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91 LX 5.0 getting hot when a/c is running

  • Thread starter Thread starter 91lx14gt5.0s
  • Start date Start date Jun 29, 2026
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91lx14gt5.0s

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#1
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91 LX 5.0 getting hot when a/c is running. Even going down the road, it gets hot. Fan runs and radiator was replaced a couple of years ago. What’s the most common issues I should check first?
 
F

fasterthangas

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#2
  • Jun 29, 2026
  • #2
What temps are you seeing? I assume you have an aftermarket gauge.
 
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91lx14gt5.0s

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#3
  • Jun 29, 2026
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fasterthangas said:
What temps are you seeing? I assume you have an aftermarket gauge.
Click to expand...
Everything is original. Only has 80k on it. Getting up to 230
 

gkomo

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#4
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I just want to clarify the 'everything is original', but how do you know the temps are getting up to 230degrees? You are using an aftermarket gauge then?
 
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AeroCoupe

lube between the nut and the face. I know my lubes
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Is the car missing the air dam and or side deflectors?

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

https://lmr.com/item/LRS-8310B-K/5-0resto-mustang-frame-rail-air-deflectors-79-93
 

Mustang5L5

That is…until I whipped out my Bissell
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#6
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Is this mostly while at idle? Or while driving or both?
 

Mindseye007

10 years still blowing strong
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#7
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  • #7
91lx14gt5.0s said:
91 LX 5.0 getting hot when a/c is running. Even going down the road, it gets hot. Fan runs and radiator was replaced a couple of years ago. What’s the most common issues I should check first?
Click to expand...
What thermostat do you have might be stuck closed?.
 
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91lx14gt5.0s

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#8
  • Jul 4, 2026
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Yeah, I meant to reply back to everyone, but my mom’s on chemo and it’s been a trying week. I’m going to change the thermostat. Also, it has only the one deflector, but everything I read, the LX only had the one piece, the gts had to have the others because of the ground effects.
 
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91lx14gt5.0s

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#9
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Also, my temp gauge goes from 130 - 270 with 5 increment lines between and a small red line at 270. It is original
 
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91lx14gt5.0s

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#10
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Mustang5L5 said:
Is this mostly while at idle? Or while driving or both?
Mustang5L5 said:
Is this mostly while at idle? Or while driving or both?
Click to expand...
Both when it’s really hot outside but pretty fast at idle. Cut ac off and cools down pretty quickly while driving
Click to expand...
 
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91lx14gt5.0s

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#11
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gkomo said:
I just want to clarify the 'everything is original', but how do you know the temps are getting up to 230degrees? You are using an aftermarket gauge then?
Click to expand...
The gauge goes from 130 to 270. Red line at 270 and 5 graduated increments between. The only things that are not original are the parts I changed myself, like the radiator and upgraded alternator
 
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91lx14gt5.0s

Member
Feb 26, 2025
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Jul 4, 2026
#12
  • Jul 4, 2026
  • #12
91lx14gt5.0s said:
The gauge goes from 130 to 270. Red line at 270 and 5 graduated increments between. The only things that are not original are the parts I changed myself, like the radiator and upgraded alternator
Click to expand...
91lx14gt5.0s said:
The gauge goes from 130 to 270. Red line at 270 and 5 graduated increments between. The only things that are not original are the parts I changed myself, like the radiator and upgraded alternator
Click to expand...
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
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#13
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SO what's the state of the cooling system? I assume stock?

WIth engine off, grab the cooling fan and try to spin it. How much resistance do you have? A decent amount or does it almost freewheel?
 
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fasterthangas

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#14
  • Jul 4, 2026
  • #14
The factory cooling gauges are notoriously incorrect. That won't be a good indicator of actual coolant temp. For reference, 220° or even 230° is not overheating. New factory cars are designed to run at higher temps. The 94-95 GT's low speed fan doesn't even switch on until 220°.
My '94 coolant temps climb with a/c on as well. My cooling system is all new and optimized with some mods.
 
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manicmechanic007

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#15
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  • #15
Are you able to check the ECT sensor voltage output hot?
When hot, that voltage will be down around .64v where the electric cooling fans should come on if equipped
** Just know that your gauge will fluctuate as it heats up and the thermostat cycles once or twice
Also, the gauge will go nuts if the coolant is low and air is on the sender
 
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