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Steve, Auto Service Technician
Category: Car
Satisfied Customers: 5494
Experience:  25+ yrs experience as a professional working technician; ASE L1 master technician
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Why is my car engine temperature running low?

Customer Question

Why is my car engine temperature running low? It does not reach the operating temperature. What do I do?

Submitted: 9 years ago via
Category: Car
Expert:  Steve replied 9 years ago.


The engine temperature is controlled by the thermostat. The thermostat closes to shut off the flow of engine coolant through the radiator until the engine reaches the temperature at which the thermostat opens. If engine temperature is not coming up to 195 degrees, then there is coolant circulating through the radiator for some reason... This can be a defective thermostat (out of calibration/ opening at the wrong temperature/ not closing fully) or by coolant leaking around the thermostat inside the housing because of an incorrect diameter or a faulty seal. The problem is definitely in that area, because the thermostat is the only component involved in causing the engine to reach normal temperature. All of the rest of the cooling system's parts are there to cause the engine to loose heat after it exceeds normal temperature.

To be able to help you in diagnosing this, we will need some more information:

(1) What temperature is the engine reaching at it's highest?

(2) How are you measuring the engine temperature?

(3) Are there any other unusual symptoms? How did you first notice the engine temperature was below normal?

(4) What brand of thermostats have you installed? What temperature measurement did you obtain before and after the replacement?

Any further information you can provide may be helpful.

Customer: replied 9 years ago.

At idle, the engine reaches 150 F on the highway. It is steady around 120 F. I have measured engine temp, using a scanner, infra-red thermometer(compared to a normal operating car), and checked against sensor resistance. I am very confident that this is a mechanical problem, and not electrical. Complaint is not necessarily the low temp reading on the gauge but the check engine. Light is set for engine not reaching operating temp. I have installed Stant, GM, Murray thermostats. I then installed a thermostat out of a vehicle that is operating properly. I have had the same results with all the thermostats installed.

Expert:  Steve replied 9 years ago.

I agree that it is mechanical. Obviously coolant is flowing through the radiator core because you describe a 30 degree drop in temperature when air is flowing through the radiator. It will be that the upper radiator hose is hot to the touch as well when the engine is not yet at operating temperature. That would be another indication that the thermostat is not shutting off the flow of engine coolant (the coolant inside the upper rad hose comes through the thermostat a few inches away. When the stat is closed to shut off flow, the upper hose will not be hot.

Although it seems unlikely, there is a very good possibility that the thermostat is the cause of this problem...

(1) You are installing the thermostat with the bulb end facing towards the engine?

(2) Have you examined the water outlet (stat housing) to make sure there are no cracks or defects that could allow coolant to flow past the thermostat? It is keeping the thermostat pressed tightly against the engine when it is installed?

(3) This thermostat should have a rubber seal that encircles the edge to seal it inside the housing. If the rubber seal is missing then coolant will flow around the thermostat when it is closed.

Customer: replied 9 years ago.

The thermostat can only be installed in one direction on this car. So I am certain it is installed correctly. I have examined and compared to another intake and thermostat housing and no defects were found. The rubber seal is also installed on the thermostat. Without that seal I am sure an external leak would occur.

Expert:  Steve replied 9 years ago.

Then by default the only possible cause left is an out of calibration thermostat. There simply is nothing else that can cause an engine to run too cold. If you remove the present thermostat and suspend it in a pan of water on the stove along with a thermometer, you can check to see what temperature it is opening at. You should not see any movement untill it reaches 195F.

I would suggest installing a genuine GM thermostat. In our shop we have had many issues with after market brands and no longer sell them for that reason.

Customer: replied 9 years ago.

In my previous reply I had mentioned we used a genuine GM thermostat. I have taken the thermostat directly out of this car and installed in another and it operated properly.

Expert:  Steve replied 9 years ago.

Have you checked calibration on any of these thermostats?

Customer: replied 9 years ago.

Verified operation in another vehicle. Same make model and engine.

Expert:  Steve replied 9 years ago.

All I can suggest is to measure the calibration of the current thermostat as I described previously, that will either confirm that the thermostat is opening at 195 or disprove it.

Another thought, any chance of a head gasket leak allowing gas bubbles to collect in the head cooling passages? If the hot liquid coolant is not contacting the temp sensor, it will cause the ECM to read the temp too low.

Customer: replied 9 years ago.
p0128 Coolant Thermostat is the only code I have ever retrieved. The cooling system is building proper psi, no coolant loss, and no over psi. I did compare the two temp readings (scanner and actual eng) are similar and considerably lower than a similar make and model car.
Expert:  Steve replied 9 years ago.

You sound as if you may be a professional tech; do you have access to an airlift/ vacuum cooling system filling tool? Also, you didn't happen to put any kind of sealer or teflon tape on the coolant sensor threads by chance?

After each time the cooling system is opened, have you performed the cooling system bleed procedure?

Customer: replied 9 years ago.
I do have an air lift, and I did follow the bleed procedure. The level in the resevoir keeps a consistent level. I dont feel like there is an air pocket trapped due to the actual temp readings taken from the engine are lower than a normal? I guess I was just hoping some one had ran into this issue before, when a thermostat didnt fix the problem.
Expert:  Steve replied 9 years ago.

Really, there is no other possible cause for the engine to be actually running too cold other than a faulty thermostat. You can easily test the thermostat by boiling it as I described. I have checked through many repair databases and for TSBs; every single hit in the repair databases (dozens!) agree that a thermostat is the only possible cause. All of the other techs in our shop agree as well. Many things can cause an engine to run hot, but the only thing that can cause it to run too cold is a thermostat.

So, knowing that there are only a couple of possible explainations for what is happening in this vehicle:

(1) You may somehow be measuring the engine temperature incorrectly, the ECM is getting a low temp reading as well due to high resistance in the CTS circuit, an air pocket in the head, or an internal ECM fault,, and the 2 seem to agree.

(2) You may have gotten a string of out of calibration thermostats (the odds are you did not, but the only way to know, is to do the test to verify that the thermostat is operating normally)

(3) Coolant is somehow bypassing the thermostat.

Logically, there are no other possible causes for what you are experiencing...

Customer: replied 9 years ago.
Just tested the thermostat in a pot of water. I performed as suspected. Opened around 200f and closed promptly when removed from the water. I cycled the thermostat several times, all with the same result.
Expert:  Steve replied 9 years ago.

Thermostat should open at 195; 200 is actually a little late but that is warmer than normal so I would agree the thermostat is not causing the problem. Now we know for sure, instead of assuming that it is OK.

That brings us back to the other possibility; the ECM may be incorrectly reading engine temperature. The next step is to reinstall the thermostat, refill and bleed the cooling system, and bring the engine up to operating temperature. Then, once coolant has started to circulate in the system (thermostat has opened) use a contact type thermometer to measure the temperature of the engine at the water outlet. You can use a non-contact type (temp gun) for this, but I have found that these often are off by as much as 10 degrees from the measurement taken with a contact type thermal probe; so for the type of problem you are chasing that amount of inaccuracy isn't acceptable. Once we have an accurate temp reading and have verified that the engine temperature is reaching 195, we can then check temp sensor resistance with an ohmmeter to see if it is close to what it should be at that temperature.