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TDI Power Enhancements Discussions about increasing the power of your TDI engine. i.e. chips, injectors, powerboxes, clutches, etc. Handling, suspensions, wheels, type discussion should be put into the "Upgrades (non TDI Engine related)" forum. Non TDI vehicle related postings will be moved or removed. Please note the Performance Disclaimer.

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Old September 3rd, 2006, 02:44   #1
Join Date: Nov 2005
Location: Victorville, CA
TDI(s): 98 Jetta
Fuel Economy: 40mpg, but I drive it like MAD!!
Default Why does the car go into Limp Mode?

To begin, I imagine that this is NOT the correct place for this thread, but since I couldn't find any location I thought suitable, I put it in the area it seemed least offensive. If anyone who has the power to do so knows a more correct place for it, please move it and accept my apology for posting in the wrong place!

Also, let me add that I have searched for about 4 hours on this, and I can find 500 threads that say "oh, you must be going into limp mode" but not one that discusses WHY it is going into limp mode. If this has already been fully addressed in some other section, I can't find it and would someone please link me to it?

On to the problem:

I got my 98 Jetta right at a year ago. It is a stick shift, if that matters.

Months before I ever put a VO fuel system in it, there would be times when it would suddenly have no power. At the time, I described it as "like the turbo suddenly was gone off the system". I mean it would run smooth, idle nicely, and take off and drive like normal, but even with my foot on the floor it could barely accelerate. Next time I would drive it, that problem would not be there. So I sort of noticed that if it was doing that, I just needed to shut it off and restart it and the problem would go away.

I read up on it some on this forum, and found "limp mode" and determined that must be what I was having. But right about then it stopped happening, and so I forgot about it.

Around December was the last time I remember noticing it, until about a month ago it started happening again. Sometimes I can drive 200 miles and it doesn't go into limp mode, other trips it will drop back into limp mode within 5 or 10 seconds of restarting.

We plugged in an OBD sensor (not a vag-com). Among the readings it gives is something about the glow plug circuit, which was throwing a CEL last year. But that too stopped happening, and so I also forgot about it. And it gave several codes that said something like "undefined drive train problem" which was useless information.

Aside from the hassle it is to have to restart the car, I know that it must be going into limp mode because it is detecting a problem that I really ought to fix. But the problem is that I don't know what the problem is!!

What kinds of things will cause the computer to go into limp mode? Without the use of a vag-com, I wouldn't know what to start checking, and would at least like a little guidance on where to look.

Or, is there anyone in Southern California with a vag-com and knowledge that would be willing to plug it and tell me a few things if I came to their place?


98 Jetta TDI, Frybrid VO fuel system , Installation thread here. 27,000 miles on VO as of 9/1/06
91 F-250, 7.3, 5-Spd, blending in rear tank while working on a homebrewed conversion. 8,000 miles on VO as of 9/1/06

Installing VO conversion kits (all makes) in Southern California, e-mail me
JeffNLisa is offline   Reply With Quote
Old September 3rd, 2006, 06:38   #2
Veteran Member
Join Date: May 2003
Location: Between Toronto & Montreal
Fuel Economy: 5.3 l/100 km (at 115 km/h)

>>What kinds of things will cause the computer to go into limp mode? <<

This is a protective mode that the car's computer defaults to if it detects problems with the boost (turbo) system. On the A4's this is typically caused by a sticking turbo actuator, but on your A3 you must look elsewhere... the N75 malfunctioning will cause it, a bad MAP or MAF sensor would cause it, a vacuum or boost hose leak may cause it... (oh, yes, a bad turbo also...)

You really should get your own Vag-Com, btw... I'm sure there are folks around your area who have one - try the Vag-Com list at the top of the page...

Good luck,


PS: if the car's rolling when the limp mode creeps up, you can just turn off the key while remaining in gear, wait a second, and turn the "ignition" back on... assuming that traffic will allow this...
ymz is offline   Reply With Quote

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