Faulty coolant temp sensor?

My Golf is never getting up to proper temperature - according to the temp gauge.

However the heater blows hot air inside the cabin relatively quickly after start up, which makes me think it is likely a faulty coolant temp sensor.

Would a faulty temp sensor produce error codes? My car had no codes when the mechanic scanned it.

eberg said:

My Golf is never getting up to proper temperature - according to the temp gauge.However the heater blows hot air inside the cabin relatively quickly after start up, which makes me think it is likely a faulty coolant temp sensor.Would a faulty temp sensor produce error codes? My car had no codes when the mechanic scanned it.

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After starting a cold engine and before the coolant temperature gauge indicates 90C feel the thermostat outlet hose to make sure it's still cold...
If the hose is warm then the thermostat is stuck open and not allowing the engine to reach normal operating temperature.

What is the outside temperature? Having the heater blower on will definitely rob the engine of heat; the heater core acts like a second radiator. I experienced this a couple of days ago when ambient temperatures were close to 0 with a wind chill of -10F. My scan gauge read ~ 154F, and the dash gauge never reached 190. I needed heat.

the outside temperature seems to make little difference - below freezing or above freezing, the cabin warms up nice a quick - actually significantly quicker than my Jetta.

This is why I strongly suspect the coolant temp sensor, but the question is whether the temp sensor would trigger any codes in vagcom, or not? My car has no codes.

I scanned my coworkers 2000 Jetta gasser with VCDS on Friday and he did have coolant temp sensor fault code. His symptoms were intermittent coolant gauge operation on the cluster.

There's two sides to the CTS, one for the gauge, one for the ECU. It's common for the gauge half to go bad and not register correctly. Usually though the gauge will 'drop out' and then return to 'normal'... as normal goes. No codes unless it's the ECU side.

Eberg,

My car behaved just like you and FlyTDI Guy are describing, I changed the sensor and all is well.

If the gauge reads low I check the temp with vagcom and if it is also low then I know it's the t-stat but if the engine temp looks fine then I know it's the gauge side of the sensor that's wonky.

I had similar readings.. my gauge sat at about 45 degrees but cabin air was fine/hot. Took the thermostat off and saw that one of the little tabs from the thermostat housing(the ones used to hold the thermostat in place while seating it back against the block) was broken off and was stuck holding the thermostat open by just a little... I removed the plastic bit, reinstalled(with the original housing without tabs) and it went back to operating fine. Spent no money and fixed the issue... I like these kinds of days.

Drain your coolant so you can take your time with the t-stat. Just open the valve on the bottom of your rad(drivers side lower), catch it in a CLEAN bucket and go at it.
Good luck.

Great input guys! I had ruled out the thermostat because I get warm air in the cabin fairly quickly, even on a cold day - but based on what 01whttdi is saying - it could still be the thermostat.

My torque app gives me a low reading too - seems to correspond with what the gauge is reading.

Have look up some tutorials for how to replace the t-stat.

the t-stat could still be in good shape and re-useable, it just may be stuck open a tiny bit... Its a simple procedure to take it off and check out. I sure you'll find the process on here somewhere.
keep us posted.

http://www.myturbodiesel.com/1000q/a4/thermostat-ALH-VW-TDI-engine.htm

But have a new one standing by, actually don't be tight like him ^^^^, just fit a new one and be done with it lol

Seatman said:

But have a new one standing by, actually don't be tight like him ^^^^, just fit a new one and be done with it lol

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hahaha.... yes, on a scale from 1 to cheap, Im cheap. lol.
I agree with money bags though(Seatman), for the $30 to just put a new one in while its out, thats good money spent.

I'm not that much of a money bags but I do know the right people in the right places which helps lol

cheap. then cheaper...

01whttdi said:

hahaha.... yes, on a scale from 1 to cheap, Im cheap. lol.
I agree with money bags though(Seatman), for the $30 to just put a new one in while its out, thats good money spent.

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You forget that you are going to loose 1/2 gallon of coolant, and at $40 bucks at the stealership, thats another $20 just for the coolant.

So now the $30 part plus the $20 in coolant you are looking at $50 bucks in parts. add Shop time at $80 per hour, (around hear anyway), Toronto is Higher, Dealership is 100-120) , Probably 7 units of labour time to re-re. It typically is more advantagous for the shop to sell you a new T-Stat, than worry about a partial.

Once you replace the T-Stat, then you know you will have proper cooling / heating flow through the system.

Depending on the Mileage, welcome to the world of VW.

PS. OP was regarding throwing a CEL , the ECU won't know that it is being lied to by the sensor. It just will assume a certain temperature, if the FUel Sensor, Radiator Sensor, say something different than the ECT Sensor (Dual Sensor, one output for dash, other output for ECU).

Since it probably is the Black plastic (original Version), the New ECT sensor has been up graded (Green Plastic Body) in the '00, It could be BOTH went out. According to Fuelly you only have a few fillups using fuelly, this means the vehicle is not 1st owner, and you are now getting all of the fun of VW ownership. Who knows if the ECT was replaced ever before. Check the Date code stampe on the Hex flange or plastic of the Sensor when you remeove it. If it xx00 or xx01 where xx is the week of the year or julian Day ofthe year, might give an indication if its original (99 or 00), or been replaced once or twice in its life. (07, 08,09). Sometimes the sensor parts just fail after 1 year, others 10.

Sounds like you have some decisions to make. If Torque is reporting low as well, it could just be outside temps or your thermostat is going bye-bye. You need to figure out which. When the CTS fails, like I said before, it will usually drop out or appear to quit working. Then, on next glance, it will be back up again. That is usually... If the ECM side is wonky, the dropping out will trigger a "implausible signal" or "open or short to ground" type error as shown in VCDS. If it's just reading low, then no error will be logged.

Another factor is our funky coolant gauge. Mine doesn't even start to move until around 130°F (54°C). It also will register 190°F (90ºC) from around 165ºF (74ºC) all the way up to well over 200ºF (94ºC). Basically it is not to be trusted or reliable for diagnostic purposes.

All things considered, with your outside temperatures lately, it almost sounds like you're OK. Torque reads low as does your dash gauge. If Torque reads below 160ºF or so, your gauge will too. Armed with all this info, the ball is in your court.

Thanks again guys!

I already have the temp sensor on hand, but since it appears that there is a significant chance that the faulty part is the thermostat, it appears to me to make most sense to just replace both at once
.
Reasoning for this thinking is that I will only have to set aside time to work on this issue once (hopefully!), and I will only lose coolant once.

Speaking of coolant. How much coolant should I expect to replace for a thermostat / sending unit job? I have seen it sold in 1.5L and 5 L jugs.

Where do I get the best deal on approved coolant?

I just drained mine into a clean container and reused it. With the placement of the radiator drain, it's entirely possible. I did apply some air pressure to the reservoir to help evacuate more coolant than gravity alone.