Why is my coolan temp too low?



I own a 2001 VW Jetta TDI, that’s a 1.9L turbo diesel engine, with 170,000 miles.

For the past couple years now, I have noticed that my engine temperature reads correctly on the instrument cluster, but when I plug it into a diagnostic computer program, it reads low. The software I use is called VCDS (formerly Vag-Com), and is by www.ross-tech.com.

The repair manual states normal operating coolant temp is 194F. When the car was much younger, it would always run at this temp, within +/- 2-3 degrees. For the past two years, I have noticed that the car typically steadies out at 172F, regardless of city/highway, summer/winter. Just this past week, I drove a 60 miles, and logged the coolant temp sensor. The average running temperature was 170F, after it had warmed up.

If I sit in the driveway at idle, it will eventually reach this temperature of 194F after about 5-10 minutes.

I have always noted that the hose to the radiator, downstream of the thermostat is always cold, unless I sit idling in the driveway. At this point, I can watch, as the hose gets warm right when the thermostat should open (187F according to the repair manual).

Does anyone have any idea why my coolant temp would be so low? I’m just worried that I could be getting an extra mpg if it was running hotter than it is. MPGs are within the normal range though.

Some notes to answer questions regarding this problem:

1. I had replaced the coolant temp sensor a year ago because the dash gauge stopped working. I had witnessed with VCDS that the temps ran cool before and after the replacement.

2. I have never replaced the thermostat.

3. I don’t block the radiator. Some have recommended this, but my low temps are observed in the hot summer and winter.


Why havne’t you changed the thermostat? It’s probably stuck wide open, allowing too much flow. That is what you should do.


Agree; the only thing that can make your car run too cool is a thermostat stuck open. Blocking the radiator is a poor “solution”. Replace the thermostat and then see what temperature you run at.

I have worked in the Arctic where it’s -50F and we did have little curtains to block the air flow through the grill. This was done to make the engine heat up faster and to keep the heater hoses warmer so the cabin could be comfortable. The engine itself did not need these once it was warmed up.


Thanks for the replies guys. I have tried to backwards “test” whether the thermostat is stuck partially open. This is what I have done, tell me what you think:

With the engine warmed up as high as it will go (~172F), I sat in the car in the driveway with it idling. I had the car plugged into my diagnostic software, so I could discretely monitor the actual coolant temperature.

Every time I saw the temperature go up 1 degree, I’d go under the hood and put my hand on the coolant hose that connects from the engine block to the radiator. The thermostat is right at the engine block, and coolant won’t flow through here unless it’s open (obviously). At 172F, this hose is cold to the touch.

As the temperature increased, the hose remained cold to touch, until the coolant was measuring 187F. Then this hose got hot, much much hotter. 187F also coincides with the temperature that the repair manual says that the thermostat is designed to open at.

So this is why I haven’t replaced the thermostat, because it “appears” to be working correctly.

Am I still missing the point here?


Am I still missing the point here?

Yea. You are doing it the hard way. Try replacing the thermostat and see what happens.


Agree with Joseph; your temperature is supposed to go up to 212F or better. Tody’s cooling systems run hot to maximize efficiency.

Just replace the thermostat with a good new one specified for your car and you will get to 212F or better.


Just curious, where do you live? If you’re in Winnipeg, a TDI won’t get warmed up in the winter even with everything operating correctly.


I live in Massachusetts. Not quite as cold as Winnipeg.

I think I’ll take the advice and just replace the thermostat. Thanks guys.