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1995 3.8L Mustang unusual overheating issue

2977 Views 6 Replies 6 Participants Last post by  modestmustang
I have a 95 v6 mustang and about a year ago I completely rebuilt the engine. Everything on the motor was replaced with new parts (water pump, thermostat, ect.) But I did not replace the radiator or heater core since this was my first motor overhaul and I was a bit naive. I flushed out the core and radiator and water came out clean so that was why I didnt replace them. Long story short a few months ago I noticed a lot of rust buildup in the radiator so I decided to replace it.

The car had no overheating issues at all, I just wanted to replace the radiator for a new one. Now my car overheats to the point where steam comes out of the radiator cap. I have a 180 degree thermostat in it that I have replaced numerous times. I have bypassed the heater core. I have a brand new radiator cap. I even replaced the CCRM (constant control relay module) because of other reasons but also since I found out it controls the fan I wanted to replace it.

Here's what is most unusual about my issue, I can drive on the highway and the engine will stay nice and cold but once I get off the highway and driving around typical city conditions, it will start to overheat. Also if I have the AC ON it seems to reduce the overheating issue.

This is the most backwards thing I have ever dealt with, with any car and any help whatsoever will be greatly appreciated.

A few side notes: no it is not the head gasket (pressure tested), not a thermostat (after 5 replacements in the past 2 months, I don't think so), the fan does work (no grinding or electrical issue)
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First off thanks to heavy metal, I finally got around to creating an account after lurking on this site for a couple years!
Secondly I really appreciate the thought out replies to my situation! They have been incredibly helpful, especially the "burping procedure." When I fill the system with coolant i just squeeze the upper rad hose which seemed to work at getting air out. But I guess not. And the fan circuit breaker is a great thing to check, I would have never thought of that!
As far as the fan situation, I turn on the AC and the fan does work. I haven't watched it to see if it ever shuts off though. I will check that tomorrow. Next point, would the lower temperature thermostat be causing problems with the PCM and fan operation? Since the fan kicks on at 212 degrees and the thermostat I have is 180 could it not be getting hot enough to trigger the fan to come on?
Also where is the ECT sensor? Is that the small sensor with one wire attached to it, and mounted near the vent plug?
Another point about the heater core, I did not just block the inlet and outlet hoses. I have connected the two together with a proper fitting and hose clamps. Therefore water is still circulating just not through a heater core.
Finally I know this may sound like a silly question but what is considered 50/50 coolant mixture? Do I completely drain the radiator and add half antifreeze and half water? Or does the entire engine need to be filled with 50% water and 50% antifreeze?
Sorry about the late replies I have been at work all day, but thank you again everyone for your help!
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