Coolant temp sensor causing hard start

[snakeye]

Glow plugs didn't come on this morning in -15°C, causing a long crank and a smoke bomb. Looked up the cel code I got on vcds and it turns out the coolant sensor is giving bad readings. I confirmed this by going into the measuring block, and got 20 something degrees C with a warm engine.

I guess I have a bad sensor? My question, though, is how come the temp gauge on my instrument cluster is displaying the accurate temperature (needle in the middle when engine is warm)?

 

[BobnOH]

There are multiple temperature sensors. Not sure what they all do, but the coolant temperature sensor does not feed the dash.

 

[JETaah]

The coolant temp sensor on an ALH engine has two sensors internally (4wires). One goes to the ECU and the other goes to the cluster.

 

[snakeye]

Interesting, so if the dash is getting the correct temperature then maybe it's just a bad contact?

 

[OlyTDI]

You can have normal dash reporting with a bad sensor.

 

[UhOh]

If you have another occurrence then unplug the CTS connector and check how it starts next time.

If your CTS is black then replace it.

 

[Vince Waldon]

Interesting, so if the dash is getting the correct temperature then maybe it's just a bad contact?

Perhaps... but pretty rare.

Just to be clear: the coolant temperature sensor has two internal sensors.. and thus four wires.

One sensor feeds the dash and one feeds the glow plug circuit. Either can fail on its own, although it seems to be the glow plug side that fails the most.

VCDS can see both sides... so you can check it out... but if your glow plug light is only coming on briefly when it's cold, and stays on for a long time when you unplug the sensor... you've pretty much confirmed the glowplug side is bad.

Meanwhile the dash gauge continues to work just fine.

 

[snakeye]

That's exactly what's happening. Thanks for the info! I guess I'll just unplug the sensor on cold days to maximize glow plug time until it gets warm enough outside to fix. Not hard to do once you break the clip off the connector

 

[slick rick]

Is this the issue on 99 alh tdis.mine takes 5sec of cranking before firing after warmed up?when cts is unplugged fires rite up but have no check engine lights??

 

[BobnOH]

Yes, the early TDIs had a not-so-good fuel map. Needs a tune to correct hot starts. Your 99.5 may need a chip soldered in to tune, so unless you want to solder that thing in, you need to send the ECU to the tuner.

 

[slick rick]

Okay would that also explain poor fuel mileage?

 

[slick rick]

And power loss when at operating tempeture

 

[Vince Waldon]

No and no, it's strictly an issue with the fuel map used for starting. IIRC.

 

[BobnOH]

What Vince says, no and no.
I bought a 99.5 new, it was fine for 4 or 5 years, then started the hot start bullshit. Tune fixed it. Fuel economy and power not affected.