Engine 1986 GT Overheating issues

86 5O GT

86 5O GT

New Member
Jan 25, 2026
9
0
1
Miami, Fl.
#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.
 
#2
#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?
 
#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.
 
#6
#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)?
 
#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.
 
#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.
 
#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.
 
#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.
 
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#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
 
#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.
 
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