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1995 3.8L Mustang unusual overheating issue

2973 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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Have you made sure the system is burped correctly? If theres an air pocket in the system itll overheat quick. When youre moving it keeps it cool enough and with the ac on the high speed fan comes on constantly moving air. I would try that first.

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