Intermittent limp mode diagnosis

panda

Veteran Member
Joined
Jan 1, 2002
Location
Chichester, NH
TDI
2002 Jetta, 2015 Golf Sportwagen, 2005 Jeep Liberty CRD
For sometime now my 2002 TDI has been going into limp mode when under heavy acceleration in 3rd gear. It revs easily to red line in the lower gears. Usually this does not set a CEL but when it does the code is for intermittent overboost. This weekend since I was doing an oli change I decided to look further into this problem. The car has 310K miles. I recently changed all the vacuum lines. So I put the mitty vac on the acculator and seemed to get good smooth movement with around 17-18 inch producing maximum movement. I then used the Vag-Com VNT to graph N-75 turbo response. The results consistently indicated more boost than was requested when the turbo was in use. The 2 lines pretty much tracked each other separated by about a tick mark. Except under heavy acceleration when the actual boost increased further and then into limp mode. Since the turbo was obviously responding I assumed it was likely the N-75. To be sure I swapped with the EGR N-75, but unfortunately the car can still be made to go into limp mode a little over 3,000 rpm in third under heavy acceleration. At this point I'm thinking it must be the turbo but have a hard time explaining whats really happening.

BTW the linkage has never been adjusted so that's not likely the problem. Have i missed anything? If it's the turbo is it likely to fail soon or will it continue on like now?
 

harry3

Veteran Member
Joined
Apr 2, 2010
Location
wisconsin
TDI
2002 jetta tdi auto converted to 5 speed. 2003 tdi jetta 5 speed.
Sounds like sticky vanes to me. I think I would disconnect the linkage at the actuator and move the lever by hand. Mine would stick occasionally due to the fact that an internal part in the linkage had a grove worn on it. Mine had 200k miles on it. When working properly the lever moves oh so freely.
 

panda

Veteran Member
Joined
Jan 1, 2002
Location
Chichester, NH
TDI
2002 Jetta, 2015 Golf Sportwagen, 2005 Jeep Liberty CRD
So my diagnostic approach seems valid and the likely cause at this point is a problem with the turbo? With the number of miles on this car if I have to remove the turbo I'm tempted to just go ahead and replace it with a VNT15 for a mild upgrade. I just don't want to discover after doing that that I overlooked some other obvious cause and wasted $$ on a turbo that I didn't need. I have always feared the possibility of injesting a blown turbo.
 

panda

Veteran Member
Joined
Jan 1, 2002
Location
Chichester, NH
TDI
2002 Jetta, 2015 Golf Sportwagen, 2005 Jeep Liberty CRD
Solved

For sometime now my 2002 TDI has been going into limp mode when under heavy acceleration in 3rd gear. It revs easily to red line in the lower gears. Usually this does not set a CEL but when it does the code is for intermittent overboost. This weekend since I was doing an oli change I decided to look further into this problem. The car has 310K miles. I recently changed all the vacuum lines. So I put the mitty vac on the acculator and seemed to get good smooth movement with around 17-18 inch producing maximum movement. I then used the Vag-Com VNT to graph N-75 turbo response. The results consistently indicated more boost than was requested when the turbo was in use. The 2 lines pretty much tracked each other separated by about a tick mark. Except under heavy acceleration when the actual boost increased further and then into limp mode. Since the turbo was obviously responding I assumed it was likely the N-75. To be sure I swapped with the EGR N-75, but unfortunately the car can still be made to go into limp mode a little over 3,000 rpm in third under heavy acceleration. At this point I'm thinking it must be the turbo but have a hard time explaining whats really happening.

BTW the linkage has never been adjusted so that's not likely the problem. Have i missed anything? If it's the turbo is it likely to fail soon or will it continue on like now?
I had one suggestion in another forum that the problem may be a partially clogged CAT or muffler. This sounded like it made sense so I ordered a new complete exhaust system and finally got it installed. WOW what a difference, the car now pulls strong in every gear. No more intermittent limp mode. At 320,000 mile I guess a partially clogged exhaust system is a good assumption. I'm finally able to feel my "new" larger injectors.
 

4JimmyVee

Veteran Member
Joined
Sep 17, 2005
Location
Thornhill Ont Cda
TDI
1999.5 Silver
I have had similar problems, although I never thought it could be the turbo or the exhaust. If the car just does not want to go fast say above 50 mph check the MAF. If is it does get going but is slow would could replace the fuel filter, remember to prime the filter and put a pencil in the hose so air stays out of the engine. I hed this last month, sluggish acceleration and a bit of a stutter when idling.
 
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