1997 Jetta won't glow/won't cold start

[PradoTDI]

Hi all, new here! I have a 1997 Jetta AHU with around 195k miles on it. I have an odd problem with the glow system or coolant temperature sensor. Under normal conditions the engine refuses to cold start (even on hot summer days), it just stutters and puffs huge clouds of white smoke. However, if the coolant temperature sensor is unplugged, it starts just fine.

No matter how cold it is outside, the glow plug light never cycles for more than a half second on startup, but if the coolant temp sensor is unplugged it cycles for the maximum time. I usually let it cycle for a couple of seconds before starting. The ECU doesn't throw any codes except when I unplug the sensor, and then it's just for the temp sensor. I have tried swapping the coolant temperature sensor from another '97 AHU with no change in symptoms.

Any thoughts on what it could be? I have had it plugged in to my VCDS to check for codes and see if there are any issues it shows at startup, but to be honest I don't really know what to look for; I've mostly worked with old mechanical diesels.

 

[garciapiano]

Could be a condition related to the cold advance solenoid. Some more information in the thread below

Cold Starting? Not really...

Over the past couple of months it has been getting increasingly harder to start my Golf. First, the problem started with longer cranking times, to then taking a few 20 second cranks to get it started, to then taking 5-10 minutes of 20 second cranks. This week I haven't been able to get it...

forums.tdiclub.com

Are you starting your car in freezing temps? I the glow plugs only trigger below 40°F.

 

[PradoTDI]

I'll have to check that solenoid, thanks! Not starting the car in freezing temps right now, but soon will be. It has always started fine in the winter with the coolant temp sensor unplugged, down to -15ºF. I know the glow plugs aren't supposed to glow unless its pretty cold, but even on really hot days (90ºF ambient temp) the car simply wouldn't start with the sensor plugged in, and when I unplug the sensor it starts fine even if I don't give it more than a second or two of glow.

 

[garciapiano]

Is your timing set correctly?

There is no way to service that solenoid particularly easily. They don’t often go bad.

 

[Steve Addy]

In temps above 40F the engine GP's don't operate anyway, but the engine should still start. If it's not starting then I would suggest that there's a timing issue that is preventing it from starting.

I would check cam / crank and IP alignment and see if that's in order, if it is and it still won't start I would move the IP into the center of the range of adjustment and that should get your engine started. From there you will have to fine tune the pump timing as described in the Bentley manual.

I'm fairly confident that you're going to find a timing issue when you try and check for cam / crank / IP alignment.

Steve

 

[PradoTDI]

As predicted, the timing is advanced off the charts. Unfortunately I don't have the time/shop space to fix it until the end of November, but I figure I've been driving it like this for well over a year so another month can't hurt too much! Any ideas if this can be adjusted out at the IP or is there a timing belt misalignment?

Screen Shot 2020-10-01 at 8.44.45 PM.png

• PradoTDI
• Oct 23, 2020

Jetta Timing Graph

 

[KLXD]

Be aware that if the timing is too late VCDS will give a too advanced reading so don't necessarily believe what it says. Also, unplugging the coolant sensor will cause the plugs to light even if the engine is hot so that can mask a bad timing issue.

How long has this been a problem? If it got progressively worse the crank sprocket may have started walking causing the timing to get late. Not a good thing.

 

[PradoTDI]

It has been this way since I bought the car over a year ago, with pretty much no change whatsoever. Supposedly the injector pump was rebuilt and the timing belt changed 10-20k before that, so presumably whatever shop did the work failed to set it all up correctly. I could be mistaken, but I would have guessed that having the timing belt off a tooth would cause larger problems.

Good to know about the VCDS discrepancy; any way to tell based on behavior if the timing is advanced or retarded?

The "fix" so far has been to leave the coolant temp sensor unplugged, I only let it glow for a short period before cold starting, but the car won't start with it plugged in.

 

[KLXD]

Yikes.

I'd set it at TDC, pull the valve cover and see if the slot is parallel to the head. If it's off a tooth it'll be obvious. A little off is not ideal but might be ok. Also check the pump. The pin might not go in on an AHU or the earlier (1Z?) motor even if properly timed but shouldn't be too far off.

To time the AHU you loosen the pump bolts and move the pump CCW to advance. Can do it while it's running if you're careful and don't loosen the bolts too much. Don't forget the bolt behind the injector pipes.

No way to really tell with VCDS where it is when it's that far off until you adjust. Looking at the position of the hole in the sprocket vs the hole in the mount doesn't tell you since the pump is moved for adjustment, not the shaft like the ALH.

 

[PradoTDI]

I finally got around to adjusting the timing to within spec, it is now slightly advanced. The car starts great with the coolant temp sensor plugged in now, and fuel economy has increased. Whatever shop had last done work on the timing belt/injector pump obviously had no idea how to set timing. Here's how far I had to move the pump:

IMG_3783.jpg

• PradoTDI
• Sep 7, 2021