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1998 Ford Explorer Temp Gauge Acting Goofy--6 years later

Six years ago, I had this same issue, and it was resolved.

However, the gauge is acting goofy in the same manner now. Started on 8/22. This time, there is no loss of coolant, and the engine is running just fine.

While sitting in stopped traffic, the needle would jump up to hot, and the check gauge light would sometimes come on for a couple of seconds. The needle would then fall back to the mid-point. This would happen quite often.

When traffic would move, and I would start to accelerate, the needle would head back up, but not as far, and then settle down again.

On the freeway, everything was hunky-dory. The needle would stay where it is supposed to be.

This hasn’t happened when driving around town. Just after many highway miles.

We got home yesterday just fine.

I checked fluid levels today, started the truck, and watched for any sign of leaks, and saw none.

I did notice that the temp gauge was sitting very far below the cold level.

Are you going to tell s how the problem was resolved 6 years ago or should we guess? Also you checked the fluid level and checked for leaks. You saw no leaks but was the level OK? Low? or High? Was the color muddy or the proper color of the coolant?

Based on your description I’d say; failed thermostat, or a failing temp sensor, or a failing temp gauge, or a failing head gasket forcing combustion gasses into the coolant - so not leaking - but causing air pockets in the cooling system.

If you post answers to my questions, my diagnosis might be different (read-less expensive) :smile:

Six years ago:
There is some sort of housing on top of the engine. A by-pass hose runs from it to the pump. I dunno what it’s for. Anyway, this crummy plastic housing was cracked, and that’s what caused all of the coolant to leak out.

Coolant level was good, and the correct color. No sign of contamination.

Overheating when stuck in traffic is often caused by the radiator fan not turning on when it should. Next time you notice this, stop and see if the radiator fan is spinning like crazy, it should be.

The housing you are talking about in reference to the problem 6 years ago is probably the thermostat housing. If the photo of the gauge above was taken after the engine had recently been running and warmed up, very likely a gauge problem is involved. Even if the thermostat was stuck open the gauge would still move up a little with the engine running.

My guess is the problem is one of these

  • coolant temperature switch for radiator fan or fan itself faulty
  • radiator cap isn’t holding pressure
  • thermostat faulty
  • air in cooling system
  • water pump malfunctioning
  • radiator needs replacement
  • head gasket starting to fail

If I had this problem I’d start by making sure the radiator fan is spinning like a banshee when the gauge shows overheating, and if it was I’d replace the radiator cap (do that when engine is cold).

The photo was taken less than 5 minutes after I started the truck in my driveway.

Far as I can tell, the fan is belt-driven, and has no “on/off switch” like I’ve seen in other vehicles over the years.

Ok, forget it’s a larger vehicle, many of those don’t use electric motor operated radiator fans. They use a fan operated by the engine but controlled by an electric clutch. You should still make sure it is spinning as fast as it looks like it can, when the temp guage shows overheating. This design sometimes allows the fan speed to be dithered a little higher, a little lower. Special equipment is required to verify the fan is turning the correct speed.

If the truck had just been started, the gauge as shown in the photo above is probably normal.

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OK. Next time I’m in town—probably Monday—I’ll get a new radiator cap and try that.


Got a new radiator cap, and things are still the same. I have noted that the gauge shoots up under certain conditions that I hadn’t noticed earlier. To wit:

While blowin’ & goin’ down the freeway, everything is good. Gauge is where it should be. After coming off the freeway, even after just 15 minutes (the time it takes me to get to town from home), and slowing down to town speeds, the thing will shoot up, and bounce up to max, and down to normal. This will continue until I get back on the freeway.

All that said, it didn’t happen yesterday. We went to Seattle (about 60 miles on I-5), got off the freeway to drive the last bit on surface streets. The gauge stayed in the normal range then, and all the way home. From the highway to home is about 10 miles.

The only major difference was the weather. It was much cooler, and some rain here and there.

what does this mean?

Slang for driving

Was temp sending unit, thermostat, or water pump replaced at that time?

How was it “resolved”?

No, no, and I don’t think so.

A plastic housing that sits on top of the motor was cracked, and that’s what caused all of the coolant to leak out.

Well, you might as well replace that stuff, and when that doesn’t work, it’s head gasket time.

I’d try these ideas before considering the head gasket as the culprit.

  • Remove all the air from the cooling system. On some cars all you have to do it put the front wheels on a ramp, open the radiator cap, and idle until the thermostat opens. But that method doesn’t work on all cars, so follow the manufacturer’s recommended procedure.

  • Replace the thermostat.

  • Replace the water pump.

  • Chemical test for exhaust gasses in the coolant.

All fixed.

The thermostat had died, and took the housing with it. And, a bypass hose had succumbed to age.

Thanks for the help, everyone!

Good for you for sticking with it, getting a correct diagnosis, and a presumably inexpensive repair. Glad you are back on the road with a reliable ride.