Mysterious Drama with my Coolant T Sensor

Well, here's what's going on:
Here in PDX OR, we have a mild climate, and it's always 40*F and raining. Or at least, 8 months out of the year. Now, it's going under 32*F overnight in the overnight hours.


Basically, my glow plugs (in my '00 ALH 02J Jetta) don't usually need to work this hard. But I've been perplexed as I try to solve my hard cold starts.


-It won't start within a reasonable time frame with only one cycle when it's cold.

-Running the glow plugs 5 times will make things better.
-The issue is not present when the engine is warm.
-The temperature gauge works fine.



I unplugged the Coolant Temperature Sensor:


-The glow plug light acts like my parent's old diesel van, staying on for 20 seconds or so.
-The cold engine starts right up post-glow plug cycle.
-The temperature gauge sits at 0, regardless of actual engine temp.
-When the engine is warm, it will rev at startup (Evidently, a cold engine would need that extra fuel)


So my question is, what do I do?


A) Put Bosch Glow Plugs in. That NKG one is garbage!
B) My relay is bad. Replace it!
C) A coolant temp sensor from NAPA won't suffice! Get something better!
D) If the harness has exposed wires, replace it.

E) All the above
F) Something Else that I have not thought of yet

Thanks for the help.



Thanks for the help, it's much appreciated.

When you unplug the coolant temp sensor, the glow plug circuit goes into default mode to get it running. It could be your glow plugs are getting on the tired end of things or poor connection at the harness. That four way connector does wear out. Also make sure the contacts are clean on the glow plugs themselves. They can get corroded over time.

Also, don't use autolite glow plugs. I know you said Bosch, but just in case

If there was a problem with your GPs or GP relay, you'd have a check engine light at least.
Does your GP light stay lit noticeably longer on the freezing mornings? It should.
Fact is, when that light goes out, the GPs are still glowing. Just sit at "ON", let it go out, and wait the amount of time it would take when the temp sensor is unplugged and try that. 31F overnight tonight!
Also agree with Nero, it would be a good idea to clean the plug ends where they seat in the harness. And BOSCH all the way.

You can change the glow time, it's a temp based map that determines the amount of time the glow plugs glow/light on...you'll need vcds to change it.

I turned my 2000 up a little to help it start in the cold, it's 35 this morning and it'll probably have a 3 or 4 second glow and fire right up.

Really recommend reading the glow plug 101 thread a time or 2.

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There are 1000 threads like this. SO MANY people point to the glow system as the cause of a hard cold start.

As already stated, the glow system is monitored by the ECU. All year round. Hot, cold, makes no difference. The glow system is monitored. If you have a bad glow plug, you'll have an MIL on with a DTC stored. Of all the things these cars are good at, glow system faults are very good at being detected. By far THE #1 reason any ALH TDI ever has its MIL on.

What is probably happening is, an artificial lengthening of the preglow period (by either repeated cycling of the key before starting or disconnecting the CTS forcing the maximum preglow period as its default) is masking some other problem.

Poor engine health (low compression)

Poor injector health (bad spray pattern)

Poor quality fuel

Incorrect cam and/or pump timing (belt not installed correctly)

Or, incorrect temp values being sent to the ECU, in which case no DTC may be generated. If the ECU thinks the coolant is 70 C, it will not command a preglow. This is of course easily checked with a suitable scan tool.

Pretty sure mine is worn nozzles as I have 308k and probably the ones it was created with.

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Well it's 30°F this morning, and I'm not far from OP, I'd have to say this is the first morning my car didn't do its typical "a little hard to start" nonsense. But I also noticed that my glow plug light stayed on for 5ish seconds instead of its typical half second. Guess I need to go in and adjust my glow time a pinch

The light just tries to guess when it's ready to start, plugs continue to glow.

BobnOH said:

The light just tries to guess when it's ready to start, plugs continue to glow.

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Something that drove me crazy on my 7.3 PSD. I think glow time could run up to 90 seconds but the light turned out after like 10? I always wanted to run a light off the glow plug relay to tell how long they were really on (watching voltmeter worked somewhat)

On my older TDIs, I turn the key on, then fasten my seatbelt, plug in my phone, put the coffee in the cupholder, etc. - then start.

My understanding is they do an initial glow and then switch to after glow which isn't as hot but still provides some heat until the coolant reaches a certain temp or when you exceed 2500rpm

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oilhammer said:

There are 1000 threads like this. SO MANY people point to the glow system as the cause of a hard cold start.
As already stated, the glow system is monitored by the ECU. All year round. Hot, cold, makes no difference. The glow system is monitored. If you have a bad glow plug, you'll have an MIL on with a DTC stored. Of all the things these cars are good at, glow system faults are very good at being detected. By far THE #1 reason any ALH TDI ever has its MIL on.
What is probably happening is, an artificial lengthening of the preglow period (by either repeated cycling of the key before starting or disconnecting the CTS forcing the maximum preglow period as its default) is masking some other problem.
Poor engine health (low compression)
Poor injector health (bad spray pattern)
Poor quality fuel
Incorrect cam and/or pump timing (belt not installed correctly)
Or, incorrect temp values being sent to the ECU, in which case no DTC may be generated. If the ECU thinks the coolant is 70 C, it will not command a preglow. This is of course easily checked with a suitable scan tool.

Click to expand...

Booyaw. Can’t add much to that. My money is on late timing.

It’s also possible to lengthen the glow time across the board. Look for a really old post by username Wingnut. “Extend glow time” something like that.

When you get your hands on a VCDS, check your injection timing in Basic settings 000 on the timing checker and make sure you aren’t in the bottom of the graph. Also look at Measuring block 001 with the motor fully warm and verify a plausible reported coolant temp as well as an injection quantity of between 2.0-4.0

If you meet the timing, temp & IQ conditions then go ahead and crank up your glow time by a second or two. I did it to my ~400K mile golf the last winter that I was driving it extensively and it certainly took a lot of the rumble out of colder starts.

Make sure you didn’t get slipped an H5 battery by some careless shop lackey at some point. The big H7 ALH battery does make all the difference

I checked his injection timing a few weeks ago, it was right on the money.

Nero Morg said:

I checked his injection timing a few weeks ago, it was right on the money.

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What about static timing? My first bout with starting issues was a cam to crank timing issue.

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I didn't check static. He had the timing belt off twice recently doing a normal service. I'd assume it was done right, but you know what assuming gets ya...

Nero Morg said:

I didn't check static. He had the timing belt off twice recently doing a normal service. I'd assume it was done right, but you know what assuming gets ya...

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So true.

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Okay, here's the verdict:

Based on what I know, it sounds like the most likely cause is the injectors and contact points. I'll look into improving them.


Thanks for the help.

More Details said:

If I am remembering the timing of events correctly...
Around the end of February 2019, Nero Morg tested my compression. It was actually above the manual's listing of "new". He also assisted me with installing new injector tips.
This sort of helped.

Glow plug codes kept coming up occasionally and steadily more frequently. I tested and found the J52 glow plug relay to be bad, so I replaced it (this was around January of 2020).
This got the codes to go away and since have not returned.

However, what seemed to be excessive cranking time continued.

I eventually tweaked the glow plug timing with VCDS. I may have gone way overboard, but here's what I have seen with my current settings:

-At 60°F, it glows longer than necessary, but I can just crank and go.
-At -10°F, it required ten cycles to start the engine.
NOW DON'T COMPLAIN! Customers at the tractor dealership had been calling us off the hook as their off-road fuel had gelled up that day. And some co-workers were late because of fuel gelling.

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