Temp sensor broken?

Hi Folks,

I have a 2003 Jetta TDI at 106k miles. The car has been running super-smooth since I got it, consistently getting about 43-47 mpg.

Question, I got my timing belt changed and water pump replaced at 100k miles, just a few months ago. The job went smooth, I took it to a "trusted-list" TDI mechanic who only does VW.

Ever since the car-work, I have noticed that my car's temp gauge will raise up to 190 like before-the-TB-work, but then, once I start to cruise at 65 mph for my daily commute, the temp gauge will fall to about 100-120 and stay their [with very-minor and slow fluctuations up and down].

If I rev the engine, go up large hill, or overtake someone, my temp will rise to 190, but then quickly fall back to 120.

My fluid levels are all normal and I have no leaks.

Before I got the car-work done, my car would take about 7 minutes to reach 190, but would stay at 190 for the entire 40 mile trip to work.

Is my temp sensor or something else jarred, dirty or broken?

Thanks,
Fabian.

did the new water pump have a plastic or metal impeller?

and my next thought concerns the thermistat.

This is the waterpump i got:

http://www.metalmanparts.com/displayProductDocument.hg?productId=69&categoryId=55

My failing temp sensor did the exact same thing, makes you think the head gasket is bad Floor it to pass a guy, falls off the chart, same deal.

Anyone have the how-to on replacing the sensor?

Thanks,
Fabian.

In the how-to section I think.

http://forums.tdiclub.com/showthread.php?t=60705

http://www.inmud.co.uk/page28.htm

It could be the thermostat stuck open...it would reach normal operating temperature at slow speed town driving and once you reach steady state on the freeway, the increased airflow could cause it to drop a little lower. However, in my experience this is unlikely...when thermostats are stuck open (I actually had one break at one of the two supporting arms in my Dodge Ram) the temperature gauge will usually remain at the lower end of the scale if it reads anything.

More likely than not, listen to "slave2school"

Slave2school,

Did your gauge ever go back to showing the right temp after it started acting strange, without trying to fix it?

Thanks,
F.

Flamaest, if you do use an infrared temp gun don't be suprised if your temp is not the 190 deg. stated by your temp gauge. These gauges seem to show 190 even though the temp may be 175 to 195. I think the computer holds it at 190 when it is relatively close to keep the driver from worrying when in reality the temp is fluctuating, at least this is what my ScanGuage II shows.

ahh, 175 to 195, ok, i will remember this...

F.

I tried the TEMP-Sensor-replacement instructions this past weekend, and found that a few changes to these instructions would further reduce the amount of coolant loss through the procedure.

http://www.inmud.co.uk/page28.htm

My version:

Open the coolant reservoir to remove line pressure in the system by unscrewing the blue coolant reservoir cap. A few seconds later, close the coolant reservoir cap.

Thoroughly clean a 2 liter bottle of soda and cut out the bottom so the side-walls are about 1.5 inches high.

Place this soda-bottle-bottom under the sensor plug by gently pushing aside the hoses around the sensor. The space is tight, so the bottle will not move once in place.

This bottle-bottom will catch the pint-or so of coolant so it doesn't spill all over the engine.

Next, Pull off the sensor-plug [leaving the temp sensor in place]:
TIP: Try removing the MAF sensor to see how the plug "closure latch" works.

HOW-TO: Push the plug towards the sensor first, then try pulling/squeezing the "closure latch" away from the sensor, similar to the MAF sensor. This will release the latch and the plug will start to wiggle off.

Remember the direction and orientation of the plug, the release latch should be on the top.

TIP: You may want to clean the plug with some compressed air to get any dust off the sheath.

If the plug looks clean, place the new temp-sensor into the plug with the new o-ring installed and ready to go. The plug-latch should click and/or seat firmly in place.

Next, remove the plastic PIN which is holding the old sensor to the engine. The pin can be pulled gently downwards with any small plier. Once the PIN is out, the sensor should still not fall out [line pressure was removed in the first step].

The PIN is plastic and should be treated with care, unless you got a new one; set it aside to be reused.

At this stage both sensors should be in the car, one in the engine and one connected to the sensor plug.
With a single hand, and fingers around both sensors, pull the old sensor out the engine making sure you pulled the old plug and old o-ring [it should be very easy to pull off], and switch in the new sensor with new o-ring.

This can be done in a single motion to reduce the amount of lost coolant to almost a few table spoons.

Ensure the plug is seated snugly and refit the holding-PIN.

NOTE: You should be able to feel the PIN into place using the groves where the pin seats itself into the engine. Why? Sometimes the PIN may seem to be in place buy the inside PIN-leg may not be in the groove; you can feel for this with you fingers after the PIN has been put back in place [to see if you need to re-seat the PIN].

Next, remove any coolant from the Soda-bottle-bottom using the turkey bastor. If you only lost a little, just discard it. Remove the Soda-bottle-bottom and discard.

Next: Look at your coolant reservoir level and confirm that the coolant level is still around the MAX mark. If not, you may need more coolant.

When refilling the coolant reservoir and closing the coolant reservoir cap, you can help the coolant move around by gently squeezing the main coolant hose a few times [right behind the fan, near the battery] You should hear coolant gushing through the system. The will help get any bubbles through the system.
Clean off the coolant sensor area. Once dry and clean, put a shop-towel snugly under the coolant sensor plug to ensure coolant is not leaking. Wait a few minutes.

Remove the shop-towel from under the coolant sensor plug.

Drive around and confirm the new temp sensor is working properly, the engine and dash-temperature-gauge can take 6-10 minutes to reach 190, depending on the outside weather.

As a general precaution, periodically recheck your coolant reservoir levels for the next few weeks and gently wiggle the temp sensor plug to make sure things are in place.



Thanks,
Fabian

My sensor worked fine but only when it "felt" like it. For $5 it'll be alot less trouble to just swap the sensor and see what happens (if you ask me). Sorry for the tardy reply, on vacation

I alwasy seem to use too many words..

Someone else wrote down a simple version of my novel:::

Let a warm engine cool down for about 2 hours.
Next , open the coolant reservoir to release any pressure, then close it
again (so the coolant won't run out so fast when you remove the sensor).
2. locate the sensor and remove the electrical connector (squeeze really
hard and pull)
3. get the new sensor ready, complete with o-ring.
4. Pull the plastic retaining clip which holds the sensor in straight down
to remove.
5. as fast as you can, remove the old sensor, make sure you got the o-ring
out, and put the new sensor/o-ring in it's place.
6. replace the clip and the power connector.


Enjoy,
F.