I'm having another problem. When I bought this truck 93 4x4, 5.0. it was overheating when i drove it on the highway. I had a coolant flush done and a new thermostat put on. It ran fine without any cooling issues until now.

Now when i turn the heat and turn the thermostat all the way to hot the truck will overheat. If I put it back to the cool setting or about half way between cool and hot the truck cools right back down to where it's supposed to be.

What would cause this problem? it doesn't really make sense to me.

 

Well it souunds like you may not have enough coolant in the system, the wrong thermostsat, or a bad head gasket.

 

Thats a weird one, once the heat is on, moving the temp **** should only be moving the air mix door back and forth and doesn't have anything to do with coolant flow.

Maybe there is an airpocket somewhere in the cooling system??

-Jon

 

Upper and/or lower radiator hoses may be soft and are collapsing. Usually, turning the heater on lowers the temp.

Possible heater core leak...check floor on right side...other possibles are heater water valve, bad or leaking heater hoses...t'stat installed upside down.

 

Just out of curiosity, how are you determining that it's overheating, and that it cools down when you turn the temp ****?

 

Mine does that time to time, but never to the point of overheating. If I turn the heat on the temp gauge rises from the regular "o" in "normal" up to the "a" or "l", but then eventually starts to drop back down and reaches the regular temp again. Doesn't do it every time, in fact rarely, but I have noticed it and since the temp never reaches an unsafe level, I really don't care.

Do you notice a smell of antifreeze when you put the blower on high? If so, even faintly, the heater core may be leaking causing a low coolant level. Check the overflow tank's level and add if necessary.

But if the heat itsn't on, the gauge runs completely normal. That is an odd one.

 

I think what PKRWUD might be getting at is that you can't trust the factory temp gauge, you may have a gauge issue and not a cooling system problem.

-Jon

 

Very good, Grasshopper.

The older I get, the more I realize that you should always confirm there is a problem before you try and fix it.

 

Tell me about it, i'm a shop foreman at a dealership and ive seen guys buried in wiring diagrams for 2 hours trying to figure out why the dome lights won't come on when the rear hatch of an SUV is opened. Then they finally figure out that the lights arent supposed to come on when the hatch is opened!

-Jon

 

double post.

 

What is it with Ford gauges? Short of the voltmeter, I've heard pretty much every factory gauge now is in-accurate. The pressure gauge is just an idiot light, the speedo's tend to read low, the fuel senders spaz out, and I've even heard factory tachs give false readings? Now a simple thermostat Ford can't even get right? A bi-metallic strip with a current passed through it. How hard can that be?

Sheesh.

But if the gauge "malfuntions" every time you turn on the heat without fail it seems kinda unlikely that it's just the gauge messing up.

I saw in the JC Whitney catalog the other day radiator caps with a probe and small dial on the cap that read the coolant temp. They were only like $10, pick one of those up if you're not sure. Easier than going to the trouble of replacing the factory sender/gauge.

I used to try and verify problems before I fixed them, but 99% of the time I wasted hours "verifying" the obvious so I just stopped bothering. Of course, these were problems like a spark plug that kept launching out of it's hole past 3k RPM(that one was a major hassle), oil spurting out of valve covers, and the transaxle going into reverse instead of second(you know how european trannys have that stupid "push-left-down" thing? Yeah, that didn't work).
Though if the only sign of problem is a pegged FORD gauge, then it probably isn't a whole lot of cause for concern.

And BTW, does anyone else's speedo needle "bounce" at high speeds? Anything above 60 or so and it does a small bounce endlessly. Maybe bouncing up 1-2 mph or so. The faster you go, the more bouncy it gets.

 

well it was a really easy fix, it was just low on coolant. I should have known better to check that first I looked at the overflow tank it looked like it was good but when I opened the radiator cap it actually needed a bit of coolant to fill it up the whole way. stays nice and cool now.

yeah my speedo jumps a little bit on the higway too nothing too major though i can live with it, not sure why it does it though

 

It's not just Ford, and it's nothing new. Factory gauges have never been known for being accurate or reliable, primarily because the manufacturers don't have enough faith in their customers to use them properly. People get all worked up because the oil pressure gauge points to a different spot on the gauge in their new vehicle than it did on the gauge in their old one, so something MUST be wrong, and they bring it in for warranty service that it doesn't need. If it was up to the manufacturers, no vehicles would use gauges, they would all have idiot lights, but some customers feel better if they see a needle move, so many now install idiot lights that move a needle on a fake guage, rather than than turning on (or off) a light.

When servicing a vehicle, you should NEVER rely on the factory gauges for information. I don't know a single person in my line of work that does.





Actually, that makes a lot of sense. When you turn your heater on, your cooling system fluid level actually goes down, and if you are already low, this will cause an air pocket to form up top, near the gauge sending unit. When there is no fluid, and only air, affecting the gauge, it responds much more quickly, and would appear to be overheating almost instantly.

Air pockets are bad.