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Any suggestions here? Boost is a few hundred RPM delayed, spikes, then dips, followed by a slow rise back up to normal just before I need to shift. Here's an example, green is target boost, yellow is actual:


I've tried adjusting the VNT rod out longer to about 20 in-mg, and also to the factory spec of 18.5 in-mg, but it doesn't seem to affect it any, not even the intensity of the spikes.

Lots has been done on this car to get it running right already:
New MAF
New N75
All new vacuum lines
New air filter
Cleaned snow screen
New VNT actuator (moves freely)
Clean EGR/intake manifold
Adjusted injection timing to optimum slight advance
 

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It looks like a badly lagging turbo. New VNT means a new turbo? How fast are you stepping on the throttle? Not surprisingly, a big stomp will cause a spike. Have you double checked the vacuum routing?
 

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Discussion Starter #3
It looks like a badly lagging turbo. New VNT means a new turbo? How fast are you stepping on the throttle? Not surprisingly, a big stomp will cause a spike. Have you double checked the vacuum routing?
Sorry, edited that to read "VNT actuator". Turbo is same as when I bought the car, original I believe. I've tried to gently roll on the throttle and the same spike happens, albeit the line starts at a different angle.

When I redid all the vacuum lines, I was pretty meticulous - remove and replace one piece at a time. It's possible something could be off, but I doubt very much - I matched up with diagrams and pictures best I could to make sure everything is/was proper. Any suggestions of anything in particular to look for there? Check valve directions, etc.?
 

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Discussion Starter #4
Just tried making the arm real long (23in-mg), and it came down quite a bit, but something definitely still looks wrong to me. Massive delays in boost delivery (which I guess I would expect with the rod so long - movement doesnt start until 5 or 6 in-mg now), and strange tapering:

 

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Discussion Starter #5
Some questions: would a boost leak cause a similar curve? How about a clogged breather? I see where some people have disconnected the vacuum line to the air filter box and cured spiking (mostly on modded cars though) due to pressure in the airbox.

Some observations: on an uphill, if I ease into throttle from 1500 RPM in 4th gear, my boost curve is absolutely perfect right up to redline. This is the only way I get it perfect, anything other than this (different starting RPM/gears/throttle stabs) and I get the spikes/dips. Perhaps my boost curve is fine, but when I use different gears or methods it is off? I would expect it to be similar (i.e. not as far off as it is) however, but I could be wrong.
 

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It is critically important that the turbo actator is set correctly. it should start moving at 4.5 to 5 in of mercury and move to the stop screw at 18" of mercury. A mity-vac is a good thing to have. you can also check the N75 you should have 19''+ going to the actuator at idle.
Is the car throwing any codes?
 

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Discussion Starter #8
It is critically important that the turbo actator is set correctly. it should start moving at 4.5 to 5 in of mercury and move to the stop screw at 18" of mercury. A mity-vac is a good thing to have. you can also check the N75 you should have 19''+ going to the actuator at idle.
Is the car throwing any codes?
The actuator is currently set to 3.5 - 18.5. I've tried starting at 4/5 (ending at 19/20) with the same results. No codes, everything other than the boost curve seems ok in VCDS (IQ, injection timing, MAF values, N75 cycles).

How do I verify the N75 vacuum output? I do have a MityVac, if I connect it to the output of the N75 will it show vacuum or is that backwards? (i.e. the MityVac needs vacuum INTO the pump, not OUT of it). Thank you.
 

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There should be an output test in VCDS for the N75. If you connect it to the line between the turbo and the N75 it will show vacuum. It will show the same on the line going into it because the N75 controls the vacuum to the turbo. I can't remember what the 3rd port does but it should be obvious when testing the N75.
 

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the 3rd port is a bleed to clean filter air thru thru the air filter box
disconnect the line to the acutator and measure the vacuum to the turbo actuator
 

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I'm also looking for a resolution to this problem
 
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