Coolant Temp Sensor

VagPhillips

Well-known member
Joined
Sep 13, 2007
Location
Suwanee, Georgia
TDI
2005.5 pkg 2
I had been experiencing hard cold starts, leaving my garage in a cloud of smoke and embarrassing my kids ever since the weather turned cooler. I
was finally able to disconnect my coolant temp sensor, (damn that connector) and ever since then it has popped right off each morning while disconnected.
Last night I finally got around to replacing the sensor with one purchased from ID Parts. To my surprise, this morning it was back to its old ways, long time to turn over, lot of unburnt fuel smoke. Starts fine all day afterwards.
Oh, its a mk4 2001 Jetta.
No Vagcom handy.
Any ideas?
 

VagPhillips

Well-known member
Joined
Sep 13, 2007
Location
Suwanee, Georgia
TDI
2005.5 pkg 2
Thanks for the prompt reply. Not sure if it means anything, buy my temperature guage seems to be working, if anything it seems a little more sensative. Used to warm up and stick to straight up 190, Seems to respond to how its bring driven now, just under or up to 190.
Thanks again for your input
 

sootwagen

Veteran Member
Joined
May 25, 2003
Location
Orange County, New York, USA
TDI
2003 Jetta Wagon, 5 Speed, Indigo Blue, 2013 Audi Q7 TDI, Silver
Thanks for the prompt reply. Not sure if it means anything, buy my temperature guage seems to be working, if anything it seems a little more sensative. Used to warm up and stick to straight up 190, Seems to respond to how its bring driven now, just under or up to 190.
Thanks again for your input
The CTS has 4 pins. 2 of them go to the instrument panel, the other 2 go to the ECU.

Pull the connector again. If it starts right up, the CTS/wiring to the ECU is the probable culprit.
 

n1das

TDIClub Enthusiast, Veteran Member
Joined
Jun 11, 2002
Location
Nashua, NH, USA
TDI
2014 BMW 535xd ///M-Sport, 2012 BMW X5 Xdrive35d, former 3x TDI owner
I had been experiencing hard cold starts, leaving my garage in a cloud of smoke and embarrassing my kids ever since the weather turned cooler. I
was finally able to disconnect my coolant temp sensor, (damn that connector) and ever since then it has popped right off each morning while disconnected.
Last night I finally got around to replacing the sensor with one purchased from ID Parts. To my surprise, this morning it was back to its old ways, long time to turn over, lot of unburnt fuel smoke. Starts fine all day afterwards.
Oh, its a mk4 2001 Jetta.
No Vagcom handy.
Any ideas?
Injection timing is retarded. (late injection) When was the timing belt last changed and who did the work? If a VW dealer did the work, suspect epic FAIL.

Find someone in your area with VCDS (VAG-COM) who can look at the injection timing. The timing is probably retarded enough to make cold starts hard with the normal amount of preglow time. By unplugging the CTS, you made the ECU think it's -40F (-40C) out and it gave the maximum amount of preglow time available.

Note that this does not necessarily mean you have a bad CTS. The CTS actually has 2 sensors in the package...one for the instrument cluster and the other one for the engine ECU. It's possible to have flakey instrument cluster readings and the ECU still reading temps correctly and vice versa. But since you unplugged the CTS and gave it longer preglow time resulting in easy cold starts, there's probably nothing wrong with the CTS.

You can also extend the preglow time with VCDS but this is a band-aid approach. You're better off to find and fix the real problem first and then the extra glow time isn't needed. The usual cause of what you're describing is caused by retarded injection timing.

Good luck and Welcome to TDIclub. :cool:
 
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VagPhillips

Well-known member
Joined
Sep 13, 2007
Location
Suwanee, Georgia
TDI
2005.5 pkg 2
TB was done 40k ago by a trusted guru that I found on this site, not a dealer.
Sounds like I need to get it hooked up to a Vag-Com.
Really appreciate the advice.
 

n1das

TDIClub Enthusiast, Veteran Member
Joined
Jun 11, 2002
Location
Nashua, NH, USA
TDI
2014 BMW 535xd ///M-Sport, 2012 BMW X5 Xdrive35d, former 3x TDI owner
TB was done 40k ago by a trusted guru that I found on this site, not a dealer.
Sounds like I need to get it hooked up to a Vag-Com.
Really appreciate the advice.
Whew! <breathes sigh of relief!> OK so TB was likely done correctly and is OK. The TB does stretch some during its life so it is normal for the timing to retard slightly over the course of a TB change interval. I've witnessed this with my 02 Golf several years ago over a complete TB change interval...started out slightly advanced but in spec and was at the retarded edge of the spec when it was time to change the belt.

VCDS (VAG-COM) is what you need. It's well worth the investment to get yourself set up with VCDS. :cool:

Good luck.
 

VagPhillips

Well-known member
Joined
Sep 13, 2007
Location
Suwanee, Georgia
TDI
2005.5 pkg 2
Checked timing today and it was retarded. Actually literally the chart.
Noticed some fuel on top if the fuel filter and went to take tee connector out of filter to inspect o rings and it came apart, leaving the bottom half in the filter. "There's your problem".
Trying to patch it until new one arrives. Dealer has it but wants $56.
Am I correct in assuming that this fuel leak would affect timing?
 

Kristopher

Well-known member
Joined
Aug 20, 2008
Location
Calgary Canada
TDI
99.5 Jetta
I've read no less than 20 threads to find out if the CTS ECU reading will default to the same as the Fuel Temp Sensor reading if the CTS is unplugged/disconnected.

So far from this thread I've learned that it should read -40f/-40c when unplugged.

... By unplugging the CTS, you made the ECU think it's -40F (-40C) out and it gave the maximum amount of preglow time available...
My 1999.5 Coolant Temp had a reading of 18C but when it was unplugged it defaulted to the same as the Fuel Temp reading of 9C. Non the -40 like I had expected.

Ambient temps are much lower...-15C (5F and below) but the car was plugged in with my frost heater. MAF Air Temp read -7C so that seemed in line with a warmish engine and cold outside air.

It's supposed to get real cold tonight so I'm going to have to check on a stone cold car another day just to see what temp the ECU sees when the CTS is unplugged and everything else is also super cold.

The issue I'm tracking is a hard cold start/stumble. GP light stays on for ~4-5 seconds but I have to cycle ignition at least twice to get it to fire. Even then it's difficult to start. Ohms are all between 0.6-0.7 on the GPs. No codes.

Either my ECU is reporting a strange connection with the Fuel Temp and thus isn't getting the right starting map, or it's normal behaviour and my GPs are actually toasted even though they read OK.

In sum:
Fuel Temp 9C
Air Temp -7C
CTS connected 18C
CTS disco'd 9C


thanks for any input guys and gals.
kris
 
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UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS
Kristopher, that's some excellent reporting there.

I don't profess to know much about these problems. I've got a couple of cars that are less than stellar starters in my mildly cold climate (I have noted that when it's down near the threshold of 40 degrees that the cars start harder- GPs not fulling commanded yet). One I'm figuring needs its IQ bumped up: this one actually fires right up, but stumbles a bit afterward (compression is good). The other is fully advanced and everything else seems fine- ironically, I'm thinking that the IQ might also be a factor here in that it's HIGH (the threads are confusing; I've read of people claiming that high IQ settings can do this, and then I've read of others stating that low IQs can cause hard starting!). For this later car I'd already replaced the starter, which wasn't spinning the engine more than about 175 RPMs (I think the new one bumped it up to around 250 RPM). OK, enough about my issues..

Compression is very important. If your car's compression is compromised then you're going to have issues starting no matter how everything else is (unless, perhaps, you get a starter that cranks 500 rpm or such!). I was looking to buy a UTV that had a diesel in it and that owner stated that it started hard; he was a big diesel engine mechanic and wasn't familiar with GPs; so, I told him I'd check his GPs, hoping that it was a simple GP issue; well, I also knew that it could be a compression issue so I told him that and he was fine with doing a compression test on it- turned out it had one cylinder (out of three) that was way down on compression.

Cranking RPMs are also key. If you're not spinning the engine at 200+ RPM (I'd prefer to see 250) then that'll be a cause for harder starting. And off course, the battery is important here! (with GPs pulling power in addition to the starter there can be a fairly substantial draw- weak battery and or cable/ground issues will diminish effectiveness).

Fuel temps. Not a lot one can do here other than run winter diesel and anti-gelling additives.

So:

1) Timing;
2) Compression;
3) Starter RPMs & battery;
4) Fuel temps.

You could verify that you have proper voltages along your GP harness. Since you're not getting any codes/CEL for GPs I wouldn't think you should find a problem here.

Can't say that I've heard of anyone having temperature-related issues with their ASVs, but since they are known to cause starting problems it's worth a look.
 

dweisel

Top Post Dawg
Joined
Jul 28, 2006
Location
Wheeling, West Virginia
TDI
dweisel isn't diesel anymore!
I'd try to get someone with VCDS to set your glow plug time longer. My son's 03 Beetle was startin hard and with some blue smoke. Set glow plug value to 32650 and now starting better. NO SMOKE.
 

UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS
I'd try to get someone with VCDS to set your glow plug time longer. My son's 03 Beetle was startin hard and with some blue smoke. Set glow plug value to 32650 and now starting better. NO SMOKE.
Where is that set in VCDS? I'd be interested in trying this on the wife's car. Hit freezing tonight and I toggled the GPs twice and it started right up. Maybe one day I'll do a compression test just to see if compression might be the source of the problem.
 

pdq import repair

Veteran Member
Joined
Nov 6, 2016
Location
idaho
TDI
09 Jetta
If your timing is way off, something is wrong. They will retard a little as they wear but usually not enough to cause problems. And no the fuel leak will not affect timing in any way.
Get your timing on again and watch it. I have seen the crank pulley wear and let it slip a little. If the timing moves again soon, inspect the bottom pulley. If it retards enough you will get hard starting, smoke, etc. It will act like not enough glow plug time, or at least forcing the plugs to run longer will help it start, but is not really the underlying problem.
 
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Kristopher

Well-known member
Joined
Aug 20, 2008
Location
Calgary Canada
TDI
99.5 Jetta
(haven't figured out how to attach a jpg...nowhere to host it just yet)
Coolant Sensor anomalies cont...
99.5 TDI
cold start issues

A few new developments;
  1. The ASV is operating normally (at least on a warmish engine)
  2. Found my CTS is actually leaking a little. Might have been doing this for some time, I don't check down there often.
  3. Noted the Coolant Temp (G62) definitely mirrors the Fuel Temp (G81) when the CTS is unplugged. In this case it defaulted down from 54.9C down to the 19.8C fuel temp and the Glow Period Preheat jumped to 9.95s (just changed Grp 12 is 32668). Still baffled why it's not showing -40
  4. IQ Idle Stabilization (Injection Quantity Deviation Gp13)
    #1: 0.33,
    #2: -0.47,
    #3: -0.09,
    #4: 0.19 mg/str
    It doesn't look like the IQ deviation is far off between cylinders. If those numbers are any indication it doesn't look like compression or the injector/nozzles would be anything to worry about.
  5. Finally got a code of P0380-35-10 Intermittent though the harness tests with 11.8v across all 4
  6. GPs tested at 1.0ohm on all 4 (cold they test 0.6-0.8 ohm each)

For what it's worth, the car starts just fine when warm. Timing belt is a couple years old and timing checker shows within spec (tad bit advanced).

It never flagged a P0380 GP MIL code all last year but started the same as it does now; very very hard to fire. I noted last year it stumbled a lot on cold starts so by disconnecting the EGR vacuum line it cleared that up a lot.
 
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Kristopher

Well-known member
Joined
Aug 20, 2008
Location
Calgary Canada
TDI
99.5 Jetta
Ok so I've found a few things that can cause the ECU to report the Coolant Temp to be the same as the Fuel Temp:

1. DTC. - I've cleared mine and still the two temps match each other.

2. Once CTS unplugged and reports -40, the ECU changes from the -40 to match the fuel temp after the car is running (I've not verified this on any working car)

3. A bad T14A connector under the battery tray can short a couple wires leading the ECU to think it's reached conditions of #2 above. In this case, the guys found their GP wait time was less than 1 second. thread

In my case, my GP light stays on long enough (9 seconds), it just doesn't seem like they're glowing (yet all test okay incl nearly 12v at the harness).

It's plausible my CTS is acting 'normal'.
The fact my CTS and Fuel Temps are matching with CTS unplugged is likely moot anyway because I've had the Fuel Temp down low enough to ensure GP time is over 5 seconds and still very rough starts.

Could the harness still be bad even though I'm getting nearly 12v at the glowplugs?

If not, I'll have to pull the plugs and see if they glow hot enough. If that checks okay, I suppose a compression test is in order though I'm not convinced.
 

UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS
If you've triggered a P0380 then there's an issue in your GP system. If GPs' resistance is good and you're getting good voltage then the issue lies with the connectors. Son's Golf tested all good yet had an issue with one GP- I wrapped some foil over the GP connector and then seated the harness on it and have not had the error return: it was meant as a temporary fix, but it's been holding up for quite a while now (it had been a repeated problem before).

Might check your thermo-T (fuel filter). Could be that you're getting a bit or air in your lines.
 

Kristopher

Well-known member
Joined
Aug 20, 2008
Location
Calgary Canada
TDI
99.5 Jetta
bah...lost my previous reply.

99.5 jetta with funny CTS reporting.

Long story short;
- the glow plugs are working. I removed all and tested...all red hot (checked one by one though)
- temps seem accurate
- CTS -2.7c
- fuel temp -0.9c
- air temp -0.9c
- with CTS disco'd it read the fuel temp of -0.9c which was unexpected again so I don't know what's going on there.

Funny enough it's now starting better after I removed/tested all the plugs. I kept the GP dwell at 9.95s and only cycle it once (instead of the usual 2 times).

Anyway thanks for the help :)
 

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagen, 2014 Tiguan,2003 Jetta 261K Sold but not forgotten
bah...lost my previous reply.

- with CTS disco'd it read the fuel temp of -0.9c which was unexpected again so I don't know what's going on there.

Funny enough it's now starting better after I removed/tested all the plugs. Anyway thanks for the help :)
Fuel temp is a sensor inside the injector pump.

Bad or corroded connection on the glow plugs before you took them out ??

Rich W.
 

Kristopher

Well-known member
Joined
Aug 20, 2008
Location
Calgary Canada
TDI
99.5 Jetta
Fuel temp is a sensor inside the injector pump.

Bad or corroded connection on the glow plugs before you took them out ??

Rich W.
I think there might have been....even though I had the GP harness on/off a bunch of times previously. Perhaps a bad connection in the harness wiring as opposed to the terminals themselves.

Just wanted to update:
increasing GP time does help. I used to cycle the key a couple of times in the past and although this is giving about the same amount of time when cold (~10s) as when I used to cycle the key, somehow this is working better. Again, could be that one or two plugs weren't heating also.

Pretty sure I have a sticky injector, and the EGR also makes it smoke a lot on a really cold start. I just disconnected the EGR and did some adaptation to minimize the time it's open and all is pretty good now.

330,000 km and it's actually still a really solid performer.
 
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