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1993 Dodge Caravan -- temp gauge goes up and down

Normally, the temp gauge in my 4-cylinder 1993 Dodge Caravan stays a half-division below the middle, once it warms up.

Recently, though, it sometimes goes all the way to hot and then back down to the middle, taking maybe 5 seconds to go up and then 5 seconds to go back down. This seems to mostly happen after the car has been sitting for a long time (like a week), and when it happens, it only happens once, usually around the time the engine gets warmed up.

I also noticed this weekend that it would frequently go up a division or so and then go back down again, again, taking maybe 5 seconds. It happened maybe 10 times in a 50-mile trip, often (but not always) when going uphill.

I’ve checked the coolant level – the reservoir is close to max. I also checked that there isn’t an air bubble by loosening the radiator cap.

Any idea what this is? Thermostat? Fan (or fan relay)? Water pump? Faulty temperature sensor? I don’t know enough about how the cooling system works to hazard a guess.

I would say that the thermostat is probably sticking after it sets for a few days. I would probably change it out since it may stick permanently some day and that would be bad for your engine. If you didn’t notice it…it could cause some serious overheating.

That’s what I thought about the temp going up when the engine was warming up.

But then I saw it going up and down (but not all the way to hot) long after the engine was warm (even after an hour of driving continuously at 60+ mph.)

Does the thermostat also adjust the temperature after the engine is hot? Or is that a different problem?

Theoretically…the thermostat would begin to close once the coolant temperature reached a point below the rated temperature of the thermostat. Since the interior of an engine is always subjected to heat from various sources…I doubt that that ever happens much at all. The cooling fans for the radiator switch off and on (if electric) and driving the car provides coolant cooling by the airflow passing through the radiator.

Given that I didn’t get below 45 mph the whole time, I doubt it matters whether the radiator fan is on or off.

First, X2 missleman on changing the thermostat. When reassembling the 'stat be sure to use the same orientation (bleed hole or pin up or down, etc). After the 'stat has been replaced, correct the coolant levels in the reservoir and the radiator (if the radiator has it’s own cap), and drive the van for several warmup/cooldown cycles and recheck the coolant levels.

When warming up, and driving, the gage coolant temp should rise to a level (the thermostat opening temp) and then level off. Once warmed up, the gage indication will hardly vary at all, even up or down hills.

The pin (or hole), when assembled up allows air to bleed out of the block and head(s) on first fill, minimizing the need for burping.

On a 93, I’d definitely replace the t-stat with this symptom. Also have a shop pressure test the radiator cap. And verify that the engine cooling fans are coming on at the coolant temperature they should.

I’ve placed two marks on my Corolla’s temp guage. The first is where it sits when I’m driving on the freeway, after it warms up. That’s the t-stat temp, 180 deg. The second is where the engine cooling fan comes on when I’ve been idling for some time. That’s set by the coolant fan switch, 190 deg.

I agree with you, @GeorgeSanJose, except I would just replace the rad cap and not even bother to test it. They’re inexpensive enough. Get a jug of full-strength coolant and a gallon of distilled water, do a flush-n-fill, and check the hoses, replacing any that look suspect. On a '93, there could be a few.

Years ago I found that probllem because the thermostat was installed upside down. the temp would peak on hot and then open up a work normally. This is a first time start and would work normally for the rest of the day.

You cannot completely vent the cooling system through the radiator. Your engine has a threaded pipe plug in the thermostat housing, and you need to remove it to vent trapped air in the cylinder head. Unfortunately, many of these plugs got so stuck that they stripped out the hole for the allen driver. I had a 93 sundance, and I had to drill out the plug and replace it with a 3/8" brass plug.

The symptoms you describe can also be caused by a bad head gasket. Try the other fixes first and see what happens.

I took it to the dealer. They say it’s a bad head gasket. No definitive estimate, but the guy said in the thousands. Does this sound right?

Also, is this the sort of thing that just gradually gets worse, or will I encounter a catastrophic failure if I don’t fix it?

Oh, and the up-and-down thing is getting more frequent. It happened several times on the 45-minute drive back from the dealer. But it isn’t going all the way to hot.

Did you get the head gasket replaced? I would say an 93 is not worth investing thousands into.

You’ll destroy your engine if you don’t fix it. The breech will get bigger as it erodes, the head will cool erratically and warp, making the leak even bigger, coolant being drawn into the cylinders will cause accelerated corrosion, coolant getting into the oil by bypass from the combustion chamber will cause the oil to fail to properly do its job, etc. etc. etc. There is no good outcome.

With any expensive repair it’s wise to get a second opinion. But I personally think it’s time to go shopping. This buggy has just plopped one whole leg into the grave.

I got a second opinion. The second shop couldn’t find anything wrong. He said he did a compression test and it looked fine. If there’s no problem with the
compression, does that rule out a head gasket problem?

I looked at the oil and it looks like it always does after the oil has been in the engine a while. No white smoke.

One site I found says that another test is to take the radiator cap off
and start the engine – if antifreeze shoots out, it’s a head gasket problem.

The overheating never lasts more than about 5-10 seconds, and sometimes when it goes back down it goes down below the normal temperature before returning to
normal. Almost all of the time, the temperature is right on halfway or a half a
division below halfway. There are just these occasional 5-10 second episodes of
heating up.

I’m planning to take it to a different Dodge shop and ask them to replace the thermostat (and give me the old one) and check the water pump. Also check the belts, since the one that runs the alternator and the water pump looks shiny.

Anything else they should check?

don t go to a dealer with a car that old, all they will think about is selling you a new car. have the guy who did the compression test change the thermostat and see if it helps. it should be a cheap fix to just change the t stat.

Does your heater work? If the heater core is restricted or plugged up the thermostat will open late, at 240-250 F engine temp. The flow circuit through the heater is also the by-pass on this engine. You will see the return hose is connected to the cylinder head next to the thermostat, this allows circulation of coolant passed the thermostat during warm up. With out this circulation the thermostat is in stagnant area of the cylinder head.

the above post about the heater core is worth checking along with the t-stat. it seems to fit the symptoms

The T Stat in this thing is under 20 bucks…replace it and motor on. It may take several hot n cold cycles to purge all of the air out of the system after the T stat swap out…so keep that in mind… An air bubble in the system can wreak Havoc on a cooling system…its nothing to play games with either. Removing the rad cap cannot and does not burp the cooling system.

You may even have a burp nipple on the system somewhere…if so USE IT…If not…Fill the reservoir and then idle the vehicle till warm/hot…then shut down until engine is COLD… It may take up to 3 Hot/Cold cycles with a full res… to purge all the air out. Good luck this should solve the problem