I will preface this by saving I'm not a huge vehicle repair guy. I tinker, but not to excess. But I will also say I'm not paying the $374 dollars I was quoted by a repair place to get what I think is a bad thermostat replaced. Here's the story...

A week or so ago, I noticed the temp gauge on my 97 F150 go up, then fall back down to where it always was; just a hair below the mid-line. I had just gone up a hill, so I didn't think anything about it. It's gotten progressively worse of late. Now when you drive it, there no heat, but the gauge is half way. Then all the sudden it zooms up to red and the idiot light comes on. Then after 20-30 seconds, the gauge plummets back to less than half. Before, this only happened once. Then the heat came back and it was fine until the next time you drove it.

Now it just repeats the cycle I described above over and over. The gauge will stay below half for a minute, then quickly rise back up, stay on red for a minute, then almost like the thermostat is blowing, cools back down.

All I'm asking is, it sounds like the thermostat to me, how about you?

 

It could be primarily the thermostat, especially if the current one may be the original. You should do a good flush out on the system also, just be prepared for additional work. Wouldn't be the first time that a heater core starts leaking ( after the flush ).

Also check for a pressure reducing bushing in the " in " heater hose to the core. That could be plugged up a little. The fan is functioning normally ?

I can't remember having a thermostat cycle itself like that. Usually they either do not open at all, or they do not close all the way.

Dave

 

Thanks Dave, yes as far as I know the fan and everything is fine. I'm 99% sure it's the original thermostat.

The thing that made me think it was possibly think it was something different than a thermostat was the heat. Just like you said, anytime I've had one fail, it always just failed there was never any yo-yoing like this. It opened all the way and there was no heat. Now, when the gauge falls from the red, the heat will come on for a second or two. Then as it starts back up towards high, the heat shuts off.

 

Make sure the level in the tank is between the marks....check when cold. Lets just hope it is the thermostat...try a new one and see. Mine does the exact same thing and it is a head gasket problem. I have driven it all winter like that, but my heat will eventually stay on and the gauge will settle in after a while. Mine blows coolant out the tank if I don't keep the level about 3" below normal (combustion gases getting in coolant)

 

Have you checked your coolant level ?

 

Guys.....Dudes....

Um...I'm hesitant to even post this. I went out, check the coolant level. the res was empty... (smashing head into tailgate)

I went in the garage, got some antifreeze, filled the res to the line, and took it for a drive. The gauge moved slowly up, settled just below half, and the heat came pouring out.... (bashing self in the nuts with a wrench) Drove 10 miles thinking it would start yo-yoing.....Nothing.

Thanks for the advice. Turning in my nuts for some ovaries.

I guess the only other questions I have is, where did the antifreeze go that was in there(didn't notice any leaks), and did I do damage by losing it and running the truck?

 

monitor the level for 3-4 weeks. Every couple days, open the cap in the morning, see if it has residual pressure from the night before, and check the level. See if your loosing pressure overnight and loosing coolant level over a couple weeks. Does it startup and run rough first thing in the morning, or o.k.?

 

Seems to run fine. I have noticed that it skips a little bit once in awhile, but it had a gas leak, and that combine with the bitter cold lately I figured there might be some water getting into the gas. I just got the tank replaced yesterday. So I really haven't had time to notice if that was resolved.

I will check when I go to work this morning and see if it lost pressure.

 

I don't think you have done damage. Keep an eye on the level, check underneath each morning for drips. Just drive it and keep an eye on it......glad it's OK.

 

I popped the cap off the reservoir this morning before work to check the level. It's down a little, but what I noticed was it looked like black stuff on top of the antifreeze. I gave it a sniff.... ... smells like exhaust/gas. That can't be good.

 

A neglected cooling system will tend to smell funky . I would flush ( old way ) with water , no chemicals . Then refill with proper coolant % for your area . 50% works great here . Then see how it goes . Good luck !

 

sounds like bad head gasket to me. throw some bars stuff in and call it a day.

 

I have had to fix 4 different units in the last 3 months with this problem. To begin with the Ford V engines have a problem that occurs when you loose about a gallon of liquid from the cooling system the tank will show about 1" low not really anything problem is the engine is empty that is why the temperature will switch from high to low water in steam out one other thing that can happen is a engine safety mode which means it will only run on 4 cylinders this will come and go. Now we have decided things may look good but are not many times when the engine is filled with gas you may have to remove the Thermostat housing to get water back in the engine.
What goes with the water hang on: Crack in left rear of intake manifold with this you may think any water is normal. Cracks on the Left or Right front intake Manifold. Bad right front intake manifold 5.4. A small dissolved spot on the head very hard to locate 5.4. A tiny crack in the bottom of the Thermostat plastic housing 4.0. If you by chance have a bad intake Manifold it would wise to use metal.

 

My head hurts a little, but I think I got all that...

 

wow.... Hard to read through