Fuel temp, charge air temp, coolant temp???

Hello all. I have a 2001 Jetta ALH TDI with 83,000 miles. Recently I've begun to notice that at least every other day when I get in the car and begin to warm the engine up, the coolant temp gauge on the dash gauge cluster is sporadic. Meaning sometimes ill drive around and it will read zero, then suddenly go to full temp. Or it can also start moving up then stop, then move down, and fluctuate like that. This has happened only 1 other time, about a year ago to date.

If you go to Measuring Blocks, Group 7, it displays fuel, air, and coolant temp. I would think that if you left the car outside overnight, all three of these values should be equal to the ambient air temp, within some manufacturer’s tolerance. I’ve checked these values periodically since I first had this sporadic coolant gauge issue last year and this is what I witnessed. Fuel and coolant temps are usually close in value, but air sensor temp is usually around 5-10 degrees Celsius less. Today for example, my car had been shutoff for 6 hours in constant 28F degree weather. I logged into Vag-com before starting it and the readings were actually as follows (converted to F):
Which is with the car outside for these 6 hours, not in a garage.

Readings in the past have been similar.

I’m going to replace the coolant temp sensor anyways, since at least the sensor portion that displays temp is shot. But has anyone ever read these values in cold weather (40F or less)? Would anyone be willing to take a log of these before starting one morning, and reporting back?

I just don’t understand why:
1. All three aren’t equal?
2. Why fuel and coolant are equal but not to air?
3. Why none are the actual ambient air, even the charge air sensor reading (which is not being boosted with the car off, so it should be equal to ambient?
4. Could this be contributed to hard starting if the coolant temp sensor is reading way above the ambient air like this?

I just have a ScanGauge, which displays "Inlet Air Temp" (I assume this is charge air temp), and "Water Temp".

In the morning, after my 2003 ALH Golf has been parked outside overnight in 40-50F air, I too notice the "Inlet Air Temp" being several degrees (F) colder than the Water Temp. So far I've put this down to residual heat in the block, which by gut feel could easily be a couple of degrees or perhaps more. I've no idea where my fuel temp fits into all of this.

I drive this car to work every weekday, and occasionally notice it being a little slower to warm up Monday morning (if parked over the weekend) compared to Tuesday-Friday. I've been meaning to see if the temperature discrepancy is there on Monday, but, Monday mornings being what they are, haven't remembered yet.

Meanwhile, it sounds to me like you have a flaky Engine Coolant Sensor, a part with a bad reliability rep around here it seems. It's a cheap part and easy to change; why not at least get one on order?

Eddy

They all should be the same

I have looked at Air, coolant, and fuel temps at cold start up for other reasons. They are usually within a degree C or 2 of each other and when I do the math they are near the local ambient temp as measured by other thermometers.

Are you sure you are converting from C to F correctly?

C x 9/5 + 32 = F

Eddy, thanks for the reply...thats weird though don't you think? The only thing I could think of was also residual heat, but it doesnt make sense, especially metal is a conductor and has a high heat transfer coefficient (the opposite of an insulator).

I tried checking the temps after work today, after sitting for 10 hours, and the temp never got above 34 degrees. Fuel and coolant registered 36F while air was at 35F. And the car started respectively, a little better then this morning. This morning was 28F remember, and as a side note the glow plug light did stay on for much shorter.

Thanks as well Jeff. Yes I was converting to F correctly, I actually had my calculator do it.

Either way, a temp sensor will be on the way.

Please keep any input coming guys, especially if youve measuered any of this values before.