Short Glow-plug Cycle

D

dean_f

New member
Joined
Jun 14, 2018
Location
Rhode Island
TDI
Jetta and Beetles
'03 wagon. Great shape with some mods: turbo, tune, clutch. Issue with glow-plug cycle on cold start. Cycles too rapidly, not giving plugs enough time to warm up. The glow-plug light comes on for less than a second... Plugs, harness and module all new. Is there a thermostat in the loop? Suggestions?
 
JDSwan87

JDSwan87

Black Swamp Thing
Joined
Feb 9, 2014
Location
Michigan near Toledo
TDI
2001 Jetta TDI, 5 speed Lagoon Blue Metallic(sold); 2005 Jetta TDI Wagon auto
dean_f said:
'03 wagon. Great shape with some mods: turbo, tune, clutch. Issue with glow-plug cycle on cold start. Cycles too rapidly, not giving plugs enough time to warm up. The glow-plug light comes on for less than a second... Plugs, harness and module all new. Is there a thermostat in the loop? Suggestions?
Click to expand...
Anything above 40*F, your glow plugs will not cycle. If you are having hard starts when cold, check your static timing.
 
S

steve6

Veteran Member
Joined
May 25, 2010
Location
Beaverton, ON
TDI
2003 jetta tdi
If you unplug the coolant sensor on the back of the block it should make the glow plug light stay on for 10 seconds, if it does not that sensor is bad.
 
oilhammer

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis, MO
TDI
There are just too many to list....
Glow plugs work all year round, despite the persistent myth. :rolleyes:

However, the PREglow period varies, and is not very aggressive. In any event, the glow system is always monitored by the ECU, so if there is an issue with it, you'll have a DTC, and the MIL will be shining happily in your face. :p

So let's rephrase this, since all too often the glow system is blamed for something it has no part in: the car is hard to start cold. So, if you have no glow related DTCs, then we already know the glow system is OK. But, we do not know if the ECU is getting the correct info to properly use it. Coolant temp is the main input here. Verify this value with a scan tool. You can also look at preglow parameters, but that isn't really necessary.

Next I would look for things like loss of fuel prime (leaking pump, thermo-tee, etc.), poor timing (M&P belt install, etc.) or in some cases just a sick engine.

Oh, back to plugs though... not usually an issue on the ALH but the WRONG plugs can cause problems too. And may not ever show any DTCs if they are all the same. This is more of a PD thing, though.

Also, often slow cranking gets overlooked. Worn out starter, weak/wrong battery, poor connections, etc.
 
Last edited:
JDSwan87

JDSwan87

Black Swamp Thing
Joined
Feb 9, 2014
Location
Michigan near Toledo
TDI
2001 Jetta TDI, 5 speed Lagoon Blue Metallic(sold); 2005 Jetta TDI Wagon auto
oilhammer said:
Glow plugs work all year round, despite the persistent myth. :rolleyes:

However, the PREglow period varies, and is not very aggressive. In any event, the glow system is always monitored by the ECU, so if there is an issue with it, you'll have a DTC, and the MIL will be shining happily in your face. :p

So let's rephrase this, since all too often the glow system is blamed for something it has no part in: the car is hard to start cold. So, if you have no glow related DTCs, then we already know the glow system is OK. But, we do not know if the ECU is getting the correct info to properly use it. Coolant temp is the main input here. Verify this value with a scan tool. You can also look at preglow parameters, but that isn't really necessary.

Next I would look for things like loss of fuel prime (leaking pump, thermo-tee, etc.), poor timing (M&P belt install, etc.) or in some cases just a sick engine.

Oh, back to plugs though... not usually an issue on the ALH but the WRONG plugs can cause problems too. And may not ever show any DTCs if they are all the same. This is more of a PD thing, though.

Also, often slow cranking gets overlooked. Worn out starter, weak/wrong battery, poor connections, etc.
Click to expand...
I stand corrected.
 
D

dean_f

New member
Joined
Jun 14, 2018
Location
Rhode Island
TDI
Jetta and Beetles
Thank you all. The car starts every time; cranks fine; just cranks longer than it seems it should. My wife's BRM starts almost instantly every time. Maybe more an ALH thing, but will check coolant temp sensor. Beyond my skill to assess at this point - on an learning curve - but have an excellent mechanic/friend to help. Will let you know what we learn... Interestingly, after the first start of the day, all subsequent starts are immediate. Even if the temp reads cold.
 
G

Genesis

Top Post Dawg
Joined
Feb 26, 2003
Location
Sevier County TN
TDI
'03 Jetta Wagon
Look at the clear line before you crank it; if there's a material amount of air in there, or if you see an air bubble go INTO the line when you start cranking find the airleak and fix it. That is often the cause of this sort of thing and it can be a lot of "fun" to find -- although one of the more-common causes is the thermo-T at the fuel filter either being cracked (VERY hard to detect as it frequently doesn't leak fuel in this condition) or the O-rings being bad.

As noted low cranking speed often is implicated too; the most-common causes are either bad grounds or the starter itself.
 
jettawreck

jettawreck

Top Post Dawg
Joined
Aug 2, 2004
Location
Northern Minnesota-55744
TDI
2001 Jetta and 2003 Jetta
dean_f said:
Thank you all. The car starts every time; cranks fine; just cranks longer than it seems it should. My wife's BRM starts almost instantly every time. Maybe more an ALH thing, but will check coolant temp sensor. Beyond my skill to assess at this point - on an learning curve - but have an excellent mechanic/friend to help. Will let you know what we learn... Interestingly, after the first start of the day, all subsequent starts are immediate. Even if the temp reads cold.
Click to expand...
This sounds like your static injection timing is not advanced as much as it should/could be. Late (retarded, or not advanced enough) timing makes for poor "cold" starts.
 
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