Reader Question: My 1996 Ford Mustang 3.8L V6 has some symptoms of cooling/heating issues:
The temperature gauge reads cold for about 5 miles after starting and running the car after which the needle jump to almost off the gage. After about 15 miles it starts to come down in temperature on the gage that is when the heat or defrost starts to get warm.
Then the temperature goes back up and the defrost or heat goes cold. At about 20 miles then the temperature drops to about the “O” on the normal range of the gauge and stays about there with no problems.
Prior to this, the car overheated and I took it in for service and a pressure test was done. The intake gaskets were leaking which I replaced along with a new water pump and three thermostats. The car still seems to still be using coolant and there are no visible leaks. I have checked the oil for the presence of water and there is none. I have also captured exhaust fumes and there is no evidence of coolant there.
The cooling system never seems to be charged unless the car is really been run for many miles but after the car sits overnight the upper hose is completely empty of fluid.
This is about to drive me crazy!
I am a little confused here, so let me just start with the basics again, which is what I would recommend you do in this situation. Get another cooling system pressure test to rule out any internal or external coolant leak. You did not mention anything about coolant loss, but let us rule that out first.
Second, run the engine long enough that the coolant temperature should be hot. Then with the heater ON, feel your upper and lower radiator hoses and both heater hoses. They all should be hot to the touch. If only one RADIATOR hose is hot, then I would suspect you have either a bad thermostat or a restriction in the radiator itself.
You can run your hand down the middle of the radiator starting at the top, if you hit a cold spot as you make your way down the bottom of the radiator you probably need a new radiator as there could be a restriction inside. A faulty radiator cap can also cause a coolant leak on the system and not allow needed internal pressure to build up in the system.
If you only have one HEATER hose that is hot, you probably have a heater control valve problem or you have a restriction inside the heater core itself. You can sometimes back flush a dirty heater core, or you have to replace the core if it is full of rust and debris that will not flush out.
If BOTH heater hoses are hot but there is no hot air inside the vehicle, you have a heater blend door control problem inside the dashboard or a problem with the heater control switch in the dash.
It would be great if you had access to one of those hand held laser heat guns so you could take some readings of the engine and the radiator and compare them with the dash gauge readings.
Part of your email suggests a faulty temperature sending unit, which would explain the erratic temperature gauge, but would not have an effect on no heat inside the vehicle. I like to use those heat guns to locate coolant restrictions as well.
Last thing I would do is feel the temperature of your intake manifold, could there be a chance that there is a problem with the intake installation causing some kind of restriction in the intake? If you did not have this issue before the intake gaskets, I would TRIPLE check the installation.