Role of temp sensor in starting

elicon200

Veteran Member
Joined
May 16, 2005
Location
Southwest CT
TDI
2001 Golf GLS TDI
I'm having random hard starts and read into the temperature sensor as a possible cause.

If it is bad, what does that do to starting? I assume the computer is looking for a reading, the hotter or colder the engine the more it must retard or advance the timing - is this right? Does the sensor influence the fuel quantity?

Golf is chipped with Malone Tuning stage 1.5 and performed hammer mod a few years ago to decrease IQ and thus reduce engine shudder on heavy acceleration.

Thanks.
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
You shouldn't have hard starts in the weather we've been having regardless of your temp sensor's condition. I'd look for another cause: fuel draining back into the tank when the car sits, air in the fuel lines, clogged fuel filter, worn out injectors, timing off...one or more of those things.

The temp sensor will tell the ECU to increase fueling when the engine is cold. It will also tell the ECU to cycle the glow plugs. But your car should start fine in mild weather regardless.

How's your fuel economy? Has it declined recently? That would be a tip-off that something's up with the injectors. How high is your IQ setting? 3.0 or so shouldn't cause any rough running if the injectors are in good shape.
 

elicon200

Veteran Member
Joined
May 16, 2005
Location
Southwest CT
TDI
2001 Golf GLS TDI
Thank you Indigo. I am suspecting fuel draining away from the filter in some way. I've been checking the clear tube form the filter before starts and have notice the bubble size changes a lot form a tiny bubble to more than an inch long.

The injectors were replaced about 100k ago. I did the timing belt 35k ago and timing was half way to the top advanced line. I replaced the seals in the upper pump due to leaking about 20k ago. No difference in mileage recently.

My question about an air leak is: if there's an air leak, after driving, shouldn't I lose fuel during driving? There has been no fuel noticed on the ground after my 1 hour plus drive to work.
 

KLXD

Top Post Dawg
Joined
Aug 22, 2009
Location
Lompoc, CA
TDI
'98, '2 Jettas
Pressure can cause seals like o-rings to seat and not leak while running but let air in.

Also, some seals, like those in the filter, don't see fuel pressure. They can leak enough to cause trouble starting but still allow the engine to run.
 

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
I'm having random hard starts and read into the temperature sensor as a possible cause.
If it is bad, what does that do to starting? I assume the computer is looking for a reading, the hotter or colder the engine the more it must retard or advance the timing - is this right? Does the sensor influence the fuel quantity?
Golf is chipped with Malone Tuning stage 1.5 and performed hammer mod a few years ago to decrease IQ and thus reduce engine shudder on heavy acceleration.
Thanks.
As mentioned, in the ambient temps currently you shouldn't have CTS related starting issues. In starting situations the engine starts generally w/o timing imput from the ECU or CTS, relying on the mechanical static timing settings. The main job of the CTS at startup is to provide data for the ECU to "decide" how long to cycle the GPs and perhaps alter fueling. After the engine is running the CTS data determines how the ECU adjusts timing, fueling, etc. Unplug the CTS and timing goes very advanced, for example, because the default temp is something like -40F.
Air leaks are strange. Seldom will fuel leak unless there is a major fault. Watch the clear line for bubbles while running. An inch size bubble after sitting a few hours is fairly typical of the ALH cars I've had as long as it stays in place. The most common source of air leak starting issue after sitting ("cold") are at the fuel filter T fitting. Sometimes even new O rings there will "leak", it seems. A thin coating of heavy grease before inserting the T into the filter has sealed a rogue leaker for me on occasion.
 

IndigoBlueWagon

TDIClub Enthusiast, Principal IDParts, Vendor , w/
Joined
Aug 16, 2004
Location
South of Boston
TDI
'97 Passat, '99.5 Golf, '02 Jetta Wagon, '15 GSW
I'd vote for air getting into the IP when the car sits, allowing fuel to drain back to the tank. That's were I'd start.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
I'm having random hard starts and read into the temperature sensor as a possible cause.
If it is bad, what does that do to starting? I assume the computer is looking for a reading, the hotter or colder the engine the more it must retard or advance the timing - is this right? Does the sensor influence the fuel quantity?
Golf is chipped with Malone Tuning stage 1.5 and performed hammer mod a few years ago to decrease IQ and thus reduce engine shudder on heavy acceleration.
Thanks.
Define this please, as anyone throwing out possible solutions is guessing since we really do not have enough information.

Cold? Hot restart? Long crank? Slow crank? Lots of smoke during cranking?

All three temp sensor values are easily found in the ECU data if you want to see what the ECU sees.
 
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