Temperature Fluctuations

[Delta]

Car is in question is my 08 Jetta. After moving I've been having to do alot of driving up and down mountain passes to get to work or shop. Thing is, when I go down, say 8% grade, my temp gauge goes from the normal (190) to almost the 120 mark. I still have the same amount of heat. I can only assume if the computer is reading a lower temp it's dumping more fuel. After leveling out the gauge will go back to the normal range. Coolant is right at the min mark, I've been thinking about adding a little more, but not sure. This has done this since I moved back to PA back in March, but now it's an everyday thing. Any guesses from you guys?

 

[kschachn]

Well the engine isn't producing nearly the same amount of heat at a low load than it is at a high one. And by "dumping more fuel", what is it that you mean by that? Going downhill the engine may not be much above idle and is neither using a lot of fuel nor producing nearly as much heat as it would be going up the same hill.

Also, the thermostat operation is not instantaneous so if it was open wider to compensate for going up the hill, it will not close back up right away. But if the temp comes back up from 120 in a reasonably short period of time then you should be good to go.

 

[Rand]

Bad thermostat?

 

[CT8]

The heat rise is due to the higher amount of heat generated by the need for increases power going up the hill and going down the hill requires less power so less heat. More power more heat.

 

[Quattro Pete]

Agree with Rand. Your thermostat may be on its way out.

 

[OneEyeJack]

If you're going down hill with no load on the engine your fuel system might shut off all the fuel at times. That's just normal operation. That's part of what makes fuel injection so much better than a carburetor. That carb will keep "dumping" fuel into the intake manifold just like on my Honda.

 

[Delta]

I was wondering if the thermostat was on its way out, I do not look forward to that repair. Pretty much everything on top has to come off for that. The temp comes back to normal within 3 minutes. It does however take forever to warm up in colder (25* or below) and produce and to produce decent heat it takes about 10 minutes, although it is a boat anchor of an engine, so I expect that. I will keep an eye on it, luckily this VW is easy to bleed off air if I do have to do work on the cooling system. I wish I had a VagCom to view live data. I just thought it was all odd, the Equinox, Camry, or Expedition wouldn't fluctuate.

 

[kschachn]

Originally Posted By: CT8
The heat rise is due to the higher amount of heat generated by the need for increases power going up the hill and going down the hill requires less power so less heat. More power more heat.


Heat rise? He said the temp goes down from 190 to 120.

 

[HerrStig]

Originally Posted By: kschachn
Originally Posted By: CT8
The heat rise is due to the higher amount of heat generated by the need for increases power going up the hill and going down the hill requires less power so less heat. More power more heat.


Heat rise? He said the temp goes down from 190 to 120.
Most of us made the inference that he meant 220.

 

[Delta]

Yea it seems to never get above the 190* mark even lugging the engine up the 8% grade mountains. Everything might be ok, I might just be "freaking" out. The CEL has not come on like most VW's do when the thermostat goes.

 

[bullwinkle]

Sure sounds thermostat related. It'll cost you extra gas money when it gets really cold out, it'll never come out of open loop, theoretically the extra fuel could dilute your engine oil & damage your catalytic converter. I'd bite the bullet & change it.

 

[rslifkin]

The temp gauge in those doesn't really read actual temp. When it drops to 120*, that's really just saying it's not at full temp. Without hooking up a scanner, there's no way to know if it's at 170* or 120*.

As far as the temp dropping when going down a long, steep grade, that's normal, especially with the cabin heat on. Once decel fuel cut kicks in, there's very little heat being produced.

After climbing a long, steep hill and then descending a 10% grade for several miles (in 3rd gear, ~2800 rpm / 65 mph, in fuel cut), I've seen the coolant temp in my Jeep go from 200* down to 180* (195* t-stat) on a 70* day (cabin heat off). And that's a much bigger, heavier cast-iron motor that takes a lot more to cool down. I could easily see a smaller aluminum motor (maybe 1/2 the weight) cooling down a good bit more in the same situation (especially with the cabin heat on).

Usually the ECU will disable fuel cut to get some heat back in the motor if the temp drops below a certain point. I'd hook a scanner up and see what the temps are doing. If it gets up to normal temp in a reasonable amount of time and stays there when driving and only drops going down steep hills (and isn't an overly fast drop once below t-stat temp), then I'd say it's fine.

 

[HerrStig]

What happens to cooling system load when a semi driver engages the Jake on a long downhill? Or, what happens in the resistance elements on a diesel electric loco going down hill?

 

[Kuato]

What is the location of the coolant temperature sensor in this vehicle?

 

[KrisZ]

Before changing anything, find out if your Jetta has a dummy temp. gauge, meaning heavily dampened, or a real one that is fairly accurate.
If the gauge is not heavily dampened, it is not unusual to see small temp. gauge movements as engine load changes. Most people freak out by this, therefore most manufacturers heavily dampen their gauges to only indicate big temp. changes.

 

[Delta]

Originally Posted By: Kuato
What is the location of the coolant temperature sensor in this vehicle?


IIRC it's by the lower radiator hose in front by the radiator.

And I was wrong about the temp. Pictured here is after 3.25 mi down a 8% grade. Looks to be 150-160ish. 190 is normal, never gone past it much before.



Like said it might not be a dummy gauge. Might be the reason my other vehicles never showed a difference.

 

[Quattro Pete]

Hard to tell from the pic but it looks to be about 170f. Maybe its normal.

 

[silveravant]

My Jetta 2.slow warms up supper fast and the gauge stays rock solid at 190 once warmed up. Scan gauge indicates some temp. movement, but not much. My guess is your thermostat is bad.

 

[Kuato]

Originally Posted By: Delta
Originally Posted By: Kuato
What is the location of the coolant temperature sensor in this vehicle?


IIRC it's by the lower radiator hose in front by the radiator.

And I was wrong about the temp. Pictured here is after 3.25 mi down a 8% grade. Looks to be 150-160ish. 190 is normal, never gone past it much before.



Like said it might not be a dummy gauge. Might be the reason my other vehicles never showed a difference.



So since it's not in the block behind the thermostat, and is instead indicating the temperature in the lower radiator hose, up front near the radiator, then I'd say there is nothing wrong.

When the thermostat closes and there is no circulation through the radiator, the coolant there cools down. IMO you are seeing an indication of a properly operating system.

 

[TFB1]

Originally Posted By: Delta
Originally Posted By: Kuato
What is the location of the coolant temperature sensor in this vehicle?


IIRC it's by the lower radiator hose in front by the radiator.

And I was wrong about the temp. Pictured here is after 3.25 mi down a 8% grade. Looks to be 150-160ish. 190 is normal, never gone past it much before.



Normally one would expect that each hash would be 10* but there is a extra on that scale so each is closer to 8.7* not 10*... Backing up two hashes from 190* would be approx 173*...

A thermostat that's not fully closing is likely the culprit... BUT if the lower hose is the inlet to engine from radiator, I can see how you're getting a false reading as it's reading radiator temp, not engine...