98 Jetta with high idle

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
Hello I just bought a 98 Jetta that has some mods from the previous owners.
It has Bosio DLC 1019 nozzles, a rocketchip RC3 tune, and EGR delete. I'm not sure exactly what else has been done to it.

It is idling around 950 RPM hot or cold, it's always the same. Using VCDM, it appears the temperature sensor is working since that comes up to temperature correctly. I'm not sure what else to look for.
I've spent most of my life working on mechanical diesel engines, but I've never touched anything with a computer, so this is all a bit new to me.
Can someone please point me in the right direction on how to diag/fix this?
thanks
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
You can change the idle speed.

Should be the same as the ALH, if I am wrong someone please correct me:

Go into VCDS in the Engine Controller, go to log in and type in '12233' in the box, and click 'do it' or enter.

Now go to Basic Settings, and VCDS should have a drop down menu you can click on that displays things that can be changed.

One of those will be idle speed.

With the engine at hot idle, you can change the number on the right side (the default I think is 32768.. but yours may have been changed, hence the higher than spec idle which is 903). Just make small changes up or down, the click Test, Save, and see how much the idle speed changes. You can keep doing that until you find what you want.

Although honestly 950 is not that high. These are not giant diesel engines, they idle the same speed or higher than many similar displacement gasoline engines. But some folks bump the idle speed up to make them a little smoother, especially if they have had some mods that might make them a little shudder prone at lower idle speeds.
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
I'm from the Midcoast area in Maine.
I went to basic settings and I'm not getting the drop down, I looked in the instruction manual, and that shows the dropdown you explained above.
Not sure what's going on, but this is what I see basic settings.
I have a HEX-V2

Do I have to select a specific group to get the drop down?
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
Whoops, instead of Basic Settings go to Adaptation. Sorry, my bad (you will have to turn the key back off then start the engine back up and re-enter the 12233 login).
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
So when I go to the adaptation menu, it will let me make changes, but for some reason I'm not seeing a drop down menu, it just lets me select some channel to change the value for. Is there some documentation that correlates the channel numbers to specific attributes? I have the service manual, would it be in there?

This is what I'm seeing. Obviously the engine is cold here, I'd warm it up before making any change.



EDIT; I have found this page that explains some of the channel numbers. Still not sure why I don't have the drop-down
 
Last edited:

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
It may be your version of VCDS, or what that label file for that ECU has tagged to it.

But channel 01 should be idle speed. And yours does appear to have been changed from the default value of 32768.

So I'd first try just putting it back to that, and see how it idles. It may be that there is something rattling or buzzing in the 20+ year old interior that drove someone crazy and by simply adding a few RPM to the idle speed it made it go away.

Normal idle speed is 903, and yours is showing 924 at 10.8 C. So it is not off by hardly anything at all, but if you want to change it you certainly can.
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
According to the ross-tech page I liked above in my edit, idle adjustmet should be channel 02, 01 should be injected quantity. I know injected quantity was turned down because apparently the clutch can't handle the full power of the rocketchip RC3 tune. I'll get a chance to go back out to the garage in a bit and take a look.

I guess I just assumed the idle should be lower, but it makes sense, it's a small, high compression, fast revving engine with low rotating mass.
I'm used to marine diesel engines that have massive rotating mass, rev slowly, and have a low redline. Even a 500cc marine diesel idles at 700rpm, but now that I actually think about it, that's such a different beast it can't be compared. Even my Mercedes 300TD has a massive rotating assembly and much lower redline.
 

oilhammer

Certified Volkswagen Nut & Vendor
Joined
Dec 11, 2001
Location
outside St Louis (where it's safe)
TDI
There are just too many to list....
Yeah, sorry again, I got the channels messed up, LOL. It has been a while since I was in one. My head is starting to fill up with all the newer cars' stuff and the older stuff gets shuffled about in there.

But yeah, idle spec is about 900, and they typically idle rock steady right above that. Most are 903 it seems. The ECU does some smooth idle compensation to add/subtract injected fuel to each cylinder, too.

I know you said this is new to you, but your car's technology was actually over a half decade old when it left the factory. It's ancient now. But it actually works very well. Bosch EDC15 is a solid, reliable way to make an engine run. They made some more refinements for the 2000 model year.
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
It's pretty damn modern to me, I drive cars from the 50's and 60's, and my mercedes is a fully mechanical diesel, so...
Anyway, I put the value back to the default. Now it idles at 903, nice and smooth, no rattles or anything. It actually also shuts off a lot more smoothly. Not sure why that value was messed with, but it's way better where it belongs!
 

chrisgt

Active member
Joined
Apr 29, 2021
Location
Maine
TDI
1998 Jetta TDI
Thanks for all the help on my noob questions!
I'm having fun getting the car back into nice shape. I have to reseal the pump and it's really cool being able to set the IQ correctly by just looking at vagcom and tapping the QA until the values are correct. I'm used to mech diesels where there is a certain level of greybeard hackery going on to get things back into the right place after a reseal.
 
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