Coolant temperature gauge dips after 10min drive and returns to normal

[defever]

Good evening B5 drivers,

Couple of things are bugging me with my car; I want to maintain this car as long as I can so I want to keep it problem-free. And I get to learn more about car mechanic stuff.

The coolant temperature gauge (it's a blue square sensor) is becoming temperamental. I did some homework and I now know that this is quite common, either caused by the coolant temperature sensor or the thermostat just above the oil sump.

The symptom is:

• The coolant temperature gauge goes up to 90c within 10min of town driving and all the sudden drops rapidly down to 60c as if something got disconnected.
• The gauge stays down at 60c for few minutes.
• Then it returns up to 90c as if nothing happened. It's fine for another 10min or so until I reach my destination.

I haven't done a long drive since I noticed this so I don't know if this is only a start up problem or if it repeats itself later on.

I learned about the climatronic trick and I checked the figures on 49c (I read that "49c" reading is the live coolant temperature detected by ECU, is this correct?) this evening. The reading is consistent with the gauge behaviour; it starts around 40-45 when I start the car and then gradually increases to around 88; there's a lag with the reading and the gauge needle but roughly the readings are the same. Then, when the drop happens, the reading goes down at equal rate to 40-45. Then, after few minutes of staying in 40s, it picks up to 88-92 again.

I only have VCDS lite and the only fault code I found on engine (which didn't re-appear after I cleared it) was:

00575 - Intake Manifold Pressure
17-00 - Control Difference

Is this familiar to anyone?
Is this the beginning of a failure of the coolant temperature sensor or the thermostat?
If the climatronic reading and the gauge are roughly consistent with each other, is it likely to be a thermostat problem rather than the coolant temperature sensor?

Thanks very much in advance and please let me know if there is any other investigation I can do to help you help me.

[Doctle Odd]

I'd change the thermostat first, genuine or a brand like Gates. It sounds like it's opening too soon. Is the CTS genuine? The fault code could be an old one

00575 - Ross-Tech Wiki

[defever]

Thanks very much Doctle,

I looked into how thermostats works (shut until the coolant is hot enough then opens to regulate the temperature at around 90), and yes I'm convinced that perhaps it's opening up too soon which cools down the engine too early, hence the dip in the coolant temperature? Also this makes sense as my MPG hasn't been as good at it used to be few months back.

I vaguely remember seeing stats replacement in one of the receipts kept by one of the previous owners, and I don't know about CTS. Just been looking around on the web on how to go about replacing it; I think I can do it myself replacing the stats and CTS (might as well?) and maybe the flange (it's plastic and known to break off easily?). Also, I need a replacement coolant.

Are these the right part numbers?:

Thermostat: 044121113

Thermostat O-ring: 059121119

Connecting socket (flange): 048121121B

CTS (I have the square blue one): 078 919 501B – not sure about this one, VAGcat.com doesn’t show one so I had to Google a number.

CTS O-ring: I don’t know

[Martynas26]

Thermostat or new sensor and water pump on the end plus bleed properly all coolant system. Its just my guess


A4 B6 1.9 QUATTRO 07463021112

[Martynas26]

On audi a4 coolant temp sensor should be black factory fitted or green new. But its all about there u get it. Loads of green but just audi have real green u need

A4 B6 1.9 QUATTRO 07463021112

[Doctle Odd]

It could also be the water pump impeller has disintegrated. If it has front of the car needs removed and timing belt also needs to come off.

[defever]

Update:

Called TPS and will be picking up the thermostat, o-ring and the CTS later today. I was asked which CTS and I wasn't sure so I asked for blue square 4-pin (I think) one. It's definitely blue and square that's on my car at the moment.

Been searching for a good write up on how to flush the coolant; lots of different ways to do it! As a practice, I think I will drain the whole fluid from the drain plug at the bottom of the radiator (had it refilled a month ago in a garage when cambelt was done with the belts and water pump... should've asked for the thermostat... I forgot), flush with distilled water twice, and fill it with 50/50 G12+ - water mix, engine on with hi heat for few minutes and top up as necessary to complete. Ref: Audi Addict DIY: Coolant Change and bleed on a PFL A4 B5 1.8L 1998

Is there a magic air bubble drain hole on my car?

I also learned a lot more about coolant mix... G12, G12+, G12++, G30 (Halfords claims that it's equivalent to G12), tap water vs. distilled water... I've done all the wrong things by occasionally topping up the fluid with G30 and tap water from the air pump at petrol stations... ignorance kills car!!

I think I can replace the thermostat, CTS and flush the coolant myself. I need to find an empty big bottle to catch the used fluid and how to dispose it responsibly!

[Doctle Odd]

One of the pipes in the bulkhead will have a hole I don't recall which but you just pull the pipe back a little if it doesn't self bleed.

[defever]

UPDATE!!

I did it! Well, I hope!! Yesterday, I tackled the task, easily replaced the CTS and flushed the coolant. getting the thermostat housing was a right nightmare, as there was a beam going right across it and I could only access the bolts from awkward angles...

Because of that, I stupidly stripped the hex bolt trying to unscrew and screw from wired angle, got quite distracted by that and realised that I put the o-ring on the wrong side of the thermostat!! Of course, the thermostat housing didn't seal properly when I put everything back, and I stripped the bolts further...

So! Today I went to screwfix and got M6 x 20 stainless steel (the OEM is M6x22 4S SP, N10172501). I searched for Bright Zinc Plate or stainless steel and I was told stainless steel would be the best bet for less rust and more strength than BZP.

I'm not sure about the integrity of the bolts I bought from screwfix, but I managed to get the stripped bolts out, put the O-ring the right side and secured the housing with the SS bolts. And so far, the heater is boiling hot, no coolant leak, the temperature is stable between 90-94, and the reservoir level is still!

I'm wondering if getting the SS bolts from screwfix was a good idea or not.

Are they same as the OEM bolts or should I try to get the OEM bolts and replace them eventually??

[mick01]

Stainless steel bolts (from where ever) will be of a much higher specification than those originally fitted, and will probably out live the car, even though they may be 2mm shorter than the originals I wouldn't worry about it as there's not a huge force acting on the thermostat housing even though the system is pressurised. The stainless bolts will be much superior to those originally fitted.