TheStang1999

My car is currently defying everything I've learned. Changed the thermostat recently, refilled the coolant, got all the air out of the lines. It gets to op temp, but when I turn the heat on the guage goes down. Heat still comes out in the cabin, but this guage issue has me concerned. Any ideas would be appreciated!

craig71springsport

Your car may still be having heating/cooling issues of some sort. My girlfriend had a giant POS Saturn coupe that when she turned the heat on it would drop. She had another Saturn before that, and that one had such bad problems with heat that she'd have to turn the heater on full blast when she'd sit in traffic and sweat to death to keep it from overheating.

You could still have a faulty thermostat, they are actually considerably delicate mechanisms. It is not uncommon to get a faulty one. Either that or the gauge instrumentation is bad.

TheStang1999

Thanks. It blew the original thermo a month or so ago then the new one went couple of weeks ago. It's now reaching op temp if I allow it to warm up for at least 15 minutes. The MIL hasn't come back on and it's still getting warm inside the cabin. In fact, it gets just as hot as it did with the original thermo before it died. The gauge just happens to drop down when driving at high speeds and when the heat is on higher than 1 click. Still stays warm, but that needle drops.

Either the gauge is broken or it was always supposed to do this and the old thermo was really messed up.

petrock

When you say that the gauge drops, does it drop all the way to the left, or only a little bit to the left? Does it drop suddenly or gradually? If it is suddenly then the problem is electrical. If it is gradual then the motor is probably cooling down somehow and it may not be a problem at all.

LilRoush

It could be dropping a little b/c of the heat exchange happening while the heater core is in operation. It's more or less adding another radiator to the system when starting to blow air across it to heat the cabin.

petrock

I just happened to be playing in my ’98 Mustang electrical diagnostics manual for an issue on my wifey’s car and happened to come upon the wiring diagram for the blower motor & temp gauge circuits (only 5 pages apart from each other). These two circuits are very very simple and do not share a ground, connectors or splices (at least not any I could find). That means that the only way one could effect the other is if they were shorted together somewhere. Where the short is shouldn’t be too hard to track down if you have a volt-meter or multi-meter. I can walk you through it, but it all depends on the answers to the questions I asked in my previous post:

My guess is that it drops suddenly, indicating an electrical problem.

To rule out the temp sensor, do a voltage drop test on the sensor with the car running at operating temp & the heater/fan OFF. Set your volt/multi meter to read as close to 12 volts without going under. Then back probe one wire going into the sensor connector with a small safety pin or needle. Touch the negative meter lead to battery negative post and the positive lead touching the safety pin. Remember that reading. Then back probe the other wire at the connector and touch the positive lead to the safety pin. Remember that reading. According to my manual, the ground wire should be a yellow/red wire and should be somewhere around zero volts, but not exactly zero. The power wire should be a red/white wire and should have some amount of voltage on it above the amount on the ground wire. The exact voltage value is irrelevant. You just care that there is voltage going in and that it is dropping to close to zero on the other side of the sensor.

Next, to confirm my theory, cycle through each fan speed setting and re-check the ground wire of the sensor. My guess is that its voltage reading will go up, indicating there is resistance on the ground side of the sensor. A significant increase would match up with the gauge suddenly dropping all the way to the left. Less then a couple volt increase would match up with the gauge suddenly dropping a little bit or partially to the left.

Now, you answers to the above questions and the results you get with the meter will determine what direction to start looking to find the short. Let me know what readings you get and i’ll walk you through it.

EDIT: I messed up the math initially, and corrected it.

Dr.Le

Depending to how much the drop is, it could be normal. (if its gradual drop)

when i had a stock 192 degree thermo everything was normal.

but when i changed it w/ a 180, the temp needle is slightly less than half w/ the heater on, but is normal w/o heater on.

TheStang1999

Ok, to answer some questions: It was a gradual drop. If I allowed the car to warm up to op temp before driving, it was ok. When I turned the heat on, the needle dropped slowly and stopped at about 1/4. If I had to go on the highway, it would drop to the C. Heat would flow as the needle dropped, but would stop if it was at op temp.

This evening, it overheated again. Pulled the thermostat out, stuck closed. Also noticed that it was a 180 degree thermo and my car requires a 195 degree. Installed new one with new gasket. Overheated. At this point I've swapped out the thermostat, radiator cap, overflow tank, and lower radiator hose. Tomorrow(or later today, however you look at it), I'll be getting a new upper radiator hose and performing a full flush. It's due for one anyways. This is a very bad time in Massachusetts for the cooling/heating system to break!

TheStang1999

No flush yet, but at this point it's a new thermostat, new gasket, new upper & lower hoses. Now, the upper hose isn't sealing around the water outlet with the new clamps, so I'm going to try and put the stock clamps back on there. They're different types. The pressure is building in the upper hose when I turn the heat on and the hose gets hot. After all the new parts, I let it run to op temp to make sure the fan was kicking on. Fan works great. It reached op temp, so I turned on the heat. No heat comes out, then it overheats. I'm thinking bad heater core or there's a clog in there. Opinions?

Dr.Le

If its at op temp and you turn on the heat and no heat comes out then i believe its a leak. the pressure escapes out of the leak instead of into your car vent.

Man you did almost everything already. by the way, a 180 temp is better for our cars according to this websitehttp://www.miracerros.com/mustang/thermo.htm

I trust him alot and have been driving w/ 180 for a while now w/ no problems. my car is an auto and i appreciate the lower temps so my tranny would be happy.