Overboosting / Wastegate Cycling

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
I'm having an issue where the engine hesitates under full throttle (full boost). The turbo will "cycle" and the boost will pull back, almost as if it's bouncing off a limiter. This causes some minor herky-jerkiness in the torque response as the turbo seems to backcycle.

This wasn't an issue before I swapped my turbo for another used OEM turbo. I have a Malone stage 2 tune, but aside from that all the rest of the engine is stock, with the stock turbo.

My guess is that the wastegate actuator may be to blame, or perhaps it may be something else like the N75 or the MAP sensor. I have no codes. The wastegate actuator adjustment has not been touched. It still has the factory paint markings on the actuator shaft.

The car actually went into an overboost limp for the first time ever on a long road trip the other week. I was at a constant 75MPH for about 2-3 hours and I went full throttle to overtake a car, after which the car decided it didn't want to boost anymore. Perhaps due to boost creep.

Limp mode was alarming given I had never experienced it before. I figured that there would be some kind of indication or CEL but apparently there isn't. The car started up and drove normally the next day.

Anyone know what I need to check to fix this?
 

Steve Addy

Top Post Dawg
Joined
Aug 7, 2002
Location
Iowa
TDI
97 Mk3
Check your turbo WG actuator to make sure it's free and moving as it should. These get dry and can end up sticking, which causes the erratic boost conditions...and limp mode.

Steve
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4 (sold), '97 Jetta (scrapped)
Lubing and exercising the wastegate lever and replacing vacuum lines solved my limp mode issues. It only occurred at 70-80mph.

I‘d occasionally get the intake manifold pressure code (1550?), but not always. If I really beat on it, I could get it at lower speeds, but I don’t typically never did that… only when testing the fix. It’s in the what did you do thread, but good luck finding it, lol.… probably 2 years ago.

-Todd
 

burpod

teh stallionz!!1
Joined
Nov 27, 2004
Location
cape cod, ma
TDI
82 rabbit vnt ahu, 98 jetta vnt ahu, 05 parts car, 88 scirocco.. :/
could easily be a MAF issue. can also be cause by clogged egr. the tune you have will likely exaggerate the issue. the smoke limiter is at play here helping cause the oscillations...

taking an 011+001+003 log would point you in the right direction :)

limp mode will happen if the boost deviations happen for long enough with enough deviation.
 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
This is when I facepalm and kick myself for not checking the wastegate duty cycle with the turbo off the car… like it was for months while I waited for parts for my engine rebuild….

very difficult to get to that actuator rod/lever with the turbo installed… but not impossible I suppose.
 

Steve Addy

Top Post Dawg
Joined
Aug 7, 2002
Location
Iowa
TDI
97 Mk3
This is when I facepalm and kick myself for not checking the wastegate duty cycle with the turbo off the car… like it was for months while I waited for parts for my engine rebuild….

very difficult to get to that actuator rod/lever with the turbo installed… but not impossible I suppose.
It's not a fun task if the engine is hot....but I managed to fix the actuator problem on a friends car in a parking lot by driving the right wheel up on a curb and crawling underneath and then using a 12" adjustable crescent wrench to work the actuator back and forth.

I burned myself more than once but got it done so I was happy...

Steve
 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
I got under the car today and it’s not the wastegate… it moves freely. I’m starting to think it’s a vacuum leak preventing the wastegate actuator from functioning properly. The booster plastic vacuum line cracked a while back, and though I have repaired it with a piece of hose, a spring and a couple hose clamps, I think I still have a small leak somewhere.

I’d love to fabricate something a bit more robust than the hard plastic stock piece but I couldn’t find any suitable materials at Home Depot Motorsports after checking multiple places.
 

ToddA1

Top Post Dawg
Joined
Aug 3, 2011
Location
NJ 08002
TDI
'96 B4V, '97 B4 (sold), '97 Jetta (scrapped)
Have you verified all the hoses to/from the N75 are clear? Disconnect both ends and blow through the tubing, one at a time. Is the N75 working properly?

-Todd
 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
Have you verified all the hoses to/from the N75 are clear? Disconnect both ends and blow through the tubing, one at a time. Is the N75 working properly?

-Todd
Vacuum lines are good. The only time I’m getting the issue is at full boost. I’m going to take some data logs and report back.
 

Steve Addy

Top Post Dawg
Joined
Aug 7, 2002
Location
Iowa
TDI
97 Mk3
Vacuum lines are good. The only time I’m getting the issue is at full boost. I’m going to take some data logs and report back.
turbo wastegate operates on pressure, check for N75 clicking, pull some numbers from vcds.

Out of curiosity what ECU are you running on your 97?

Steve
 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
This turned out to be a bad N75. Threw a new one in and it now boosts normally. Parts cannon works sometimes.

Symptoms:
1. Down on power
2. Lopey, rough feeling under hard acceleration (I attribute this to overboosting/turbo not running efficiently)
3. Issues described in post #1.

In the months before the N75 finally gave out, it would buzz sometimes for up to a minute after shutdown. I have discovered that this is due to residual vacuum being let out through the diaphragm. How much of that buzzing is normal is anyone’s guess, but the buzzing had actually ceased after shutdown for several months before I finally replaced the valve. The new one does not buzz.

My guess is that the diaphragm finally gave out and couldn’t hold pressure to actuate the wastegate fully, leading to overboost.

Hopefully the new valve lasts 25 years like the old one did!
 
Last edited:

ValveCoverGasket

Veteran Member
Joined
Apr 26, 2012
Location
pnw
TDI
colorado duramax, 1z vanagon, tdi touareg
good to hear you fixed it with a valve!

Lubing and exercising the wastegate lever and replacing vacuum lines solved my limp mode issues. It only occurred at 70-80mph.
i was surprised how much of a difference this made on my swapped van (though to be fair the turbo sits a lot closer to the road and to a wheel in that setup) previously some really funky and irregular boost response, along with boost plausibility codes, after some lubing alls well again.

i imagine maybe that could be an issue on a mk3/b4 that was run in a lot of road grime or didnt have a belly pan?
 

garciapiano

Veteran Member
Joined
Feb 12, 2018
Location
Southern California
TDI
1997 Jetta TDI (1Z)
good to hear you fixed it with a valve!

i imagine maybe that could be an issue on a mk3/b4 that was run in a lot of road grime or didnt have a belly pan?
The wastegate seizing is either corrosion buildup or carbon buildup. So it could be a combination of many factors.

Mostly, if it’s not getting exercised enough, it can seize up. So the best preventative measure is driving it hard on a regular basis - not letting it sit.
 
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