dterry68

Ive read another thread but wanted to post just in case to explain my situation. My gf has a 1987 Mustang GT convertible and since it has been cold she just started using her heat and when its is idling or stopped the heat is kind of warm but when she starts to drive it gets cooler. I checked her coolant and its filled to the top and looks good so thats not the problem i thought maybe a clog in the line but im not sure. Was thinking about taking both hoses off and flushing them out and filling with brand new coolant and see what happens. I was going to get a new heater core for her but wanted to wait so we wouldnt have to take the dash off and everything right now. Anybody have an idea?

RogerDodger1

Stop.

S-L-O-W-L-Y back away from the vehicle.

Put down the wallet.

Back away from the toolbox.

Don't guess.

T-H-I-N-K.

D-I-A-G-N-O-S-E.

Resist the urge to start replacing perfectly good parts, wasting your time and money.

Unless some kind of radiator stop leak product has been added to the cooling system, there is no reason for the heater core, (or anything else in the cooling system), to become plugged up.

If the heater core were plugged, it would remain plugged at all speeds.

How many miles are on this vehicle? When was the last time the thermostat was replaced? Does the vehicle have an engine coolant temperature gauge and if so, what does it indicate? What color is the coolant? Have you looked inside the coolant overflow tank and if so, what do you see?

Information + diagnosis = correcting the problem.

Guessing + replacing parts = waste of time and money.

mattdel

cooling systems can become plugged from general wear and deteriorating conditions of the copper radiators that they flow through, this includes the heater core. electrolysis inside the cooling system is the primary cause of this. if the coolant is still a nice grass green color, then this is probably not the case.

mattdel

First item on my checklist in a case of higher rpm = lower temp would be a seized fan clutch.

RogerDodger1

This is extremely rare in a modern vehicle.

RogerDodger1

How do you figure? If the fan is still spinning at higher engine RPM's, how could that cause poor heat inside the passenger compartment of the vehicle? Pre-fan clutch vehicles had the fan spinning at all RPM's too and didn't have poor passenger compartment heat.

mjr46

not true at all, guess you haven't worked on many gm vehicles with the famous dex-cool, you'd be really suprised to see what dex-cool when it gels up did to a malibu I worked on, and that was just one example of the many I have.................so not so rare as you claim

mjr46

very simple, a locked up fan clutch could do such as it'll draw in even more air volume, coupled with the air flow at highway speeds, we now would have an overactive cooling system,.....and correct pre fan clutch cars such as my 66 mustang had a direct drive fan and spun all the time , however, most did not have elaborate fan shrouds such as my 66 did not from the factory =non-a/c car...............2 prime modern day examples that prove your philosophy wrong are 1...my 89 gt had a 8 blade flex fan and oem shroud and a 3 core aluminum radiator, car sitting still puts out warm heat but once I hit the highway and flexfan does it's job, engine temp drops and air goes cooler, took fan off = issue solved.......example 2 1992 jeep cherokee = my dads, fan clutch locked up currently and he refuses to fix it.....engine temp never reraches operating temp and goes even cooler once car hits the highway and sounds like a jet engine coming at you and heater temp becomes luke warm.....................not saying I feel this is the OP'S problem as he'd hear a locked up fan clutch but what matdel states is possible = seen it before................need anymore real world examples???????????

dterry68

thanks for all the information. Rodger dodger ive narrowed it down i havent been guessing. the temperature gauge doesnt work right now so idk what temperature it would read. Im not sure if a previous owner has put anything in the radiator. The coolant is still green and looks good. As for the thermostat im not sure if it ever has been replaced and the car has 135k miles or more on it.

88gtstang

Did you add water to the coolant, if your running straight coolant this would cause the same problem. You need to have water in the coolant for heat, I would check this first as my friend had the same problem and that is what it turned out to be. He would top off the radiator with straight anti-freeze.