04 TDI running too cool?

Hey all, First time post here, long time TDI owner. I have a 'hopefully' simple question, can a TDI engine run too cool? I noticed that my 04 Jetta TDI temperature gauge was running around 130 - 150 and not the normal 190 range today. It is very warm here in CT with temperatures around 88 with high humidity. If memory serves the temperature gauge in my car normally goes to 190 after driving for a while and stays at that temp, today was the first time I noticed it lower, much lower. Is this normal?

I am scheduled to have my timing belt and water pump replaced in a few days since my car has 95,000 miles on it but nothing is currently wrong with my car. I'm just doing standard maintenance for the belt and water pump.

Should I ask my mechanic to check the thermostat or can a stuck thermostat cause strange temp readings or sorry lower temp readings?

Thanks for any info. Doug in CT.

The first thing I thought of too was a failed thermostat.

When they fail, they're designed to stay in the open position, so I would surely check it.

Also - the coolant temp sensors crap out all the time. They occasionally cause improper readings. If it's never been replaced, you might eliminate that while you're in there.

I think if it were mine, I would go ahead with both thermostat and sensor, since you'll have the cooling system open anyway. Those are cheap parts.

When my CTS went out the in dash gauge would climb to 190 and then suddenly drop down and climb back up. My Scan Gauge II on the other hand was a consistent rise with no abrupt changes.

Thanks for the posts everyone. Tonight after getting home I let the car cool down for about 1 hour then when I went back out the temperature slowly climbed to 190 then dropped down to 140 for a minute and promptly jumped back to 190 and stayed there for the duration of my drive. Taking the car out again it went to 190 and stayed there.

Im out tomorrow for a few hours and I will keep a eye on it. I'll take everyones advise to heart and share the info with my mechanic.

Thanks again. Doug

Doug D said:

Thanks for the posts everyone. Tonight after getting home I let the car cool down for about 1 hour then when I went back out the temperature slowly climbed to 190 then dropped down to 140 for a minute and promptly jumped back to 190 and stayed there for the duration of my drive. Taking the car out again it went to 190 and stayed there.

Im out tomorrow for a few hours and I will keep a eye on it. I'll take everyones advise to heart and share the info with my mechanic.

Thanks again. Doug

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My gauge is asking the same way. On a 5 hour ride out to my parents house, the gauge reading was steadily getting cooler as I drove on the Interstate. It would go back up to 190 when on back roads, or during city driving. Does seem more like a thermostat issue, or a gauge issue?

eats--
I would guess that is a thermostat that is starting to open too early.

On backroads/slower speeds you don't have enough air flow to cool below the max. On the interstate, at increased speeds, you have enough air flow that even a small amount of coolant flow cools the sytem below temp.

My auto-Civic used to get horrible MPG in the winter because of a partially failing thermostat. The coolant wouldn't get to full temp, so the ECU wouldn't let the torque converter lock.

But remember, I'm no mechanic, I just stayed at a Holiday Inn last night.

If you place a tdi thermostat next to a plain vanilla thermostat you will not be able to pick the 2 apart unless you knew the casting numbers or some other unique identifier. So then how can anyone in their right mind say that one is designed different from another?????

If you have ever looked at a tdi engine one of the first things that strikes you is that the thermostat housing is in an unusual location compared to "conventional" placement. This system architecture is the only difference in the tdi cooling system than plain "conventional." The valve is placed such that system pressure and flow tends to lift the valve open. That big spring will still tend to close the thermostat when the eutectoid seal fail; it will fail closed. Out of millions of thermostats 1 or 2 springs will weaken with cycling and age and will be easier for system pressure and flow to open the thermostat. Or a small piece of corrosion debris has closed the pressure relief hole making it easier for system pressure and flow to lift the thermostat open. That thermostat is the exact same thermostat you saw before you cae down with extreme onset of Alzheimers. Now that you have Alzheimers you can't see that the thermostat housing is in a slightly different location. Further you no longer know how a simple automotive cooling system works. Now you can't even wipe your own drool.