Engine temp in winter

1993 Camry 4 cylinder

So it’s been really cold here in Michigan recently. Low temps in the single digits and highs in the teens.

My temp gauge is erratic. The engine will warm-up to a normal temp in roughly normal amount of time but in a few minutes the gauge will fall. (I’s say that the normal position is about 48% and the current “cold” reading is, maybe 30%)

I originally thought this was a failing thermostat even though the thermostat is only six months old (OEM part)

Now I’m thinking that the very cold weather is my problem. I think that the water in the radiator is so cold that when the thermostat opens it causes the engine temp to drop suddenly.

Should I change the thermostat or just move to a warmer place?

I live in a climate somewhat colder than yours. In the old days, companies made detachable front cover for the grill, with little openings you could uncover for highway driving.

Today’s grills don’t lend themselves to those, and I would recommend getting the hottest thermostat for this engine, and installing a block heater when the overnight temperature really drops.

In spite of what you think, you may actually still have a bad or incorrect thermostat. I’m assuming the temperature gage on the dash works, and that when you are driving a fully warmed up car you are getting lots of heat out of the heater.

I’m not sure how your temp gauge is designed to work but your coolant temperature does actually vary depending on what is going on. It will, for example rise while the car sits still at idle. It will continue to rise until the cooling fan kicks on or you start driving - which does the same thing as the cooling fan by forcing air through the radiator. If your cooling system is working normally the coolant temp will likely fluctuate between about 195 - 240 degrees. So are you sure that you’re not just watching normal fluctuations? Some temp gauges show them, some don’t.

Have your coolant checked!! Should be a 50/50 mixture…good to -34.

This is in line with what I’m thinking. I’m pretty sure that the gauge is not super-sensitive. The engine temp can fluctuate somewhat without moving the needle on the gauge.
I think the very cold air makes the “normal” fluctuations large enough that they do move the needle.

Does the gauge return to “normal” after the drop? If so there’s no problem. I think you’re seeing the thermostat open.

When the gauge drops the thermostat should close again, or partly close, to allow the coolant in the engine to reach normal operating temp. This cycle should repeat until all coolant is at normal temp.

Right, the temp does come back up to the typical 48% indication and then it’ll cycle again. So I agree with you that this is just the thermostat opening as normal.
Weather forecast calls for temps in the upper 30’s this weekend (Woo-Hoo!) I’ll see if the thing behaves better then.

Check The Coolant Level In The Radiator And In The Coolant Recovery (Overflow) Tank.

Low coolant / air in cooling system can cause fluctuations. If the coolant level is good, I’d try purging air from the system. I hate to think like this, but if the coolant level is low and causing these temperature indication “swings” then a failing head gasket would not be out of the ordinary on an old car like this.

Have you had the car for some time ? Has there been any repair or maintenance involving the coolings system recently ? Is this fluctuating gauge something fairly recent ?
How’s that coolant level ?


I’ve had this car for 18 months. I bought it from the original owner. He had all the service done at the dealer.
No cooling system work done recently. The last time I did anything with the cooling was the new thermostat last July.
The gauge has been rock solid in the middle until last weekend.
I haven’t checked the coolant level yet, hadn’t really thought about coolant getting out the tail pipe. It doesn’t leak coolant so I don’t look at the level very often. I’ll check that tonight.
I just got my tax refund and I really don’t want to spend it on a head gasket job!
I did some research on the symptoms of failing head gasket and the one thing that seems consistent is bad head gasket causes over-heating.
I don’t have that, I have under-heating. So I think I’m going to dodge the bullet on the gasket failure.

You may just be seeing the thermostat opening, but do keep an eye on this situation. I had a Chevy truck do this to me for a while. The gauge would creep up to a higher than normal reading, then suddenly drop like a rock before settling down. One day it hit the red without dropping at all. I parked the truck, called for a ride, replaced the thermostat in the parking lot where I left the truck, and never had that issue again. You could have a defective thermostat, or something is causing it to stick shut longer than it should.

" . . . the one thing that seems consistent is bad head gasket causes over-heating." “I don’t have that, I have under-heating.”

When air gets into the cooling system through a leak in a hose, seal, gasket, etcetera, then the sender for the gauge is sometimes no longer sensing coolant temperature (air pocket) and sends false information to the gauge. The air can keep coolant from circulating and it’s possible for a car to have little or no heat available in the passenger area (or fluctuating temperature), and a low temperature indication, while the engine is over-heating.

I’ve replaced head gaskets before on cars with those exact symptoms.

Anyhow, check the coolant level and get back to us. If not, you’ll need to fill it with the correct mix and monitor the level carefully to see if it drops again and to see if the gauge stabilizes.

I hope the level is up to full.


The thermostat and the engine fan or the AC fan could also contribute to a fluctuation like that. The gauge failure is usually due to the temperature sensor or its connection which may need cleaning. At least your heater seems to be working well enough. That’s a good sign for everything but the sensor that is making the gauge go all foolish.

Thermostats are not switches, they are analog devices. They open and close smoothly and the amount they open is related to the temperature level. They react fairly slowly and so dampen the reaction to change. If the rad coolant temp is super cold and the engine warm, the thermostat will only be open a crack to meter in enough cold coolant to regulate the temp. Yours is acting like a switch because it is sticking or opening erratically.

It will.

BTW you’re probably also turning the heat on as the temp gauge reaches normal the first time. When you do that, you’re dumping an extra load of cold coolant into the system as the stuff that’s been sitting in the heater core is now part of the coolant loop.

OK, today is a nice warm day. The temp is above freezing and the sun is shining. Everything is melting.
Driving my car, the temp gauge still reads low. That eliminates the cold ambient tempearature theory. That was really too much to hope for anyway.
Thinking about my options; I know that the thermostat is only eight months old. I’d be surprised if a thermostat went bad in such a short time.
On the other hand the sensor is probably original, 17 years old. I can imagine that it could be used-up.
In addition, the gauge does move. It just doesn’t move the way I think it should.
So I decided that I would just replace the sensor. See what happens.
The Toyota dealer had a part number but didn’t have the part on hand. Cost $30. Well, since I’m guessing I didn’t really want to bet $30 on this guess.
Called NAPA. They had one for $12.
Installed the part. Started the car and the gauge went right up to the middle, the normal zone.
I’m thinking "OK getting somewhere now"
But then the needle kept right on climbing. In one minute the needle was on the Hot peg.
I’m sure that the engine isn’t really that hot. It hadn’t been running long enough to reach that kind of temp.
So I think the sensor is the likely problem but the NAPA part may be wrong or somehow faulty.
I replaced the NAPA part with the original and returned the part to NAPA. They were very good about it. Gave me a refund even though I told them that I had installed the part.

Going to try a Toyota part.