too, as it means there is always a little coolant flowing through the system.

 

This problem is why GM smartened up and started putting the thermostat on the inlet side of the engine.

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You guys suggest putting it in the upper or lower hose? Feel like the upper would be more effective but the product suggests lower. Seems if the coolant is flowing too fast from the radiator to the motor you'd want to slow it down coming out of the top. Might be completely off here though.

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On 2/2/2018 at 6:14 AM, SBpunk said:

You guys suggest putting it in the upper or lower hose? Feel like the upper would be more effective but the product suggests lower. Seems if the coolant is flowing too fast from the radiator to the motor you'd want to slow it down coming out of the top. Might be completely off here though.

 

I doubt it will matter, but I'd put it in the bottom based on cooling system function theory.  You may need to 'adjust' it later though.

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