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1995 3.8L Mustang unusual overheating issue

2976 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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Harbor freight sells infrared thermometers for about $25, pretty cheap. I diagnosed clogged radiator once with it, top and bottom had different temperatures. You can see thermostat opening, you can see when fan starts (point next to fan switch or sensor).
I always check thermostats putting them in boiling water.
Some radiator fans have several speeds.
Based on your description you have problem with fan or radiator.
Most likely fan because AC turns on fan.
Welcome to the site!!
+1 to the others suggestions to CONFIRM there's isn't any air trapped inside the motor. The V6 coolant refilling procedures need to be followed exactly.

Why is this important!! Because on the older V6's with an ECT sensor when air is trapped inside the motor, air will be in contact to the ECT instead of coolant. This will make the ECT read LOW. The motor will be overheating while the dash gauge says "I'm OK".

Turning the AC reduces the problem because when the AC is on, the cooling fan runs all of the time. It may be possible that the PCM is turning off the cooling fan because the ECT is reading low (see air touching the ECT sensor).

Next thing. Today's cars are designed to work with the water flowing through the heater core all of the time. Do not cut and block the heater hoses to bypass the heater core. Loop them instead.

Next CONFIRM that the cooling fan is actually working. Should be easy to do as the V6 has a single speed cooling fan. Turn the AC and confirm that the cooling fan is actually running.

Now here's where it gets interesting. In the V6 the cooling fan is protected by an automatic resetting CIRCUIT BREAKER!!!! This means it's possible for the cooling fan to appear to work. Stop working and then begin working as if by magic.

What's the most common reason for the CB to trip? Bad fan bearings causing excessive power draw. How does the fan feel when spun by hand? Does it spin freely? Any "growling" noises while it's running?

I strongly suggest starting with the basics.
  • Loop the heater core (don't block the heater core lines)
  • confirm the correct 50/50 coolant mixture
  • confirm the radiator pressure caps holds pressure
  • re-fill the system following the V6 coolant re-filling procedures
  • Confirm that the cooling fans runs

If after doing all of this it still overheats, pull over and check the cooling fan. The CB may have tripped.
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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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