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Engine 1986 GT Overheating issues

  • Thread starter Thread starter 86 5O GT
  • Start date Start date Jan 25, 2026

86 5O GT

New Member
Jan 25, 2026
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Miami, Fl.
Jan 25, 2026
#1
  • Jan 25, 2026
  • #1
I have an 86 GT that is having overheating issues. New radiator, dual contour fan with dakota digital set up, new water neck and thermostat, new silicone hoses. Flushed and burped the system with new antifreeze, Car overheats on the highway and runs cooler when at idle or short distances from home. I'm puzzled as to what the problem could be and told it may be the water pump. Any thoughts would be great before I keep buying parts and get nowhere.
 

AeroCoupe

lube between the nut and the face. I know my lubes
Founding Member
Oct 28, 2001
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Claremore, OK
Jan 25, 2026
#2
  • Jan 25, 2026
  • #2
Does the car still have the air deflectors?

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Replace those rotted out radiator side air deflectors on your 1979-1993 Mustang with these direct replacements from 5.0 Resto!
lmr.com

Mustang Radiator Air Deflector Kit (87-93) E7ZZ-8349

Replace that broken, or missing 1987-1993 Mustang radiator air deflector with this direct replacement from DCR!
lmr.com
 

KRUISR

10 Year Member
Apr 16, 2015
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Jan 25, 2026
#3
  • Jan 25, 2026
  • #3
86 5O GT said:
Car overheats on the highway and runs cooler when at idle or short distances from home.
Click to expand...
That is exact opposite as what should happen. When cruising on the highway you shouldn't even need the electric fans.

I would check for the air deflectors as mentioned above. What temperature t-stat did you install and I would question if you got all the air out of the system.

Also, what do you define as overheating? If it is a temperature reading, where are you getting it?
 

limp

wrap a little cheese around it and its a done
5 Year Member
Oct 4, 2020
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Florida
Jan 27, 2026
#4
  • Jan 27, 2026
  • #4
Make sure your electric fans and its mounting is not blocking air flow..... Seen it before
 

86 5O GT

New Member
Jan 25, 2026
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Miami, Fl.
Jan 27, 2026
#5
  • Jan 27, 2026
  • #5
KRUISR said:
That is exact opposite as what should happen. When cruising on the highway you shouldn't even need the electric fans.

I would check for the air deflectors as mentioned above. What temperature t-stat did you install and I would question if you got all the air out of the system.

Also, what do you define as overheating? If it is a temperature reading, where are you getting it?
Click to expand...
Factory temp t-stat was reinstalled. Full system was burped as well. Overheating temp by factory dial was passed normal on the highway, and reached 214 by the Dakota Digital reader. I’m thinking water pump and maybe eliminating the thermostat all together as I’m in south Florida where winter doesn’t get as cold ever.
 

86 5O GT

New Member
Jan 25, 2026
9
0
1
Miami, Fl.
Jan 27, 2026
#6
  • Jan 27, 2026
  • #6
AeroCoupe said:
Does the car still have the air deflectors?

Mustang Radiator Side Air Deflector Pair (79-93) E4ZZ-8310

Replace those rotted out radiator side air deflectors on your 1979-1993 Mustang with these direct replacements from 5.0 Resto!
lmr.com

Mustang Radiator Air Deflector Kit (87-93) E7ZZ-8349

Replace that broken, or missing 1987-1993 Mustang radiator air deflector with this direct replacement from DCR!
lmr.com
Click to expand...
No, good point.
 

KRUISR

10 Year Member
Apr 16, 2015
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Jan 27, 2026
#7
  • Jan 27, 2026
  • #7
Not having the deflectors can easily cause overheating as air will go around instead of going through the rad.

If you replaced the water pump, did you get the right kind. There are two types and one definitely will not flow coolant right. When you burped the system, did you see coolant circulating in the rad (flowing through the tubes into the end tank)?
 

Blackhawkxx

Advanced Member
Mar 3, 2023
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Jan 27, 2026
#8
  • Jan 27, 2026
  • #8
86 5O GT said:
maybe eliminating the thermostat all together as I’m in south Florida where winter doesn’t get as cold ever.
Click to expand...
I have always read that by removing the thermostat, the water flows too quickly to be cooled in the rad.
 

86 5O GT

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Jan 28, 2026
#9
  • Jan 28, 2026
  • #9
limp said:
Make sure your electric fans and its mounting is not blocking air flow..... Seen it before
Click to expand...
The setup I got from LMR comes with brackets that mount to the fan housing, and directly to the radiator. Polarity is good and set right to temp on the Dakota. KRUSR was spot on, never seen a car overheat on the highway inner city driving brings the temp up but nothing compared to distance driving. I’m thinking it may be a flow issue with the antifreeze meaning that the water pump may have gone bad with all the new replacement parts. Everything else is working as ordered so right now it’s a purchase and apply trial.
 

86 5O GT

New Member
Jan 25, 2026
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Miami, Fl.
Jan 28, 2026
#10
  • Jan 28, 2026
  • #10
Blackhawkxx said:
I have always read that by removing the thermostat, the water flows too quickly to be cooled in the rad.
Click to expand...
Yeah, that’s possible I guess. At the point of desperation!
 

86 5O GT

New Member
Jan 25, 2026
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0
1
Miami, Fl.
Jan 28, 2026
#11
  • Jan 28, 2026
  • #11
KRUISR said:
Not having the deflectors can easily cause overheating as air will go around instead of going through the rad.

If you replaced the water pump, did you get the right kind. There are two types and one definitely will not flow coolant right. When you burped the system, did you see coolant circulating in the rad (flowing through the tubes into the end tank)?
Click to expand...
I haven’t replaced the pump yet, waiting on delivery due to inclement weather between Waco and Miami. Rad circulates perfect.
 

KRUISR

10 Year Member
Apr 16, 2015
1,801
903
143
Jan 28, 2026
#12
  • Jan 28, 2026
  • #12
If the rad is circulating the coolant, then the pump is pumping it around and the t-stat is opening. That's why I asked if you can see the coolant coming out of the tubes in the rad.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,842
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polk county florida
Jan 28, 2026
#13
  • Jan 28, 2026
  • #13
195* stat and 214* high temp at speed is only a 20* increase, without knowing specifics that's ok, a little on the high side but not overheating, is the overflow tank functioning? Right pressure cap?
 

86 5O GT

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Jan 28, 2026
#14
  • Jan 28, 2026
  • #14
Temp gauge on my 86 says Normal, Needle gets passed the L, never found that to be Normal. Everyone tells me it's not, shouldn't be getting up higher on the highway especially. Something's gotta be wrong.
 

AeroCoupe

lube between the nut and the face. I know my lubes
Founding Member
Oct 28, 2001
5,795
3,928
183
Claremore, OK
Jan 28, 2026
#15
  • Jan 28, 2026
  • #15
You need to install a temporary electric or mechanical gauge on it and know what the temperature is vs using a stock gauge that are known to be all over the place.
 
Reactions: limp

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,842
10,514
203
polk county florida
Jan 29, 2026
#16
  • Jan 29, 2026
  • #16
86 5O GT said:
Temp gauge on my 86 says Normal, Needle gets passed the L, never found that to be Normal. Everyone tells me it's not, shouldn't be getting up higher on the highway especially. Something's gotta be wrong.
Click to expand...
the stock gauge is a 'best guess' or 'close enough ' hint at what may or may not be wrong. A rise of 20* running at a higher speed, like mentioned above would not be abnormal, I've seen 215* to 220* @ 100+ mph
 

2000xp8

SN Certified Technician
Aug 8, 2003
8,015
1,613
194
NJ
Jan 29, 2026
#17
  • Jan 29, 2026
  • #17
What dakota digital device are you reading 214 degrees with?
 

86 5O GT

New Member
Jan 25, 2026
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Miami, Fl.
Jan 29, 2026
#18
  • Jan 29, 2026
  • #18
AeroCoupe said:
You need to install a temporary electric or mechanical gauge on it and know what the temperature is vs using a stock gauge that are known to be all over the place.
Click to expand...
Setup from LMR came with the Dakota Digital . Works through the app to set fans, and diagnostics. I agree completely, Factory gauge sucks, better yet, just today, took a good look under the car. It's almost 100% gonna be the pump. Sounds like a secada living in the pump, and maybe just may be the original pump. 75k miles, and 40 years, I'm changing it anyways once Waco thaws out. Appreciate the comments, and advice really.
 

86 5O GT

New Member
Jan 25, 2026
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0
1
Miami, Fl.
Jan 29, 2026
#19
  • Jan 29, 2026
  • #19
2000xp8 said:
What dakota digital device are you reading 214 degrees with?
Click to expand...
https://lmr.com/item/LRS-PAC27502K/dakota-digital-electric-fan-controller-w-sensor-70-amp-relay
 
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