Strange over heating issue.


New Member
Mar 31, 2019
Salina, KS
I have a 86 mustang lx with a 3.8L v6, the heat gauge normally sits about in the center. However, if I turn the heating on in the cab the heat gauge sky rockets and goes past the red indicator, and after I turn the heater off, it goes back down very quickly. I have replaced the water pump, radiator, hoses, so basically the entire cooling system. I have not touched the heat sending "probe" thing. Any ideas what would cause this and if its a concern?

Similar but different topic, I have read that the alternators in this car are underpowered. When I run my headlights at night they run ok until I turn on anything else that would require power such as the heat. If anyone has knowledge on this and has a recommendation for a new alternator that would be awesome.
Thanks for the help in advance.
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
polk county florida
You need to take some voltage readings at the battery.
at idle with no accessories
At idle with headlights on and turning on wipers then heater fan.
the voltage should not drop below 13 volts, make sure all the battery connections are clean and tight, battery cables not corroded inside.

This is really a tech question but since we don't have a forum for the 3.8 family I'm leaving this here to get some exposure.


I didn't really have an issue with the stink...
SN Certified Technician
Mar 2, 2015
Sounds like a weak ground strap from the block to the frame of the car. Get one of those battery cables from the parts store with bolt on lugs at both ends. Bolt it to somewhere at the engine ( engine mount, water pump stud, power steering bracket )and bolt it at the sway bar. Never can have too many grounds. I think there's supposed to be a braided type strap that runs from one of the intake studs to the firewall. Those get corroded and cause all types of stuff.