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Discussion Starter #1
I just noticed on a long road trip that the temperature gauge takes forever to reach mid-range. It used to get there in 5-10 minutes of idling. Now it takes 45 min. of 50mph driving (mid-40's outside temp) to get to mid-gauge. It will go up faster running the engine at standstill, but then driving (35 deg. outside temp) will actually drive the temp gauge back down almost to the "C" hash.

I put a new thermostat in six months ago.

My only worry is that the thermostat is stuck and if the gauge measures radiator temp instead of engine temp that the engine is actually running hot. I gave the thermostat housing a few taps with a hammer but no change.

The heater air temp follows the gauge, cool-ish until it finally reaches mid-gauge, then warm air.

Coolant level is normal.
 

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What van do you have? Are the fans stuck on?
 

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With a cold engine, remove radiator cap, start engine and see if coolant is moving, if coolant is moving, then you have a stuck open thermostat. Never use "Fail Safe" thermostats.
 

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I just noticed on a long road trip that the temperature gauge takes forever to reach mid-range. It used to get there in 5-10 minutes of idling. Now it takes 45 min. of 50mph driving (mid-40's outside temp) to get to mid-gauge. It will go up faster running the engine at standstill, but then driving (35 deg. outside temp) will actually drive the temp gauge back down almost to the "C" hash.

I put a new thermostat in six months ago.

My only worry is that the thermostat is stuck and if the gauge measures radiator temp instead of engine temp that the engine is actually running hot. I gave the thermostat housing a few taps with a hammer but no change.

The heater air temp follows the gauge, cool-ish until it finally reaches mid-gauge, then warm air.

Coolant level is normal.
I suspect your new thermostat has failed, not unheard of. That's the reason why I never pro actively replace thermostats, if they fail, I replace them, but not before.
 

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Discussion Starter #6
Is the panel gauge measuring engine temp or radiator temp? Do I have something to worry about, or just inconvenient until I can replace the thermostat?

Tomorrow morning I'll try Levy's test when it's cold.
 

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I agree with LEVY; sounds like the thermostat is stuck open. I used one of those FAILSAFE thermostats in our old 2001 Pontiac Grand Prix, and it was doing the same thing as you describe. Pulled the thermostat out and it was stuck open with the little bi-metal pieces that are designed to keep it open if it gets too hot. I used a needlenose pliers and pulled out the metal pieces, and reinstalled it. It worked just fine after that! Engine must have been running a little hot once, with the silicate from the dexcool plugging up the coolant passages in the engine. I flushed out the cooling system as much as I could and swapped coolant to the old green/yellow stuff to prevent future problems.

I believe the temp sensor is on the "front" part of the lower intake manifold, on the engine. Hmmm, are you sure it's not an air bubble trapped in the system? Hopefully it's not a slow leaking head gasket (cylinder pressure to water jacket, putting air in system).
 

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Is the panel gauge measuring engine temp or radiator temp? Do I have something to worry about, or just inconvenient until I can replace the thermostat?

Tomorrow morning I'll try Levy's test when it's cold.
It measures engine temp.
 

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Discussion Starter #9
So it measures actual engine block temp, not coolant in the engine temp? That's good, that would be the only good design.

Anyway, I did Levy's test this morning, and first thing I noticed before starting it was that the coolant level was surprisingly high, almost up to the cap. When I started it, couldn't see any circulation but it slowly rose in level until when I put the cap back on it spilled some. No bubbles or foam.

I drove it a few hundred miles like this when I first discovered the low temp on the gauge last weekend.

Verdict?
 

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Is the panel gauge measuring engine temp or radiator temp?
So it measures actual engine block temp, not coolant in the engine temp? That's good, that would be the only good design.
No, it measures the coolant temperature in the block (not coolant temperature in the radiator).
 

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It measures engine temp.
Not really...

No, it measures the coolant temperature in the block (not coolant temperature in the radiator).
Coolant temperature sensor measures only coolant temperature, not block or head temp. An air bubble can fool the coolant temperature sensor into thinking all is well when the engine is actually overheating. I have experienced this.
 

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Not really...



Coolant temperature sensor measures only coolant temperature, not block or head temp. An air bubble can fool the coolant temperature sensor into thinking all is well when the engine is actually overheating. I have experienced this.
I meant coolant in engine, not in radiator. George cleared that out already, not need for further clarification.
 

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I meant coolant in engine, not in radiator. George cleared that out already, not need for further clarification.
Coolant temperature same in engine as in radiator, if thermostat is working properly.
 

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So it measures actual engine block temp, not coolant in the engine temp? That's good, that would be the only good design.

Anyway, I did Levy's test this morning, and first thing I noticed before starting it was that the coolant level was surprisingly high, almost up to the cap. When I started it, couldn't see any circulation but it slowly rose in level until when I put the cap back on it spilled some. No bubbles or foam.

I drove it a few hundred miles like this when I first discovered the low temp on the gauge last weekend.

Verdict?
Replace the stat again.

What temp stat did you install last time?
 

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Coolant temperature same in engine as in radiator, if thermostat is working properly.
No, very unlikely if not impossible. Why? Unless the radiator is empty, then it will be cooling the coolant before it gets routed back into the engine. Under normal driving operations, the coolant in the radiator is dozens, if not may dozens of degrees lower than the coolant in the engine block and the heads.
 

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Coolant temperature same in engine as in radiator, if thermostat is working properly.
Not sure what this guy is smoking late at night, but this makes no sense at all. It wouldn't be a "cooling system" if temperature were the same on both sides of thermostat.

Not really!
I have to agree!

No, very unlikely if not impossible. Why? Unless the radiator is empty, then it will be cooling the coolant before it gets routed back into the engine. Under normal driving operations, the coolant in the radiator is dozens, if not may dozens of degrees lower than the coolant in the engine block and the heads.
This makes a lot more sense! :)
 

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My van's heat was not getting up to temp a few months back, so I decided to change the tstat......even though I just changed it about a year and a half ago. The current tstat did look fine, but I changed it anyway. Still no change in heater temp, and quickness to get even warm. I was starting to look at info on cleaning / replacing the heater core, even though the temp gauge seemed to be staying well under the normal middle location.

Finally, on a whim, I decided to just change out the radiator cap. And what do you know, I got my toasty heat back!!
 

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There's a tool I have in my tool box perfectly suited for situations like this.

Thermal imaging camera.
It sure comes in handy in a LOT of situations.

Over at my personal minivan repair website I have some videos posted of the 3.3 warming up. You can see the heat propagation and exactly when the thermostat opens.

You can also see cylinders that are running hot or cool and a bunch or cool stuff like that. Even spark plug leads that have higher resistance and relays that have problems. Wheel bearings that are needing grease....all kinds of stuff.

One of the coolest tools I've added in a long time. Highly recommended.
Mine is a SEEK thermal PRO but there are MUCH better ones out there if your pockets are deep.
 

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My van's heat was not getting up to temp a few months back, so I decided to change the tstat......even though I just changed it about a year and a half ago. The current tstat did look fine, but I changed it anyway. Still no change in heater temp, and quickness to get even warm. I was starting to look at info on cleaning / replacing the heater core, even though the temp gauge seemed to be staying well under the normal middle location.

Finally, on a whim, I decided to just change out the radiator cap. And what do you know, I got my toasty heat back!!
I don't understand how replacing a radiator cap would've solved your problem. Plain water boils at 212°, adding coolant at the equation raises the boiling point, so a cooling system with plain water should at least run your system at 212°, well above the 180°-190° thermostat opening point, in other words, even without radiator cap, you should have a good heating tempetature.

Maybe you did something else at the same time?
 
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