coolant temperature sensor problem

[doublegarage]

S4/B5 with 160,000 miles. Original owner, original turbos, well maintained etc etc.

My coolant temp gauge went "erratic" about 3 months ago. Usually reading colder than "12 o'clock" where it used to be rock solid. Doesn't warm up as fast as it used to, then "cools off" and recovers on long hill ascents and similar. Oil temp looks completely normal the whole time, usually running very cool, rises for long drives uphill, cools down again on descent. The thermostat was changed along with water-pump and timing belt at 90k miles - so it's due again.

But, I thought I would change the coolant temp sensor in case it was this $30 part that was at fault. Nothing is as easy as you hope...Bought the new part at the local Audi dealership. I removed the air pipe for a better view of the rear water-rail where the sensor fits. I snapped the plastic clip immediately, got the remains out and pulled out the old sensor, which does not look good at all. I imagined it dipped in coolant the whole time, but it looked oily all over the sensor-tip and the pipe seemed only about "half-full" of coolant. I'd also thought the that coolant would run out of the hole once I removed the sensor (since the expansion tank is higher than the sensor) but that didn't happen. Does this suggest there's a blockage somewhere? If so, where? why oil on the end of the sensor?

The motor does have oil leaks which I need to work on, and it's possible that oil is leaking down on to the back of the sensor, then past the o-ring - but I don't think so.

Last of all - the dealership sold me completely the wrong sensor, different connector shape etc, so I had to put the old one back - but now I'm thinking its erratic reading is due to not enough coolant in that pipe and NOT due to a bad sensor.

Any thoughts on any of that?

Thanks,

-Richard
Scotts Valley, CA

[mholme]

Did you try to change the sensor on the passenger side or driver side?

[Master]

It's definitely the thermostat that won't let the clock stay on 12 O'clock and won't let the car warm up properly
Just replace the thermostat and the water pump and flush the radiator along the way the you should be fine,

Also if the CTS was faulty then most likely your engine should have randomly misfired and you had a check engine light on!

[mholme]

No, it's not definitely the thermostat. It sounds like he's working with the wrong sensor. There's one on either side of the coolant pipe that have different functions. One is for the after run feature and the other relays information to the gauge and ECU about temperature readings. He's on the right track starting with the easier fix.

[Master]

He's on the right track starting with the easier fix.

I Couldn't agree anymore, Always go with the easier/cheaper fix first, but with the sludge or oil that he saw in the coolant & considering 70k miles on the old thermostat, it makes it beyond just a sensor.

[mholme]

I Couldn't agree anymore, Always go with the easier/cheaper fix first, but with the sludge or oil that he saw in the coolant & considering 70k miles on the old thermostat, it makes it beyond just a sensor.

Ehhhh...we'll see. I've seen thermostats fail open and closed, so I'll wait for him to respond. I just reread his post- Looks like he removed the correct sensor, but may have gotten the after run sensor from the dealer.

[Dayne]

I was having the exact same issues. First thing I replaced was the "green top" coolant temp sensor on the passenger side of the engine bay. It "helped" I think but didnt solve. Turned out to be exactly what I had suspected, a thermostat that was stuck open. when I was driving up mountain passes or on the throttle a lot or sitting in traffic with no air hitting the radiator the car would go up to normal temp. However as soon as I was driving normal again it would go back to cold. considering your due for a tbelt job the thermo is likely to be stuck open.

from what I've read this is sensor you replaced:

http://www.ecstuning.com/Audi-B5_S4-...nsor/ES264174/

If so then good!

If not, then replace this sensor first and this is where it goes:





Now with the oil on the sensor. If its black oil chances are you have a leak somewhere causing the coolant to mix with the oil. If this is the case then its possible some of the oil may have made its way to the thermostat and caused the thermostat to go faulty. No garentees but its possible. If the tip just seemed "oily" this is fairly normal because the coolant used in these cars (G-12) is techniqually an oil because its used to lube the system. Don't believe me just dip your finger in some coolant and rub it around between your fingers, you will notice it has a higher viscosity then normal coolant.

Now did the car overheat at all before these issues started happening? If so then it's possible the overheat caused both issues. Overheating can lead to the thermo becoming faulty and ruining some seals causing oil leaks (someone fix this next section if I'm wrong because im not 100% sure) more specifically the head gaskets blowing and causing all sorts of fun, including: oil mixing with coolant.

Just like Master said. Do a timing/tbelt job flush the radiator and do the head gaskets if they're leaking. should solve your problems.

Good Luck!! Welcome to Audizine!

[udcc11]

I would think an oil cooler issue before head gaskets.

I changed that sensor on both cars and the fluid didn't squirt out at me. That seems normal.

[Dayne]

I would think an oil cooler issue before head gaskets.

I changed that sensor on both cars and the fluid didn't squirt out at me. That seems normal.

When I did my sensor I had fluid squirt out. Not very much. but some. My coolant wasnt hot but it was warmer than room temp because I had driven the car a couple hours prior. maybe it was still slightly pressurized and that's what caused it?

[Master]

Do you have your check engine light on at all ? if i recall correctly a faulty CTS gives p1296 code,
again I think it's just sludge (not oil) due to old cooling system component

[Imola34]

The coolant temp sensor, from my experience, will cause very severe changes very quickly. It either stays in one place (not always "12") or, it drops to cold and stays there. The slow warming is also very typical. I could be wrong. The sensor should also throw a code as suggested.

[doublegarage]

Thanks everyone for the replies. To answer the questions asked so far...

- The oil residue I see on the tip of the sensor is black - 99% sure it's engine oil, not sludgy antifreeze. I did wonder whether it could be residue from the sensor o-ring (which was in a terrible state) but pretty sure it was engine oil.
- No the car has never overheated, I've never seen the temp go above 12, except maybe during....
- The only thing that has ever gone badly wrong on the car is an exploding auxiliary water-pump, the front fell off it about 2 years ago. Came to a halt in a cloud of steam and shut the car off - I doubt it overheated, was probably running for max 30 seconds after the "event".
- The one I changed is the sensor on the passenger side of the rear water rail, looks like the one in Dayne's picture. I did my homework and thought I wanted part number 059-919-501. The picture on ECSTuning's site looked like the right one (bright neon green base) - the one I got from the dealership had a dark green base and was in a sealed bag with that part number on it. Looks like the right size/shape but has completely different connector-shape and is of course, different green color.
- No check engine light recently at all.

So if there are two sensors in that water rail, which one drives the gauge? It sounds as though I pulled the right one? I guess I need to get the right sensor and at least eliminate this. Then do the timing-belt job and replace the thermostat.

As for oil in the water - this sensor is the only place I've seen any sign of it - expansion tank looks pristine, nothing creamy on the underside of the oil-filler. (althought that'd be water in teh oil I suppose). Where else would I look for signs of that?

Thanks for all the help - this is great. Good to be back on Audizine, I used to be a member but it lapsed.

-Richard

[udcc11]

After run sensor

Blue or black sensor.

Coolant sensor

[Delmed83]

The one on the passenger side is for your gauge. It is normal for the coolant not to drain out when you replace that one. If you have the reservoir cap closed, when you pull the sensor a little should flow out but that's it. Extract some coolant from your res and inspect the color (unless your res is new like mine and you can see right through it). If that looks good, keep an eye on the oil level. Fix the leaks.

If a new sensor doesn't fix it, then it's gotta be the thermostat.......unless your water pump is shot. If no dice with the sensor, plan on doing the t-stat and water pump together since it's def due anyway with the timing service.

[doublegarage]

The one on the passenger side is for your gauge. It is normal for the coolant not to drain out when you replace that one. If you have the reservoir cap closed, when you pull the sensor a little should flow out but that's it. Extract some coolant from your res and inspect the color (unless your res is new like mine and you can see right through it). If that looks good, keep an eye on the oil level. Fix the leaks.

If a new sensor doesn't fix it, then it's gotta be the thermostat.......unless your water pump is shot. If no dice with the sensor, plan on doing the t-stat and water pump together since it's def due anyway with the timing service.

Thanks Delmed - I got the right sensor at the dealership today - apparently my car (May 2000) is right on the edge of when they switched from one type to another - but yes I needed the neon-green model - thanks also for the pictures from WDC11 above. I thought about it maybe being a bad water pump and that's why this water-rail looks half empty, but if I had no flow at all then the oil temp wouldn't be so stable and low, right? The reservoir is two years old (replaced when the aux-pump broke) and the fluid looks really clean in there.

Too busy filing taxes to do the job this evening though - maybe tomorrow :)

Need to brace myself for a timing belt job - I farmed that one out at 90k miles but I need to learn how to do it. Are there any new definitive DIYs posted (i.e. with pictures) on how to get that done right?

Thanks,

-Richard

[udcc11]

There's a video on YouTube. Even goes through valve cover gaskets as well.

[doublegarage]

Just as a follow up - I finally got the right temp-sensor part and installed it (passenger side of rear water rail)

It does seem like that was the main problem - now my gauge heads up to 12 o'clock and once it gets there, it stays there pretty solidly, but does waver around a bit when descending long hills on cool days

It is MUCH slower to get there - slower than when I had the original thermostat before timing-belt job at 90k - and that one was absolutely 100% rock solid, crossing passes in the mountains or Death Valley - never moved from 12 o'clock.

So in other words, I think there are (or were) two problems - a thermostat that isn't as good as the original one - and a bad temp sensor (now fixed). There's no sign of it overheating, i.e. the thermostat is erring on the side of "open too long" - but yes, I need to do the timing belt job, boil out the radiator and get a better thermostat.

Thanks all for your help.

-Richard